Having a "fun" time at the moment trying to diagnosis a non starting M6 and thought I might post up the symptoms I've found to see if anybody else had experianced similar issues. It's a 1989 M635Csi with the M88/3 engine...... (google image) To cut a long story short she has no spark/ no fuel. A long process of investigation has led me to the pair of flywheel sensors. I had the good fortune to have access to an e28 M5 with the same M88/3 engine and parked her beside the non running m6. I disconnected the M5's flywheel sensor connectors and made a 3 wire extension loom for each one to reach the M6's connector plugs next door. I then cranked the M5 over while checking for spark and fuel on the M6 and bingo the M6's both triggers it's spark plugs and fires it's injectors. I've ruled out the sensors themselves by both swapping the "good" M5's sensors in to the non running M6 and no joy, and then putting the non running M6's sensors in to the M5 and she cranks up and runs fine, thus proving that the sensors are not the problem. So, thoughts turned to the flywheel that the sensors read off and at this stage I'm starting to think maybe the peg that one of the sensor reads off has fallen out ( I had this problem on a 325i many many years ago). Unfortunately, when i dropped the bottom inspection cover off the bellhousing to look in the peg is present....... She's a little rusty in there but i dont think that matters too much. I've checked the air gap between both sensors and the ring gear teeth/peg and they are within tolerance (1mm +/- 0.3mm) and I've even shimed both sensors to get the gap preciscley 1.0mm, no differance. Both sensors are returning an AC voltage when pinned out at cranking speed but are slightly lower voltages than the M5's similar measurments, (not sure whether this could purely be down to slightly different cranking speeds though). Only other things I can think of now are get an Oscillisope and compare the square waves from both running and non running cars sensors. I'm not mad gone on this idea because simply put I wouldn't know what I was looking for. Next up is drop the gearbox to inspect the flywheel, struggling to see what could be wrong with it though. The car's final journey consisted of a small drive from a driveway down to a garage at the bottom of a field after being washed. The owner reported "bottoming" the car out while reversing over a bump going in to the garage but says the engine stayed running he he didnt think any more of it. 2 weeks later when he went to restart it we find ourselves here. Would be interested to hear anyone else's thoughts on this one?
Hi Brian Good to hear from you, When the M5 cranks over with the wire extensions the M6 has spark and fuel but no joy when physical swapping in the sensors... my thoughts on this is the M6 battery and supply voltage to the DME is very good (12v) and no drop off from M6 starter, so I would be looking at all the supply voltages to the DME and also to the fuel pump relay. If it was the M3 I would be looking at the in line fuse along the battery cable in the boot 12411706111, the fuse cracks and causes starting issue same as you have and is the main supply to the DME. Flywheel looks good and think removing the trans will not bring any new discoveries.
There used to be a problem with the front screen seal leaking at the bottom corner, allowing water to drop onto the ECU and filling it with water ! maybe a connection if the car had just been washed ?
