TESTING DEVICE FOR THE ELECTRICAL SYSTEM OF A TRUCK WITH A PLUG FOR INSERT INTO A TRAILER SOCKET OF THE TRUCK

Description

TECHNICAL FIELD OF THE INVENTION

[0001] The invention relates to a testing device for the electrical system of a truck. More specifically, the invention relates to such a testing device with which the correct connection and correct function of a trailer socket of the truck and of connected parts of the truck's electrical system can be checked. For this purpose, the testing device has a plug for plugging into the trailer socket of the truck, which plug contacts comprise plug contacts for contacting socket contacts of the trailer socket, and a housing in which the plug is fastened, in which a testing device is mounted and on which at least one optical signal generator is arranged. The testing device is connected to the plug contacts and the at least one signal generator and controls the at least one signal generator depending on electrical voltages between the plug contacts. STATE OF THE ART

[0002] A test device that can be used to verify the correct connection and function of a truck's trailer socket and the connected components of the truck's electrical system is known as the ABS test plug from Secorüt Kfz-Electronic GmbH, Staufenberg, Germany; see https: / / www.secoruet.net / secoruet_produkt / abs-prufstecker / . At the end of the housing facing away from the plug, a signal generator in the form of an indicator light is located. This indicator light illuminates when the ABS test plug is plugged into a trailer socket compliant with ISO 7638 and a supply voltage is present between pins 1 and 3 and pins 2 and 4 of this trailer socket, and a signal voltage is present at pin 5.

[0003] The well-known ABS test plug does not indicate which of the socket contacts of the trailer socket being tested is incorrectly connected. It also does not allow testing of a trailer connected to the trailer socket. Furthermore, the test does not reveal whether the electrical system of the respective truck has defects if the respective trailer socket is fundamentally correctly connected and any supply and signal voltages are present at its socket contacts.

[0004] DE 44 92 205 T1 discloses a trailer lighting tester for a trailer, which has indicator lights in associated trailer circuits, which can be detachably connected to associated circuits of a tractor by means of a plug connection. The tester has a power source, a test lamp, and a switch arranged in a test circuit, by means of which the test circuit is closed via at least one indicator light of the trailer and the test lamp is activated when the indicator light of the trailer and the associated trailer circuit receive power. A separate test circuit is provided for each trailer circuit, and a separate indicator light is arranged in each test circuit. Each test circuit also has a diode that isolates the test circuit from other test circuits when power is supplied from the power source.

[0005] DE 199 12 732 A1 discloses a test device for a trailer socket with a test circuit, with lamps, and with the features of the preamble of independent patent claim 1. The test circuit simulates the bulb resistances of the lighting and braking system and includes a save and reset function. The test circuit comprises two resistors in series, one of which is variably adjustable. The base of a transistor is connected to the center of the resistors, and the lamp is located in the collector path of the transistor. A capacitor is arranged in parallel with the variable resistor. The variable resistor makes it possible to simulate different bulb resistances.By simulating the resistance of the bulb, in addition to the respective bulb of the test device, the corresponding bulb in the control panel of the driver's cab of the respective motor vehicle lights up, indicating to the driver that he has switched on the corresponding lighting.

[0006] Another testing device with the features of the preamble of independent patent claim 1 is known from US Pat. No. 4,249,125 A. Here, an LED with a series resistor is connected in series between each plug contact of the test device's plug and a reference potential transmitted via another plug contact. The LED lights up when a voltage so high that the forward voltage of the LED is exceeded is applied to the respective plug contact.

[0007] From AU 2007 200 862 B2, a testing device with the feature of the preamble of independent patent claim 1 is also known, in which a light-emitting diode with a series resistor is connected in series between the respective plug contact and the reference potential transmitted via another plug contact.

[0008] From Phillips: "QwikTechTips_June11 Troubleshooting Trailer Lights," June 27, 2011 (2011-06-27), XP055659309, a truck electrical tester is known for its connector plugged into a trailer receptacle and LED indicators that only illuminate when the corresponding truck lights are activated and the trailer receptacle is properly connected. If an LED on the tester does not illuminate when the corresponding truck lights are activated, it is suggested to measure the voltage of the corresponding circuit breaker at the truck's trailer receptacle.

