92 325i won't start, running out of ideas

cakken · 10-03-2007, 01:17 PM

Car has been down for a few weeks. Started up one morning for a few seconds then died. Was very similar to when my fuel pump failed a little over a year ago.

The car turns over strongly and the fan turns while cranking so I think the starter is fine. The battery is charged, have verified spark visually, fuel pump is good, assuming filter is good as I checked flow to the rail while cranking and it was fine,replaced both fuel pump and main relays, replaced crank shaft position sensor,and getting power to injectors while cranking.

getting no codes from pedal test.

Running out of ideas, any input is greatly appreciated.

David Bevan · 10-04-2007, 03:38 AM

Ok you need a multimeter and know how to use it... Firstly is you have the engine producing a spark which rules out the coils and the crank sensor. so you should be down to debugging the basics of the fuel injection system.
Here is how to do it... check for voltage pulses on the injector connectors you should see a short blip to 12volts whenever the ecu calls for fuel to that cylinder.
Check the coolant temperature sensor for between 2K to 6K ohms do the same with the air temp sensor, check the Throttle pot for resistance between the centre and outside pins this should vary when you move the throttle. if after all this you don't find anything unusual you may well be in the realms of trying another ECU.

Good luck

damonwilliams · 10-11-2007, 10:32 AM

David,

If cakken checks all this and all is well but still having problem, would a replacement chip (instead of replacing entire ECU) work?

cakken · 03-17-2008, 10:26 AM

Car still motionless however recently got access to Peake tool and pulled code 1a (control unit supply).

Started checking power distribution and found pretty severe voltage drop at main relay with ignition on. Full battery voltage comes into relay at terminals 30 and 86 of relay socket. However when ignition is switched to on there is a voltage drop of about 3 volts at all of the power terminals for the relay. Which I'm assuming is the reason for the control unit supply error code as voltage at ecu input 54(from relay) is about 3 less than battery voltage.

Is the voltage drop when ignition is on normal? Does the drop after being switched by the ecu indicate a faulty ecu? The voltage drop also increases slightly in magnitude when using switched ground on main relay as opposed to using chassis.

Would there be any other cause for a control unit supply error?