Author: Yu Yao Dongjiang specializing in luxury car plant Ye Zhengxiang
Failure phenomenon
Yi Liang The 2009 Beijing Hyundai Elantra car, equipped G4ED engine and automatic transmission A4CF1 accumulated mileage of about 86,000 km. The car was repaired in another repair shop due to a gear shift shock failure. The maintenance personnel checked the relevant wiring according to the fault code prompt, and replaced the relevant solenoid valve and power control module (PCM, which integrates the engine control module and the automatic transmission control module), but The fault persisted, so I requested technical support from the author.
Troubleshooting
Test drive after receiving the car, start the engine, and the engine fault light on the instrument cluster lights up abnormally (Figure 1); put the shift lever in D gear, the body impact is greater; carry out a road test, and the speed ratio corresponding to the same engine speed when driving Usually low. Use a fault detector to detect and find that there is a fault code "P0755 Shift control solenoid valve'B' (low-speed drive solenoid valve)-open or short circuit" stored in the PCM (Figure 2). Clear the fault code, the system displays normally, and the engine fault light is also off at the same time; put the shift lever in R, N, and D gears, no abnormalities, but as long as you start in D gear, the engine fault light will be on. At the same time the fault code P0755 reappears.
Checking the control circuit of the automatic transmission valve body of the vehicle (Figure 3) shows that there are 6 solenoid valves in the valve body assembly, namely, pressure control solenoid valve A (PCSV-A), pressure control solenoid valve B (PCSV-B), Pressure control solenoid valve C (PCSV-C), pressure control solenoid valve D (PCSV-D), ON/OFF solenoid valve and line pressure control solenoid valve, and these solenoid valves are controlled and powered by PCM. Use pico oscilloscope and current clamp to measure the power supply and current waveform of PCSV-B from terminal 2 of the valve body assembly wire connector CAG04 (Figure 4). It is found that the PCM is intermittently PCSV-B before the shift lever is placed in the D gear. Power supply, and the long power supply at first to make the solenoid valve open quickly, and then change to the duty cycle power supply to keep the spool in the open position; and when the shift lever is placed in the D gear, the PCM is no longer PCSV -B power supply.
If the PCSV-B circuit inside the valve body assembly has an open circuit fault, the power supply waveform is normal and the current disappears; if a virtual connection fault occurs, the power supply waveform is normal and the current drops; if a short-circuit fault occurs (short to ground or short to internal coils) ), the power supply waveform is normal and the current increases. So far, no abnormality can be found in Figure 4.
Reorganize the maintenance thinking. If the fault code P0755 stored in the PCM is reliable, in general, the possible causes of the fault code include PCSV-B failure, PCM failure and related line failures, etc., and PCM and PCSV-B have been Replaced, is the fault on the line? Or the mechanical failure of the hydraulic system will cause the storage of fault code P0755? Checking the maintenance manual of the car, I was surprised to find from Table 1 that the "shift control solenoid valve'B'" mentioned in the fault code P0755 refers to PCSV-A, not PCSV-B! At this point, it suddenly became clear that the direction of detection was wrong!
Use pico oscilloscope and current clamp to measure the power supply and current waveform of PCSV-A from terminal 1 of valve body assembly wire connector CAG04 (Figure 5). It is found that the current of PCSV-A will be rapid when PCM supplies power to PCSV-A. It rises to 713.6 mA, then the PCM stops supplying power to the PCSV-A, and repeats. Comparing with Figure 4, it can be seen that when PCM supplies power to PCSV-B, the current of PCSV-B increases slowly. Under normal circumstances, when the coil is energized, the current will slowly increase due to the self-inductance of the coil. Because the PCSV-A, PCSV-B, PCSV-C, PCSV-D and ON/OFF solenoid valves have the same structure, they can be used interchangeably (note: they cannot be used interchangeably with linear solenoid valves), and the resistance is 3.5 Ω±0.2 Ω (measured value at 25 ℃), therefore, based on the detail that the current of PCSV-A increases rapidly, it is inferred that the internal coil of PCSV-A is short-circuited.
Disconnect the PCM wire connector CAGA-K, and measure the resistance between terminal 71 and terminal 70 of the wire connector CAGA-K (that is, the resistance of PCSV-A), and the resistance between terminal 72 and terminal 70 (that is, PCSV-B). The resistance of PCSV-B is 2.1 Ω (Figure 6a) and 0.9 Ω (Figure 6b), respectively, which have large deviations from 3.5 Ω, and the resistance of PCSV-B is even smaller than that of PCSV-A. Considering that the automatic transmission is in a warm state at this time, it is suspected that the result of the test may be inaccurate. Artificially intermittently supply power to PCSV-A and PCSV-B at the same time, and measure their respective currents (Figure 7). It is found that under the same power supply, the current of PCSV-A is about 5 A and the current of PCSV-B is about 3. A, this is consistent with the assumption that the internal coil of PCSV-A is short-circuited.
Remove the valve body assembly of the automatic transmission, measure the resistance of the PCSV-A and ON/OFF solenoid valves in the cold state, which are 2.4 Ω and 3.6 Ω respectively (Figure 8), and measure the resistance of the other three solenoid valves, all at 3.6 Ω left and right, thereby determining an internal coil short PCSV-A.
Manually provide 5 solenoid valves with 2 power supplies and measure their respective currents (Figure 9). It is found that the current of PCSV-A is about 6.5 A, while the current of other solenoid valves is about 3.9 A, which is similar to that of PCSV-A. The current difference is about 2.6 A. Enlarging the current waveform, we can see that the PCSV-A current rises vertically at the moment of power-on (Figure 10a), while the currents of other solenoid valves rise at a certain slope (Figure 10b).
Troubleshooting
After replacing the PCSV-A, the vehicle was tested and the gear shifting and driving were normal, and the fault was eliminated. The voltage and current waveforms of PCSV-A during normal operation are shown in Figure 11.
Fault summary
Fault code P0755 since the shift control solenoid valve in the "B" mistaken PCSV-B, before maintenance personnel replace PCSV-B, and also failed to PCM troubleshooting, described as "The fierce meal operating a tiger, a closer "Look at the pestle on the spot", it seemed very wronged!
Analysis of Table 1, the main fault code P0750, P0755, P0760 and P0765 are described shift control solenoid valve (Shift Control Solenoid Valve, referred SCSV), notes for the pressure control electric solenoid valve (Pressure Control Solenoid Valve, PCSV) . Shift control is finally realized by pressure control. Therefore, SCSV and PCSV are different names for the same solenoid valve, but the sequence is not consistent, that is, SCSV-B corresponds to PCSV-B. You must pay attention to this. The VFS in the fault code P0748 is called Variable Force Solenoid, or variable pressure solenoid valve, sometimes called linear solenoid valve. According to the throttle opening and shift position, PCM can linearly change the line pressure (4.5 bar~10.5 bar, 1 bar=100 kPa) by adjusting the VFS. In addition, the fault code P0743 corresponds to the hydraulic lock-up clutch (TCC) control solenoid valve (ie PCSV-D).
Case video explanation
Disassembly-free diagnosis | Elantra fault code P0755