[0009] US Pat. No. 6,259,170 B1 discloses a test device that can be plugged between a trailer socket of a towing vehicle and a trailer plug of a trailer. The test device has two LEDs for each plug contact of the trailer socket, which are wired in such a way that one LED indicates a current flow through the plug contact, while the other LED indicates that a current-limiting element arranged between the trailer socket and the trailer plug is limiting the current flowing through the plug contact. OBJECT OF THE INVENTION

[0010] The invention is based on the object of providing a testing device for the electrical system of a truck with the features of the preamble of independent patent claim 1, with which a more detailed diagnosis of the electrical system of the respective truck can be carried out with further little effort. SOLUTION

[0011] The object of the invention is achieved by a testing device having the features of independent patent claim 1. Dependent patent claims 2 to 12 relate to preferred embodiments of the testing device according to the invention. Patent claim 13 is directed to a specific use of the testing device according to the invention. DESCRIPTION OF THE INVENTION

[0012] In a testing device according to the invention for the electrical system of a truck, comprising a plug for plugging into a trailer socket of the truck, which plug contacts comprise plug contacts for contacting socket contacts of the trailer socket, and a housing to which the plug is fastened, in which a testing device is mounted and on which at least one optical signal generator is arranged, wherein the testing device is connected to the plug contacts and the at least one signal generator and controls the at least one signal generator depending on electrical voltages between the plug contacts, the testing device controls the at least one signal generator both depending on whether or not any electrical voltage with respect to a reference potential having the correct sign is present at at least one of the plug contacts, and also depending onwhether the voltage between at least one of the plug contacts and a reference potential reaches a specified minimum value or not.

[0013] The test device of the test apparatus according to the invention not only checks at least one of the plug contacts to determine whether a voltage of the correct sign is present relative to a reference potential, but also checks the magnitude of this voltage by comparing it with the specified minimum value. Depending on the result of this comparison, the test device activates at least one signal generator. This not only checks the socket contact assigned to the plug contact for correct connection to the truck's electrical system, but also whether the truck's electrical system provides sufficient voltage at the socket contact, as required for the safe operation of a trailer connected to the truck via the trailer socket.

[0014] Typically, the minimum value for which the voltage is checked is 5 / 6 of the respective nominal voltage, i.e. 10 V for a nominal voltage of 12 V and 20 V for a nominal voltage of 24 V.

[0015] The inventive method of checking the voltage at the plug contact does not require measuring a specific voltage value. Rather, the voltage can be directly compared with the specified minimum value. The specified minimum value can be implemented, for example, as the reverse voltage of a Zener diode, so that the Zener diode becomes conductive when the specified minimum voltage is reached. However, the respective voltage can also be compared with a reference voltage generated by the test device using a comparator.

[0016] The user of the testing device according to the invention can read more precisely from the at least one signal generator what the electrical system of the respective truck is like, e.g. whether the socket contact assigned to the plug contact is simply not supplying sufficient voltage or is fundamentally incorrectly contacted.

[0017] In order to be able to output various information to the user of the testing device according to the invention via the at least one signal generator, the at least one signal generator is designed to actively output at least two different signals and / or it has at least two different activatable signaling elements. The signaling elements can be LEDs, for example. Even with just one LED, different signals can be actively output, for example by setting the LED to light up continuously or to flash. With two LEDs of different colors, different active signals with different meanings can be output very easily. For example, a green LED can light up to indicate that the truck's electrical system is in good condition, while a red LED can light up to indicate that a test carried out had a negative result with regard to the truck's electrical system.

[0018] It is particularly preferred in the test device according to the invention if the at least one signal generator is exclusively assigned to the at least one of the plug contacts, so that the test result with respect to the at least one plug contact and the assigned at least one socket contact can be read off the at least one signal generator.

[0019] Furthermore, in the test device according to the invention, it is preferred if not just one of the plug contacts but all plug contacts assigned to one of the socket contacts for a supply voltage are checked to determine whether electrical voltages are present relative to the associated reference potentials that reach predetermined minimum values ​​for the respective supply voltages. It is particularly preferred if the test device has, for each control contact assigned to one of the socket contacts for a supply voltage, a signal generator exclusively assigned to the respective plug contact, which the test device controls depending on whether the voltage between the respective plug contact and the associated reference potential reaches the associated predetermined minimum value or not.

