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.

if the car runs with the ground added to the relay, then one can assume everything else with the other inputs and outputs are ok. pin 20 is SUPPOSED to apply the ground for the relay and yours doesn't. needs an ecu. typical of the lh2.4s and other euros with bosch ecus. good luck, chuck.

Well, that car should have the 561 pink label. Did you verify it? That is a prime suspect, but you should look at other things. My way is to keep a couple of spares and rule it in or out instantly by swapping. They are not too expenxive about $75-125 on Ebay.

Hi guys, thanks for the responses. When you speak of a 561 pink label are you referring to the ECU model/version/part number or whatever??? There are a few numbers scattered about on the ECU, they are:

Jetronic Bosch
0 280 000 556
3 591 687
3501687
and 078

Thanks again for the help.

Guy P.

The 556 is known as a problem. I went to the local junkyard and found one for $15.00

The upgrade for this Ecu is the 933

Good luck