1988 535is igniton/ecm problem

[garyegloff]

I'm struggling with a thorny problem. I've owned this fine car for 2 years or so and 40,000 miles. In top running condition now, after many years of neglect. Almost a year ago, I had intermittent starting problems: sometimes I would get no power to the ignition (no clicking, nada), and sometimes I got power but no spark, it cranked but wouldn't fire. Left it for many weeks with Dave Bogert in Wyckoff NJ (Motor Works West) and he was pretty stymied, although he endlessly tested all electric components and all circuits. In the end he lent me a good replacement Motronic ECM, and it functioned perfectly for a couple of weeks, so I found a good used unit and it ran fine for the last 8 months. Two weeks ago it started to malfunction, usually in the course of long drives: no power to the ignition. I always found that by disconnecting the battery, the problem immediately went away, and stayed away, despite driving long miles. It was as if I was re-booting a computer or other electronic function. But then it would eventually recur, hours or days later. I bought a new used unit (in as-new condition, at least visually), and it worked fine initially, but the problem is back. So what to do? Any ideas?

[Mike W.]

Think low tech first. I'd go after the battery connections, I don't care what they look like, remove, clean and replace. There is also a seperate red wire that attaches to the positive terminal, it has to be well attached, clean and secure. And make sure the terminal on it is well attached and not loose. Next look at the ignition switch and possibly the main relay. All the electronics can be dead or even disconnected and it should still crank.

[ahab]

What Mike said. Also check your negative cable where it attaches to the chassis above the battery. Other grounds can be suspect too but this one is key.

[garyegloff]

Thanks very much. I 'll certainly re-clean the terminals and all leads, but I neglected to mention that when the ignition is without power, the rest of the car has plenty: dash, lights, radio, blower, etc. This is why I've focussed on the ecu or ecm unit. We've looked at the ignition and the relays too, but they checked out good after repeat tests.

I just read an interesting post in the M5/535i columns: "I sent the ecu off to Programma to have it checked and it tested fine. At this point i decided to take it to a local shop who works on a lot of older bmws. Turns out it was the ecu. He re-soldered the pins of the ignition circuit in the ecu and now it starts like a champ."

I guess this is worth a try too, taking the cover off the "computer and looking at the circuit board connections. After all, I own three of them now!

[tn535i]

Agree start simple, however, are you say no power to the ignition coil or no power to the ECM. If the later the circuit that pulls in the main relay starts at the ignition switch but then travels down to a connector under the kick panel that makes another connection behind the cluster that then connects to loom that adds in the code relay box before it goes through another connector before travelling over near the ecm through another connector before going out through ECM area but not before making it's way through the engine harness connector before finally tripping the main relay and powering up the Motronic ecm.

Well maybe not exactly like that (I might be confused with e30's a little), but an intermittent loss of 12v+ through this maze of connections and relays will kill the ECM and it won't restart until it has cycled and has a good connection again. The engineers or electricians at BMW must have had way to many beers at lunch the day they figured out this one.

[Mike W.]

Thanks very much. I 'll certainly re-clean the terminals and all leads, but I neglected to mention that when the ignition is without power, the rest of the car has plenty: dash, lights, radio, blower, etc. This is why I've focussed on the ecu or ecm unit. We've looked at the ignition and the relays too, but they checked out good after repeat tests.

I just read an interesting post in the M5/535i columns: "I sent the ecu off to Programma to have it checked and it tested fine. At this point i decided to take it to a local shop who works on a lot of older bmws. Turns out it was the ecu. He re-soldered the pins of the ignition circuit in the ecu and now it starts like a champ."

I guess this is worth a try too, taking the cover off the "computer and looking at the circuit board connections. After all, I own three of them now!

...sometimes I would get no power to the ignition (no clicking, nada), and sometimes I got power but no spark, it cranked but wouldn't fire.

That screams battery cable. You can take the ECU and wiring harness for FI/ignition, ie Motronic, out of the car, completely, and it should still crank. Not start or run, but still crank without hesitation.

[southgabimmer]

Ya, clean up all the ground points in the engine bay as well as the battery terminals and connections. if everything works perfectly (engine cranks and you get fuel, etc.) but you get no ignition, then you might want to open up the ecu and find the ignition circuit and transistor. some points and connections on this circuit may need to be re-saudered. I couldn't get my 535is to start because the ignition wasn't working. after checking everything and finding that everything was in good order, i figured that the problem must lie in the ecu. after taking it apart and saudering the connections, the car started on the first crank of the engine!