[0020] Furthermore, the test device of the test apparatus according to the invention controls the at least one signal generator depending on whether or not a current flows through at least one of the plug contacts assigned to one of the socket contacts for an electrical ground when a test voltage is applied. This checks whether the respective socket contact is correctly connected. Specifically, it checks whether or not a current flows when a predetermined test voltage is applied that reaches a minimum current value specified for one of the socket contacts. This checks not only the presence of an electrical ground at the respective socket contact but also its function in the form of its internal resistance.

[0021] Preferably, the test device according to the invention has, for each plug contact assigned to one of the socket contacts for an electrical ground, a signal generator exclusively assigned to the respective plug contact, which the test device controls depending on whether, when the associated predetermined test voltage is applied, a current flows which reaches a minimum current value predetermined for the respective socket contact or not.

[0022] Furthermore, the test device can control the at least one signal generator depending on whether an electrical voltage is present at at least one of the plug contacts assigned to one of the socket contacts for a signal voltage relative to the corresponding reference potential, which signals a fault-free operating state of the truck's electrical system. This fault-free operating state of the truck's electrical system can be a basic state in which the respective signal voltage neither signals a fault in the truck's electrical system nor transmits any control commands.

[0023] Preferably, the test device has, for each plug contact assigned to one of the socket contacts for a signal voltage, a signal generator exclusively assigned to the respective plug contact, which the control device controls depending on whether the respective electrical voltage is applied to the associated reference potential, which signals the fault-free operating state of the truck's electrical system.

[0024] Overall, the testing device according to the invention therefore has, for each plug contact and thus for each socket contact checked with the aid of the plug contacts, a signal generator exclusively assigned to the respective plug contact, which indicates whether the part of the truck's electrical system connected to the corresponding socket contact is working correctly.

[0025] If the at least one signal generator of the test device according to the invention is arranged in a flattened area of ​​the housing with a round cross-section, the housing, with its round cross-section, can be adapted to the connection cross-section of the plug of the test device attached to it. The arrangement of the at least one signal generator in the flattened area simplifies the mounting of the part of the test device connected to the signal generator, which is usually arranged on a flat circuit board, in the housing.

[0026] In the test device according to the invention, a legend for interpreting the signals of the at least one signal generator is advantageously arranged directly on the housing, so that a user can interpret these signals, for example, without consulting an operating manual even if the signals provide him with information individually for each plug contact or the associated socket contact.

[0027] The test device of the test apparatus according to the invention can have an energy storage device from which it is powered even when no external electrical voltage is applied to the plug contacts. Preferably, however, the test device comprises a power supply that is fed by any voltage present between the plug contacts. The power supply can be connected to the plug contacts in such a way that any voltage present between them is suitable for feeding the power supply. This means that the voltage present between the plug contacts does not even have to have the correct polarity, which results when the corresponding trailer socket is correctly connected.However, since it will rarely be the case that none of the supply voltages are present with the correct polarity at the plug contacts, it can also be provided to alternatively feed the power supply from all supply voltages that should be present between the plug contacts. If all of these supply voltages are unavailable and the at least one signal generator of the test device according to the invention cannot emit an active signal, this indicates that the electrical system of the respective truck has a significant deficiency with regard to the supply voltages that should be present at the corresponding socket contacts of its trailer socket.

[0028] Specifically, the plug of the test device according to the invention can be a plug compliant with the ISO 7638 standard. The test device according to the invention is then accordingly intended for testing the electrical system of a truck via a trailer socket compliant with this ISO 7638 standard. This can be a socket compliant with the ISO 7638-1 standard for a 24-volt electrical system of the truck, or a socket compliant with the ISO 7638-2 standard for a 12-volt electrical system of the truck. It is understood that, depending on the specific standard, the minimum values ​​with which, for example, supply voltages are compared by the test device of the test device according to the invention vary. For example, with a 24-volt electrical system of the truck, each supply voltage must be checked to ensure that it complies with a minimum value of approximately 20 volts, for example, 19 volts.

