No power at ECU

E28 technical advice asked and given! Troubleshooting, modifications and more.
Post Reply
Banders
Posts: 13
Joined: Nov 26, 2019 5:00 PM
Location: 02703

No power at ECU

Post by Banders »

I have a 1988 535is manual that will not run. Cranks but has no spark, and after throwing parts at it and doing some troubleshooting I found that my ecu isn't getting power. At terminal 18 I read 12 volts but at terminal 35 I'm getting nothing. I've replaced both the battery and the main relay (it was missing completely when I got the car) so I'm wondering where I need to go next. Is it a grounding issue or something in the wire itself? How would I check for continuity?
Mike W.
Posts: 26872
Joined: Feb 12, 2006 12:00 PM
Location: California Whine Country

Re: No power at ECU

Post by Mike W. »

No power to the ECU sure sounds like the OBC doing it's anti theft thing, even if not intended. Search for OBC and not running or no spark or something like that. Look on page 38 of the ETM, the OBC is between the battery and the coil of the main relay, electrically speaking.
Banders
Posts: 13
Joined: Nov 26, 2019 5:00 PM
Location: 02703

Re: No power at ECU

Post by Banders »

Would jumping the obc relay do the trick? I don't think it's anti theft as from what I've read you can bypass the code simply by disconnecting the battery. This thing has been sitting for 20 years so I feel like it's something else.
Mike W.
Posts: 26872
Joined: Feb 12, 2006 12:00 PM
Location: California Whine Country

Re: No power at ECU

Post by Mike W. »

That's what I would do for starters. The OBC anti theft seems to be getting crankier as the years go by. 20 years ago it was a rarity, if ever that there was a post about it. Now it doesn't seem to be unusual.
///M
Posts: 74
Joined: Apr 17, 2009 11:21 PM

Re: No power at ECU

Post by ///M »

While the OBC can stop the car from starting in your case no power (12 volt +) at terminal 35 of the DME is not the OBC. Terminal 35 gets its power from terminal 87 of the DME (main relay). It should be the red and blue wire. It may have been used for an anti-theft of an alarm or switch.
There is no continuity between terminal 87 of the relay and terminal 35 of the DME.
Test with a multimeter for continuity. If there is none check for a break in the wiring. You can also run a wire from the battery to the wire connected to terminal 35 of the DME.
If you put a switch on that wire you would be duplicating the switching action of the DME relay.
Banders
Posts: 13
Joined: Nov 26, 2019 5:00 PM
Location: 02703

Re: No power at ECU

Post by Banders »

I'll definitely give that a shot. What fuse should I use when I run the wire? And I'm assuming I'd have to cut the wire that leads to terminal 35 and splice it with the new one.
///M
Posts: 74
Joined: Apr 17, 2009 11:21 PM

Re: No power at ECU

Post by ///M »

It depends on what you want to achieve. My approach with wiring is that originality is important and any car with obvious wiring modifications devalue the car. The diy coloured crimps look ugly and using the right type of connector is easier nowadays.

The red/blue wire runs from the main relay to the DME. I would try to follow it and look for any obvious looking connections to the loom. When alarm fitters install alarms I find that their wiring stands out from the original BMW wiring. If that wire was cut to introduce a cut off or switch, then it has been a good security deterrent as the car has not been started and the missing relay is interesting.

If you can trace the wire or see it in the loom then you can probe the wire with a needle or probe and test for continuity. If you get a long length of wire and connect to terminal 87 main relay then bring that wire into the car. You can then use a multimeter connected between the wire and the probe. You are looking for the break in continuity.
Most people would probably go with a new wire as it can be quicker. The supply from the main relay is unfused the relay is the switch in the circuit. If you are putting in a switch then I would put a fuse in line. Start with a 7.5amp which should be okay.
You would have to cut the red/blue wire before the DME connector. Before cutting you would make sure there is continuity between the point you are going to cut and the socket in the DME connector. You would then have a situation where if your new switch is not on then the car would not start.
I have fitted switches like this in the boot(trunk).It can be additional security when parked up. You can fit it anywhere you want.
I have a suspicion that this is what was done and why I would put time into finding out where the break is.
Post Reply