I think that I have a bad ECU in my 1990 740 GL B-230F engine with Bosch LH Jetronic fuel system. I was driving to work, made it to the parking lot, and the engine stopped. It cranked but would not fire. I determined that there was no fuel pressure, fuel pumps not running. I jumpered around the Fuel Injection relay and the pumps then ran and I got fuel to the engine. I verified that the first stage relay of the Fuel Injection relay was activating when the key was turned on. I put a volt meter on output pin 20 of the ECU and it measured +12VDC when the key was on (pulled up through the relay coil by the +12 applied to the relay via relay pin 6 (circuit 85?). When the key was turned to the Start position the starter turns over but pin 20 of the ECU never pulls to ground to activate the second stage relay in the Fuel Injection relay.

I hardwired pin 20 of the ECU to ground and as I expected the second stage relay pulled in and the fuel pumps ran and I got fuel to the engine when I turned on the Ignition key. And now the engine runs OK. I sucked the Diagnostic Codes for both the ICU and the ECU and both returned 1-1-1, i.e. no errors to report. As soon as I remove the hardwired ground from pin 20 of the ECU the pumps stop and the engine runs out of gas.

Although I don’t have an Oscilloscope I measured (with my trusty voltmeter) the RPM signal that comes from pin 17 of the Ignition Control Unit (ICU) to pin 1 of the ECU and it measures (while the engine is running) approximately 6 volts on the DC scale and approximately 16 volts on the AC scale. This is, I believe, consistent with a properly functioning ICU (and the ECU doesn’t return the error code that indicates it is not receiving this signal.

So, it really looks like a bad ECU which surprises me since in the 358,000 miles I have had the car it has never given me a problem before. Can anybody think of any other checks to make before I try to get another ECU, and (assuming it is the ECU that is bad) any suggestions where to get a replacement at a reasonable price (whatever a reasonable price is)?

Any advice would be appreciated.

Thanks,
Guy P.