[0029] In a particularly preferred embodiment of the test device according to the invention, the plug contacts are connected to the socket contacts of a receptacle that is designed according to the same standard as the plug and is part of the test device. In particular, the receptacle can be attached to an end of the housing opposite the plug. The test device can thus also be connected between the respective truck and a trailer connected to it in order to check the truck's electrical system for interference from the trailer.

[0030] Even if the test device according to the invention only has a plug and no dedicated socket, it can be used not only to directly test the electrical system of a truck at its trailer socket, but also to test a trailer connection cable connected between this trailer socket and the test device. Such a connection cable typically has a coiled cable between a cable plug for plugging into the truck's trailer socket and a cable socket for connecting to a trailer plug on the trailer, and is also referred to as a SUSIE cable.

[0031] When using the test device according to the invention with an additional socket, the test device is first plugged into the trailer socket of a truck, the truck's ignition is switched on, and then the at least one signal transmitter is read. If no signals are registered that indicate a fault in the truck's electrical system, a cable plug for a trailer connection cable is plugged into the trailer socket of the truck instead of the test device according to the invention, and the test device is plugged into a cable socket of the trailer connection cable, the truck's ignition is switched on, and then the at least one signal transmitter is read.If no signals are registered that would indicate a fault in the trailer connection cable, the test device is connected in series with the trailer connection cable between the truck's trailer socket and a trailer plug, a truck's handbrake is applied with the ignition on, and then at least one signal transmitter is read. In this case, the test device, if designed for a trailer socket according to the ISO 7638 standard, also checks whether the trailer's ABS is functioning properly. In such a trailer socket, the socket contacts for signal voltages are those of a so-called CAN bus.In this case, without the CAN bus forwarding signals, the signal voltage at one of the two corresponding socket contacts is at a lower voltage and the other of the two associated socket contacts is at an upper voltage, which the test device according to the invention checks by comparing it with corresponding upper and / or lower limit values.

[0032] Advantageous developments of the invention emerge from the patent claims, the description, and the drawings. The advantages of features and combinations of several features mentioned in the description are merely exemplary and can be effective alternatively or cumulatively, without the advantages necessarily being achieved by embodiments according to the invention. Without altering the subject matter of the appended patent claims, the following applies to the disclosure content of the original application documents and the patent: further features can be found in the drawings—in particular, the illustrated geometries and the relative dimensions of several components to one another, as well as their relative arrangement and operative connection.The combination of features of different embodiments of the invention or of features of different patent claims is also possible, deviating from the chosen references of the patent claims, and is hereby encouraged. This also applies to features shown in separate drawings or mentioned in their description. These features can also be combined with features of different patent claims. Likewise, features listed in the patent claims can be omitted for further embodiments of the invention.

[0033] The number of features mentioned in the claims and the description is to be understood as meaning that exactly this number or a greater number than the stated number is present, without the need for the explicit use of the adverb "at least." Thus, for example, if reference is made to one element, this is to be understood as meaning that exactly one element, two elements, or more elements are present. The features mentioned in the claims may be supplemented by further features or may be the only features present in the respective product.

[0034] The reference signs contained in the patent claims do not represent a limitation of the scope of the subject-matter protected by the patent claims. They serve solely to make the patent claims easier to understand. BRIEF DESCRIPTION OF THE CHARACTERS

[0035] In the following, the invention is further explained and described with reference to preferred embodiments shown in the figures. Fig. 1 shows a testing device according to the invention in a side view. Fig. 2 is a schematic circuit diagram of the test device according to the invention according to Fig. 1 . Fig. 3 is a diagram of the test device according to Fig. 1 examinations carried out; and Fig. 4 is a flow chart of an inventive use of the testing device according to the Fig. 1 and 2 . FIGURE DESCRIPTION

