Need help diagnosing electrical problem
My 88 325ic stopped running in mid-stride a couple of days ago: we were almost done with a 30-minute run back home in the foothills west of Denver when the dash lit up with warning lights and the engine quit. I got her towed home and have spent a couple of days looking for the problem without much success.
What I've learned: all the auxiliary gear is working (lights, wipers, radio, fuel pump, etc.) except the power windows, and the idle air solenoid doesn't purr when the ignition is On. (The fuel pump works only when jumped directly to the battery; connecting the 30 and 87 terminals at the relay doesn't do anything, and the pump doesn't come on when the starter is engaged.)
I've replaced the main relay with a spare, to no effect.
The absence of a hum and vibration from the idle air solenoid indicates to me that this is an electrical problem further upstream (failure of the IAS wouldn't have made the engine quit in mid-run), but I'm unsure about how to diagnose the problem further. I have a spare ECU, but I've got some reservations about subbing it in, because of the possibility that there's a short in the system.
Any advice or suggestions will be greatly appreciated,guys.