Unfortunately, we don't know what power pins are used for what. It's almost impossible to isolate the problem to a specific pin. e.g. Pin 18 is hot battery power. We know that a code is generated for low volts on this pin, but is the power loss long enough for it to generate a code? The battery power fault is one which puts on the CEL, but do the light circuits respond faster than the fault-logging circuits? I'm wondering what the function of pin 32 is. Is the power on this pin used in the processing circuits or is it just part of the diagnostic circuits, telling the ECU that power is being sent to the thermocouple ECU and other items. Could momentary loss of power between this pin and splice 30008 cause the CEL and SDL to illuminate, but not cause the engine to skip a beat?
Do we have any idea where any of these splice blocks (eg 30008) are physically located ? Since this is an intermittent problem I may need to resort to a session with a Picoscope to backpin the +12v's on the ECU and drive the car until perhaps something is picked up. I see that Fuse 33 is in play here, have not checked that for soundness - might also drive with somebody stomping on the passenger footwell too
Nope. All spices are hidden under the black sheaths. Regarding the ECU plug backshell. It may be a good idea just to look at it for something obvious. Pull on the wires to see if anything detaches. I found a pin on my alternator plug a few months ago which had broken in half (the wire was still firmly attached to one half of the pin).
The problem is likely with 12v power feed to the ecm. You will likely find a crushed/damaged wire in the harness looping to the motronic itself within the engine Bay Area. Also check the pink immobilizer wiring in the passenger footwell. I don’t recall off the top of my head - but had this exact issue in the past.
No immobiliser on the 2.7 car .. and I couldn't find a pink wire in the wiring diagrams for the 5.2. There's a pink/yellow wire on the 2.7 which controls the fuel pump, so I guess this could be used in an aftermarket alarm system to immobilise the car. The ECU has multiple sources of power, thus making it hard to isolate the problem. Did your engine change behaviour when the CEL and SDL appeared? The suspected power loss is so transitory on Jon's car, it's not affecting the engine. Either that or one of the 4 or 5 power pins does not affect the operation of the ECU (other than trigger the fault detection circuits).
Missed this - not as far as I know. I'm driving when it happens and not sure if I'd hear a relay click but I'll pull the carpet up and uncover the fuse box and see if I can hear anything.
Just a follow-up on this issue. With persistence and some excellent diagnostic skills, Jon was able to track down the problem to a loose electrical connection on the exhaust bypass valve solenoid plug. i.e. the red/blue wire from the ECU which provides an earth to the solenoid coil. Whether the ECU puts on the SDL/CEL as a result of the high resistance wiring or whether it was because the ECU was sensing that the exhaust bypass valve was not opening all the time (via the thermocouple system) is not clear. Also noted was a very brief loss of voltage from relay "O" (the relay which powers a lot of the engine sensors and indirectly shuts down the fuel pump). It also wasn't clear if this was a result of ECU shutting down the relay as a result of the faulty bypass system. I thought the ECU only shut down the bank in response to exhaust temperature issues via direct control of the injectors and spark plugs, not by shutting down all the sensors (including the MAF and bypass thermocouple ECU) on that bank. So still some questions to be answered, but at least Jon has some trouble-free driving ahead of him.
Many thanks to Ian for some inspirational offline chats on this one. Just completed a 300 mile trip over two days and not one flicker of a CEL or SDL at all. The fix was to close up the pin on the exhaust bypass valve solenoid inside the rear right corner of the engine bay - it was splayed out a little, causing intermittent connection to the solenoid.