[0036] The Fig. 1The test device 11 shown has a plug 12 that is designed to be plugged into a trailer socket in accordance with the ISO 7638 standard. The plug 12 comprises a flap abutment 13 and a pivotally mounted locking element 14. The plug 12 is attached to one end of an elongated, slightly conical housing 15 with a substantially round cross-section. At the other end of the housing 15, which has a slightly larger diameter, a socket 16 is attached, which is also designed in accordance with the ISO 7638 standard. It has a cover flap 17 and locking elements 18 that complement the pivotable locking element 14. In addition, a fastening plate 19 is part of the socket 16, but is not used in the test device 11. Signal transmitters 21 to 27 are arranged in a flattened area 20 on the housing 15.The signal generators 21 to 25 each comprise a green LED 28 and a red LED 29, while the signal generators 26 and 27 each comprise only a white LED 30. A circuit board, via which the LEDs 28 to 30 are electrically contacted and controlled, is arranged on the rear side of the flattened area 20 and fastened thereto with a fastening screw 31. Inside the housing 15, the signal generators 21 to 27 are connected to a test device 32, to which the plug 12 and the socket 16 are also electrically connected.

[0037] The schematic circuit diagram of the test device 11 according to Fig. 1 , which in Fig. 2reproduced shows that each of seven plug contacts 1 to 7 of plug 12 is directly electrically contacted with a corresponding socket contact 41 to 47 via a connecting line 33 to 39. The test device 32 is connected to each of the connecting lines 33 to 39. The test device 32 controls the LEDs 28 to 30 of the signal generators 21 to 27. A voltage supply 40 of the test device 32 is fed by voltages between the connecting lines 33 and 34 on the one hand and 35 and 36 on the other. This takes into account that in a trailer socket according to the ISO 7638 standard, the socket contacts corresponding to plug contacts 1 and 2 have supply voltages different from the socket contacts corresponding to plug contacts 4 and 3.The power supply 40 is preferably designed such that it does not depend on both supply voltages applied to socket contacts 1 and 2 reaching a predetermined minimum value, or on both of these supply voltages being applied at all. Thus, only a single, lower supply voltage is sufficient to power the power supply 40.

[0038] The test facility 32 carries out various tests as described below using Fig. 3These tests can be carried out when the test device 11 is only plugged with its plug 12 into a trailer socket or a cable socket of a trailer connection cable leading to a trailer socket, but also when the test device with such a trailer connection cable is connected in series between a trailer socket and a trailer of a truck. In the latter case, the test device 32 tests not only the electrical system of the truck or the electrical system of the truck and the trailer connection cable, but also the electrical system of the trailer. This will be discussed in connection with the following Fig. 4 will be discussed in more detail. Fig. 3Tests 51 to 57, which are presented in a specific sequence, can also be carried out by the test device 32 in any other sequence or even completely or partially in parallel. During test 51, the test device 32 checks whether a voltage U 1-4 , which is applied to the plug contact 1 opposite the plug contact 4, reaches a minimum value of 19 volts. If this is the case, the test device 32 lights up the green LED 28 of the signal generator 21, otherwise its red LED 29. Accordingly, during test 52, the test device 32 checks whether a voltage U 2-3 at the plug contact 2 opposite the plug contact 3 reaches at least 19 volts. If this is the case, the test device 32 illuminates the green LED 28 of the signal generator 22, otherwise its red LED 29. During test 53, the test device 32 checks whether a current flowing via the plug contact 3 as a result of the voltage U 2-3 reaches a minimum current value I min.If this is the case, the test device 32 illuminates the green LED 28 of the signal generator 23, otherwise its red LED 29. Accordingly, in test 54, the test device 32 checks whether a current I 4 driven by the voltage U 1-4 via the plug contact 4 reaches the minimum current value I min. If this is the case, it illuminates the green LED 28 of the signal generator 24, otherwise its red LED 29. In a test 55, the test device 32 checks whether a voltage U 5 at the plug contact 5 is clearly above 5 volts, because a voltage below this value at the plug contact 5 signals an ABS fault according to the ISO 7638 standard. If the voltage exceeds 5 volts, the test device 32 causes the signal generator 25 to illuminate green by controlling its LED 28, otherwise it causes its LED 29 to illuminate red. During test 56, the test device 32 checks whether a voltage U 6 at the plug contact 6 is greater than a minimum voltage U min.If this is the case, it illuminates LED 30 of signal generator 26, indicating that an upper signal voltage of a CAN bus has reached its target value. During test 57, test device 32 checks whether a voltage U 7 is present at connector contact 7 but remains less than U max. If this is the case, it illuminates LED 30 of signal generator 27. This indicates that the lower signal voltage of the CAN bus is present and at the correct voltage level. Test device 32 determines voltages U 5 to U 7 relative to a reference potential specified by connector contact 3 and / or connector contact 4, depending on which of the two connector contacts 3 and 4 is connected to a functional ground, which is determined by tests 53 and 54.

