1985 635CSi - Crank, no start problem

[cornwallis]

Hello All - hope everyone is going ok in these interesting times with covid-19.

I've spent some time reading through the forum on past postings re: "no spark" problems. Lots of good information in the forum. I feel bad for starting another thread on this topic, but I'm stuck . I also poured through the hiperformancestore.com/motronic.htm - an excellent reference.

I've not driven my car in 2 years. Very reliable prior to that - suspension was taken apart and rebuilt. When I finished and reconnected everything, I got nothing when I tried the ignition. At least now, it cranks but there is no spark. Have replaced the battery, starter, distribution cap, rotor, new plugs, starter relay. I've been working my way through the ignition schematic, pulling and testing some relays, checking all the fuses, etc.

Below are the tests i've done so far. the article above is pointing me to the ECM and a "world of pain" - yikes!

Problem: No spark

1. Replaced the distributor cap and rotor - terminals were really dirty and cap points were eroded
2. Voltage @ diagnostics terminal 13: on ACC = 12.6 volts. On IGN/Start = 10.5 volts
3. Voltage @ Ignition coil (primary: on ACC = --- . On IGN = 12.3 volts (ground to battery)
4. Resistance of Ign Coil: 0.7 ohms (primary). 5630 ohms (secondary) (ign coil ok? - don't know the age)
5. All spark plug wires checked out to ~6 kilo ohms. The wire from ign coil to distr cap = 2.1 kilo ohms
6. Main Relay check: 30 & Grnd = 12.4 volts. 86 & Grnd (while starting) = nothing ( this is not good, right?)
7. Speed & Reference sensors - these have been replaced in last 3 years. I will report back the resistances once i get the metal clips off.

Any other tests I should run? Hoping you can save me from a world of pain. I'm in hot and humid Houston, the car seats were very moldy even with windows open. Which connectors should I can check/clean? I wanted to get some advice before looking for the ECM and messing with that.

[Fedaykin633csi]

Will it run with the main relay jumpered, pin 30 to both pins 87? I have a one-into-two spade connector wire doo-hickey I made in the glovebox for emergencies.

[daz635]

The issue is number 5 in the list. As you have said the problem is that there is no voltage at socket 86 and ground at the main relay. The car will never start with no power to socket 86 at the on or start position. The main relay is closed by grounding pin 86 to pin 85 to allow voltage to flow to the injectors and the motronic unit via pins 87 and 87a. The issue will be the OBC computer relay or the OBC itself not allowing voltage thu to socket 86. THE OBC relay is a common failure due to the age of the car. As said above jumper the relay from 30 to 87 and 87a image below

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Extract from Motronic Document
the main relay
gets its electrical signal to close from the on-board computer when you turn your ignition switch to ON or
START. The on-board computer is the thing in the dash that tells you your fuel economy, outside air
temperature, etc. This is probably the only really stupid thing in the E28's otherwise really good electrical
system. I can't believe they supply power to a critical relay through that piece of junk. Anyway you need
check for battery voltage between terminal 86 and ground WITH THE IGNITION ON. Power goes to the onboard
computer through the ignition switch. Inside the on-board computer is a little relay that closes
when the ignition switch is in the ON, or START positions. The on-board computer also has internal fuses,
but those are NOT involved in the operation of the Motronic systemrt if there is no voltage at this socket. The main relay is only

[Fedaykin633csi]

Stole my thunder!!!! If the car starts w/30 jumpered to 87's, try bypassing the OBC relay, might be the culprit

[cornwallis]

Thanks for the tip guys, I jumpered those contacts and it started right up!

For the next steps:

• 1. is it possible to test the OBC or just the OBC relay? Is this the same as ECM (located under passenger dash?)
  2. If not, where is OBC located?
  3. I’ve attached pics of main relay. It’s looks pretty old. Doesn’t sound like I need to replace this at the moment?
  4. I heard humming when I jumpered the contacts, coming from engine area near airflow meter. Would not stop. Is this normal? After I verified the car started for a few seconds, I removed the jumpers.

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Thanks again!

[Fedaykin633csi]

Sweet! Hopefully you found the problem. The OBC computer/relay is separate from the ECU, and is most likely located under driver side dash. I wouldn't replace the main relay quite yet, but a spare is a good thing to have with you on the road. The humming is most likely the idle control valve working correctly.

Here's my post from when I was chasing this down on my 535is. My specific fix was to bypass the antitheft circuit (no spark, no stolen car) in the OBC relay:

http://www.mye28.com/viewtopic.php?f=3& ... 5#p1368684

I'm not sure where it's located on the e24 yet (still a new owner), but the relay is mounted with the OBC computer. Someone here can definitely point you to it!

[cornwallis]

I think I found the OBC (black box).



Since I’m not getting 12 V at main relay terminal 86, thinking Is that OBC is not passing the voltage to the main relay.

Below I’m showing the test I want to do, which is to bypass the OBC relay, jumper across pins 3C2 and 4C2. But this looks to be a NC contact, so I don’t think the jumper test will do anything.

It should be closed at all times and pass voltage is that right?

If it is indeed a NC contact I don’t understand what the coil when energized makes it do? Go open?

Thanks for any feedback!

[baders]

Its just not drawn very well is all. Bridge the terminals 3 to 4 and see what happens.

[cornwallis]

Update:

You were right, I bridged those terminals, and reinstalled the main relay and it started up!! So main relay looks to be ok.

With jumper in place, the voltage at the OBC relay box from the ignition switch is 12.3 V (ACC), dips to 10.5 V at IGN, then up to 13.2 V when car is running. When I turn off engine, back to 0.0 V.

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Two questions:

1. It looks to be like the OBC relay box is bad? What do you think?

2. Is the OBC Module - Solid State part Of the OBC relay box?

[daz635]

Looks like you found the issue, the obc relay box

The issue will be either the relay or the circuit board that the relays are attached to that has dry solder joints or cracks. If you can find the fault you can reflow the solder.

It will be easier to just make connection between the two green wires permanent. Make sure they are joined well becuse the car will shutdown the instant the wires are seperated when driving. Joining the green wires permanently will have no effect on your OBC function

The other option is to replace the unit, the part number is 65811373726 with a second hand unit or you can still buy the unit new for about$100

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[cornwallis]

Fedaykin, Baders, Daz - thanks very much!!

[baders]

No worries ! Just throwing this out there; Are you sure the OBC and relay unit is not just doing its job ? That is, is it coded off ?

[cornwallis]

I pulled the circuit board out of the OBC relay box and saw one of the contact strips was damaged. I tried soldering it to reestablish continuity but my soldering skills were sub-par. Ended up worse, some of plastic base melted, ended up very messy. I’ll be looking for new part.

Can you please explain about “Coded off”? I’ve never had this issue or messed with the codes. Like I mentioned the car was fine 2 years before I stopped driving it. For the new OBC relay box, is there a configuration step I need to go through or just plug and play?

[baders]

The OBC can be coded with a number you enter as a security feature. You enter the number which drives the OBC relay closed to operate the main relay. The relay box is just a relay box.

[daz635]

OBC code operation from the owners Manual

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[cornwallis]

Update:

The new OBC relay box arrived, plugged in, and car started up! Wooohooo!! now to change the fluids...

I didn’t mess with any immobilized codes. To be honest don’t think my car has an alarm, like users manual above mentions - or maybe just never been activated. Or at least I’ve never heard it! Lol.

Thanks again for all your help!