Author: Xin Bao Auto Service Co., Ltd. of any new HE
Failure phenomenon
A 2018 Audi A4 Avant car, equipped with a CYR engine, has a cumulative mileage of approximately 10,000 km. The car was repaired in the factory due to a front collision accident. The radiator frame was mainly replaced during maintenance. In addition, some wiring harnesses at the front were damaged, which caused the relay on the lower part of the A-pillar on the left and several fuses on the fuse holder to blow. Repair technicians Do the corresponding repair and replacement. After the repair work of the electromechanical part was completed, the engine was started and found that the engine could not be started. The maintenance technician failed to eliminate the fault after checking for 2 days, so he asked the author for technical support.
Troubleshooting
Test drive after picking up the car, step on the brake pedal, press the start button, the starter runs with the engine, the sound is strong, but the engine cannot be started; release the start button and press it again, this time the starter makes a "click" sound , There is no rotating sound; this is the same phenomenon when the engine is started later, but the No. 15 (IG ON) power supply can be normally connected.
It was detected with a fault detector and it was found that the fault code "P068700 main relay, short-circuit to the positive pole, passive/incidental" (Figure 1) was stored in the engine control unit (J623). After the fault code is cleared, the starter can run again, but it cannot be started. The fault code reappears after one or two starts. At this time, the starter will only make a "click" and no longer rotate. Enter the guided fault query function, execute the "J271-Motronic current supply relay, electrical failure" detection plan (Figure 2), prompting that this fault will occur when the power supply terminal B3, terminal B5 and terminal B6 of J623 suddenly have no voltage. The meaning of terminal B3 is terminal 3 on J623 wire connector B (Figure 3, that is, wire connector T91 in the circuit diagram), and the meaning of other terminals can be deduced by analogy.
Looking at the related circuit (Figure 4), it is known that the power supply of J623 terminal B3, terminal B5 and terminal B6 is provided by the positive electrode of the battery through the main relay (J271) and the fuse SB14. J271 is responsible for supplying power to many parts of the engine, such as the fuel pump control unit, various solenoid valves, ignition coils, etc. If there is a problem with J271, the engine will not start.
Since the starter does not work occasionally, and the fault code points to the power supply of J623 terminal B3, terminal B5 and terminal B6, so use pico oscilloscope to measure the voltage of J623 terminal B3 and terminal B67 (starting feedback signal) at the same time (Figure 5, blue is The voltage signal of terminal B3, yellow is the voltage signal of terminal B67), the correlation analysis is as follows.
(1) In stage A, step on the brake pedal and press the start button, J623 terminal B3 and terminal B67 establish voltage at the same time, and the voltage amplitude is approximately equal to the battery voltage.
(2) In phase B, the voltage of J623 terminal B3 drops to about 5 V, J623 stops working due to insufficient power supply, the voltage of terminal B67 drops to about 0.8 V, and the starter stops working.
(3) In the C, D and E phases, the voltage of the J623 terminal B3 is unstable, sometimes it jumps rapidly from 4 V to 10 V, sometimes it can be maintained at about 10 V for a period of time, and sometimes it simply drops to 4 V. about. The starter then works for a while and cuts off for a while.
(4) In phase F, release the start button, the voltage of J623 terminal B3 is approximately the battery voltage, and the voltage of terminal B67 is 0 V.
It can be seen from the above analysis that when the engine is not started, the output voltage of J271 is normal. Once the engine is started, the output voltage of J271 is obviously insufficient, which is a typical false connection fault. If the line between J271 and J623 has a fault with poor contact, many actuators and sensors on the engine begin to work during the starting process, the current in the circuit increases, and the voltage consumed on the contact resistance also increases, which is really supplied to J623 The voltage drops.
The method to find the virtual connection part is relatively simple. Use the third channel of the pico oscilloscope to trace the upstream of the power supply. When the upstream power supply of the fuse SB14 is measured, the measured waveform is shown in Figure 6 (blue is the upstream of the fuse SB14 The voltage signal, red is the voltage signal of J623 terminal B3, and yellow is the voltage signal of J623 terminal B67). The analysis shows that the upstream power supply of the fuse SB14 is normal when starting, and the voltage of the J623 terminal B3 is unstable, and it is inferred that the fuse SB14 or the line between the fuse SB14 and the J623 terminal B3 is abnormal.
Remove the fuse SB14 (Figure 7) and observe carefully. It is found that the internal circuit has melted, but the ends of the wires are still tied together and not completely disconnected. It is inferred that the fault of the car is caused by this.
Troubleshooting
After exchanging the fuse SB14 test, hair motive smooth start the machine, and operating normally, the fault is rectified.
Fault summary
The car is not complicated fault for what service technicians failed to troubleshoot it? Afterwards the maintenance repair technicians that exchange, he was also used multimeter line measured J623 terminal B3 , the terminal B5 and the terminal B6 voltage, but in ON 15 state of the power of the number of measurements at that time the measured voltage close battery electric voltage, the power supply is considered normal. Since the fuse SB14 internal virtual connection, in ON 15 state number of power, the electrical path load is small (less current), the voltage drop generated is not apparent, J623 terminal B3 , the terminal B5 and the terminal B6 electrical pressure is normal; when the engine is started, the fuse SB14 control actuators and sensors in the system to work, the circuit load becomes large (larger current), the voltage drop becomes large, J623 terminal B3 , the terminal B5 and the terminal B6 voltage is not normal , Leading to J623The work is abnormal, so that the starter cannot be controlled.
As the saying goes, "measure the voltage on the car, and measure the resistance off the car", which means that the wire connector must not be disconnected when measuring voltage, and the wire connector must be disconnected when measuring resistance, otherwise it is easy to misjudge. As shown in Figure 8a, when the switch K is open, the circuit cannot form a loop, there is no current in the circuit, and the virtual resistor R will not produce a voltage drop, so the voltage measured by the multimeter is the battery voltage; as shown in Figure 8b, the switch K is closed When the virtual connection resistance R and the bulb form a series loop, there is current in the circuit, and the virtual connection resistance R produces a voltage drop. Therefore, the voltage measured by the multimeter is less than the battery voltage, and the greater the resistance of the virtual connection resistance R, the higher the voltage across the bulb. Small; as shown in Figure 8c, when the switch K is disconnected, the virtual connection resistance R forms a series circuit through the test lamp. At this time, it can be judged whether the circuit has a virtual connection according to the brightness of the test lamp.
It is worth noting that the virtual connection resistance of this vehicle will only appear when there is a large current demand. Therefore, if you want to measure the abnormal power supply voltage, you must meet two conditions at the same time: one is online measurement (the J623 wire is not disconnected) B), the other is in the working state with high current demand (when starting the engine). The fuse SB14 melts internally and is in a state of disconnection. If you use a multimeter to measure the resistance at both ends of the line, the continuity is normal. Therefore, whether you use a multimeter to measure the voltage online or measure the line resistance, it is difficult to find the fault point of the car. The pico oscilloscope can monitor the voltage change during the entire starting process, which provides effective information for fault analysis.