[0039] Tests 51 to 57 are preferably carried out three times with the test device 11 according to the invention under different conditions. Fig. 4illustrates that these conditions arise in a first step 8 in that the test device 11 with its plug 12 is plugged into a trailer socket of a truck and the truck's ignition is switched on. In a subsequent step 9, the conditions arise in that, instead of the test device, a cable plug of a trailer connection cable is plugged into the trailer socket and the test device 11 with its plug 12 is plugged into a cable socket of the trailer connection cable and the truck's ignition is then switched on. This means that the trailer connection cable is also tested with the test device 11. In a third and final step 10, the test device 11 with the trailer connection cable is connected in series between the trailer socket and a trailer of the respective truck. It is irrelevant in which order the already tested trailer connection cable is connected in series with the test device.The truck is then driven, stopped, its handbrake engaged, and its ignition left on until tests 51 to 57 are performed for the third time and the test result indicated by signal heads 21 to 27 is read. Any faults in the truck's electrical system identified by the test device in steps 8 to 10 must be rectified immediately. A truck with a proper electrical system, including proper connection of its trailer socket and proper electrical system of its trailer, will always have all green LEDs 28 of signal heads 21 to 25 and both white LEDs 30 of signal heads 26 and 27 illuminated. LIST OF REFERENCE SYMBOLS

[0040] 1 Plug contact 2 Plug contact 3 Plug contact 4 Plug contact 5 Plug contact 6 Plug contact 7 Plug contact 8 Step 9 Step 10 Step 11 Test device 12 Plug 13 Flap abutment 14 Locking element 15 Housing 16 Socket 17 Cover flap 18 Locking element 19 Mounting plate 20 Flattened area 21 Signal generator 22 Signal generator 23 Signal generator 24 Signal generator 25 Signal generator 26 Signal generator 27 Signal generator 28 Green LED 29 Red LED 30 White LED 31 Mounting screw 32 Test device 33 Connecting cable 34 Connecting cable 35 Connecting cable 36 Connecting cable 37 Connecting cable 38 Connecting cable 39 Connecting cable 40 Power supply 41Socket contact 42Socket contact 43Socket contact 44Socket contact 45Socket contact 46Socket contact 47Socket contact 51Test 52Test 53Test 54Test 55Test 56Test 57Test

Claims

1. Testing apparatus (11) for the electrical system of a truck, comprising - a plug (12) for insertion into a trailer socket of the truck, which comprises plug contacts (1 to 7) for contacting socket contacts of the trailer socket, - a housing (15) to which the plug (12) is attached, in which a testing device (32) is mounted and on which at least one optical signal transmitter (21 to 27) is arranged, - wherein the testing device (32) is connected to the plug contacts (1 to 7) and the at least one signal transmitter (21 to 27) and actuates the at least one signal transmitter (21 to 27) as a function of electrical voltages between the plug contacts (1 to 7), - wherein the testing device (32) activates the at least one signal transmitter (21 to 27) both as a function of whether or not any electrical voltage with respect to a reference potential, which has the correct sign, is present at at least one of the plug contacts (1 to 7), and as a function of whether or not the voltage between the at least one of the plug contacts (1 to 7) and the reference potential reaches a predetermined minimum value, characterized in - that the minimum value of the electrical voltage is specified as required for the safe operation of a trailer to be connected to the truck via the trailer socket, and - that it can be read from the at least one signal transmitter (21 to 27) whether the socket contact assigned to the plug contact (1 to 7) is just not supplying sufficient voltage or is even basically incorrectly contacted, wherein the at least one signal transmitter (21 to 27) is designed to actively output at least two different signals and / or has at least two different signaling elements which can be activated, and - that the testing device (32) activates the at least one signal transmitter (21 to 27) as a function of whether or not a current flows via at least one of the plug contacts (1 to 7), which is assigned to one of the socket contacts for an electrical earth, when a predetermined test voltage is applied, which current reaches a predetermined minimum current value for the one of the socket contacts.

