MARCH 2024 AFTERMARKET 29 Next month: A Renault Captur. They tend to come in threes, all full of trouble. The most rewarding part of diagnostics is not repairing the fault; It comes from a gain in knowledge. Old-skool Now for some old-skool scope work, and in keeping with these sentiments I have just turned the time clock back 25 years and purchased a FLUKE 123 20mhz 2-channel scope. This has become inevitable because of the compromised workshop environment in which I find myself. A VW T4, 1995/6 - I did say old-skool - started and ran for a few seconds, and then would cut out. This system employs an add-on controlling ignition and port injection. The injection system relies on a trigger or timing signal to switch the injectors. The Siemens 5WP4181 fuel control employs intermittent injection control. Fuel is injected in two events behind closed inlet valves, then all injectors fire together. So, I began with ignition diagnostics, in this case with my Pico. The base timing event was a hall sensor in the distributor This signal is sent to PCM pin 11, where the base signal is modified providing a timing advance control based on various sensor values. The modified signal is sent via pin 27 to the external ignition power module pin two, mounted on the ignition coil pack. This controls the coil primary current. Please refer to Fig.1. The Pico four channel scope was perfect for this challenge. I connect the essential pins 11,17,3 crank, run, and observed what happened. Blue channel pin 11 at PCM, red channel pin 27 at PCM, green pin three ignition module. Please refer to Fig.2. Conclusion; Loss of PCM timing advance output from the PCM. I also confirmed power and ground did not drop out. This meant we had a faulty PCM. The birthplace of my heritage diagnostic principles; Output, trigger, power, ground. Written in my notes from the early 90s and still relevant today. A quick trip back to the present in my TARDIS brought me to a Fiat 500, the new one. I do like the 695 Abarth, but my nemesis in the workshop less so. This was a trade customer. It was a loan car with the instrument cluster lit up like my Christmas tree. The serial faults were too long to list. They did however have a common thread; Loss of CAN communication with the ABS module. In fact, the TOPDON serial tool did not recognise the existence of ABS in the topography. Access to the ABS PCM was straightforward having removed the battery and tray. Please refer to Fig.2. I began my assessment with the CAN network. Although CAN hi/lo was present, there was some distortion. Please refer to Fig.3. Removing the ABS PCM edge connector gave rise to a suspicion of water ingress. Please refer to Fig.4. I then elected to bridge out the ABS module, allowing further scoping of the CAN network. Notably, the vehicle would not run with ABS out of the loop. CAN was now clean with no distortion. Applying my previous logic tree; Power ground ok, CAN hi/lo ok but distorted, no comms, therefore module faulty. A new module came in at more than £900, however we were able to get a good unit for £250, from a reliable supplier. Once fitted full comms were restored, clearing all DTCs except Stop/Start. I did conduct further evaluation and it suggested a body control issue. Following a conversation with our trade client it was decided to overlook the stop start problem, and yes, I did thoroughly evaluate the battery sensor back to the body module. Fig. 2 www.aftermarketonline.net Fig. 3 Fig. 4
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