2. Testing apparatus (11) according to claim 1, characterized in that the at least one signal transmitter (21 to 27) is exclusively assigned to the at least one of the plug contacts (1 to 7).

3. Testing apparatus (11) according to one of the preceding claims, characterized in that the testing device (32) activates the at least one signal transmitter (21 to 27) as a function of whether or not an electrical voltage with respect to the associated reference potential is present at each of the plug contacts (1 to 7), which is assigned to one of the socket contacts for a supply voltage, which voltage reaches the predetermined minimum value for the respective supply voltage.

4. Testing apparatus (11) according to claim 3, characterized in that the testing apparatus has, for each plug contact (1 to 7) which is assigned to one of the socket contacts for a supply voltage, one signal transmitter (21 to 27) which is exclusively assigned to the respective plug contact (1 to 7) and which the testing device (32) activates as a function of whether or not the voltage between the respective plug contact (1 to 7) and the associated reference potential reaches the associated predetermined minimum value.

5. Testing apparatus (11) according to any of the preceding claims, characterized in that the testing apparatus (11) has, for each plug contact (1 to 7) which is assigned to one of the socket contacts for an electrical earth, one signal transmitter (21 to 27) which is exclusively assigned to the respective plug contact (1 to 7) and which the testing device (32) activates as a function of whether or not a current flows, which reaches a minimum current value predetermined for the respective socket contact, when the associated predetermined test voltage is applied.

6. Testing apparatus (11) according to any of the preceding claims, characterized in that the testing device (32) activates the at least one signal transmitter (21 to 27) as a function of whether an electrical voltage with respect to the associated reference potential, which signals a fault-free operating state of the electrical system of the truck, is present at at least one of the plug contacts (1 to 7), which is assigned to one of the socket contacts for a signal voltage.

7. Testing apparatus (11) according to claim 6, characterized in that the testing apparatus (11) has, for each plug contact (1 to 7) which is assigned to one of the socket contacts for a signal voltage, one signal transmitter (21 to 27) which is exclusively assigned to the respective plug contact (1 to 7) and which the testing device (32) activates as a function of whether the respective electrical voltage with respect to the associated reference potential is present, which signals the fault-free operating state of the electrical system of the truck.

8. Testing apparatus (11) according to any of the preceding claims, characterized in that the at least one signal transmitter (21 to 27) is arranged in a flattened region (20) of the housing (15), which is round in cross-section.

9. Testing apparatus (11) according to any of the preceding claims, characterized in that a legend for interpreting signals of the at least one signal transmitter (21 to 27) is arranged on the housing (15).

10. Testing apparatus (11) according to any of the preceding claims, characterized in that the testing device (32) comprises a voltage supply (40) which is fed from any voltage applied between the plug contacts (1 to 7)11. Testing apparatus (11) according to any of the preceding claims, characterized in that socket contacts of a socket (16) are connected to the plug contacts (1 to 7), which socket is designed according to the same standard as the plug (12) and which socket is part of the testing apparatus (11).

12. Testing apparatus (11) according to claim 11, characterized in that the socket (16) is attached to an end of the housing (15) opposite the plug (12).

13. Use of a testing apparatus (11) according to claim 11 or 12, characterized in - that the testing apparatus (11) is plugged with its plug (12) into the trailer socket of a truck, the ignition of the truck is switched on and then the at least one signal transmitter (21 to 27) is read out, - that a cable plug of a trailer connecting cable is plugged into the trailer socket of the truck and the testing apparatus (11) is plugged with its plug (12) into a cable socket of the trailer connecting cable, the ignition of the truck is switched on and then the at least one signal transmitter (21 to 27) is read out, and - that the testing apparatus (11) is connected in series with the trailer connection cable between the trailer socket of the truck and a trailer socket of a trailer, a handbrake of the truck is actuated with the ignition switched on and then the at least one signal transmitter (21 to 27) is read out.