Power Supply to Main relay

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Like Bmer
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Joined: Nov 29, 2021 4:36 PM
Location: California

Power Supply to Main relay

Post by Like Bmer »

I am a new member to this forum and this is my first post so please show a little mercy.

I have a 1986 BMW 528e and I have been dealing with an intermittent starting problems like many others for years. Finally the car will not start at all so it gave me a chance to really do some testing.

First of all the car always cranks.

There is no spark.

After hours of reading many threads and performing numerous tests including the spark plugs, coil, cps, cpu, fuel and Main relays and also replacing most of those parts (except for the cpu) over a few years I believe I have found the problem.

There is not enough power coming from the Ignition to the Main relay pin 86. There is not enough volts coming into pin 86 on the main relay to send enough power to pin 30 to close the bridge that supplies power to the two pin 87s. One of the Pin 87s provides power to the ECU, so the ECU is not receiving power when the ignition is turned on.

I have 12 volts coming into pin 30 before and after turning the ignition to the on position. However pin 86 is only receiving 4 volts when the ignition is in the on position and that is not enough power to close pin 30 bridge that supplies power to my two 87 pins when closed.

To test my theory I jumped pin 30 to both 87 pins and the car did start.

I also hooked up test leads from the relay harness slots and connected them to the pins on the bottom of the relay and when I did this and turned the ignition to the on position pin 86 could not provide enough power to close the bridge between pin 30 to both pins 87 as expected. However when I disconnect the test lead from slot 86 on the harness (power supply coming from ignition to harness) and connect one end of the test lead directly to the battery and the other end to the bottom of pin 86 on the main relay, then turn on the ignition, pin 30 bridge now closes and provides power to both 87 pins and the car starts.

So being new to all of this and basically being an inexperienced Youtube mechanic, I believe I need to find a different power source for pin 86 on my Main relay that is currently being supplied from the ignition.

I know I can’t connect pin 86 straight to the battery because it would be sending power to ECU 24/7 and I believe running down the battery pretty quick.

Any suggestions as to a good alternative power source for powering pin 86 from the start position? I really want to bypass dealing with the ignition unless I really have to.

Any help would be appreciated and I apologize for making this so wordy.
Shawn D.
Beamter
Beamter
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Re: Power Supply to Main relay

Post by Shawn D. »

Like Bmer wrote: Dec 11, 2021 3:30 AM I am a new member to this forum and this is my first post so please show a little mercy.
Ha! No mercy for posting a tech question in "Comments" -- moving this to Tech Talk.
demetk
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Re: Power Supply to Main relay

Post by demetk »

Like Bmer wrote: Dec 11, 2021 3:30 AM So being new to all of this and basically being an inexperienced Youtube mechanic, I believe I need to find a different power source for pin 86 on my Main relay that is currently being supplied from the ignition.

I know I can’t connect pin 86 straight to the battery because it would be sending power to ECU 24/7 and I believe running down the battery pretty quick.

Any suggestions as to a good alternative power source for powering pin 86 from the start position? I really want to bypass dealing with the ignition unless I really have to.

Any help would be appreciated and I apologize for making this so wordy.
Find the OBC relay box under the dash to the left of the steering wheel and jump the green and green red wires. It looks like this,

Image

And get the electrical troubleshooting manual here: http://wedophones.com/BMWManualsLead.htm The schematics will help you understand the systems on your car.
Like Bmer
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Joined: Nov 29, 2021 4:36 PM
Location: California

Power Supply to Main relay

Post by Like Bmer »

Thank you for the excellent information.

If I am interpreting the information you gave me and also reading the schematics correctly I should do the following:

1. Remove the green wire that comes from the ignition switch and connects to the OBC, remove it from the wire harness that goes into the OBC.

2. Remove the Green/Red wire that goes from the OBC to the Main Relay, remove the wire where it connects into the OBC wire harness.

3. Then connect the Green and Green/Red wires together. What is the best method to use to connect the two wires? I know I could just twist them together but I am not sure if a more secure method is needed here.

Thank you
demetk
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Re: Power Supply to Main relay

Post by demetk »

Like Bmer wrote: Dec 11, 2021 10:36 PM 3. Then connect the Green and Green/Red wires together. What is the best method to use to connect the two wires? I know I could just twist them together but I am not sure if a more secure method is needed here.

Thank you
Take some insulation off both wires and twist them together without cutting them. That should be enough. The green/red wire only flows enough current to activate the relay, about 150 milli-amps.

But first I would probe the two wires for voltage with the ignition on.
///M
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Joined: Apr 17, 2009 11:21 PM

Re: Power Supply to Main relay

Post by ///M »

The OBC unit acts as a relay for the main relay. To stop the car from starting inputting a code in the OBC disconnects the green wire from the green/red wire. If you connect the two wires then you are bypassing the OBC.
There is a resistance in the circuit which is causing the main relay to drop out. If the resistance is in the OBC connections then you are bypassing the resistance and it should work.
Like Bmer
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Re: Power Supply to Main relay

Post by Like Bmer »

Thanks for the information, I will do some testing and make sure I have good voltage before I bypass the OBC relay box.

I really appreciate the help, hopefully the testing will go well. I will let you guys know what I find.


Thanks again!!
Like Bmer
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Re: Power Supply to Main relay

Post by Like Bmer »

Latest Update

Finally had a chance to do some testing on the power supply wire coming from the ignition and entering and exiting the OBC. With the ignition in the on position both wires (GN/RD and GRN) have 12 volts.

I then proceeded to test green wire (that powers pin 86 in the main relay) that goes from the OBC to cable pin connector C103 located under the glove box. The GRN wire there is only receiving 8 volts.

I then pulled out the main relay brought it to connector C103 and hooked it up with test clips to see if 8 volts would trip pin 86 to pin 30, it did not.

Therefore I now believe I have to go back to GRN wire located underneath the OBC (which has 12 volts) and see where I can run a new wire from the OBC through the firewall to the Main relay.

I was hoping to avoid running the new green wire all the way from the OBC to the glove box where there is a cluster of wires going through the firewall.

That seems like a waste of distance considering the Main relay is located just on the other side of the Firewall directly across from the OBC. However I don’t see any opening on the firewall to run a new wire from this location.

Any suggestions would be appreciated.

By the way I did use this project as an excuse to purchase a new tool, I purchased the Fluke Pro3000 Tone Generator and Probe Kit. I used it to help track some of the wiring. Always kind of fun to buy new tools :D
demetk
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Re: Power Supply to Main relay

Post by demetk »

Bummer. I think the the aux fuse box has an ignition on circuit that you can tap into.
Like Bmer
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Location: California

Re: Power Supply to Main relay

Post by Like Bmer »

Demet

Interesting find. Just came back from looking for a Auxiliary port on the fuse box and didn’t see any. Car may be too old, it still has the old Ceramic style fuses.

I also didn’t see any great places to run a wire through the firewall without doing some major poking to some very old wire harnesses.

However the interesting news is that I did find a disconnected wire close to the fuse box that was just lying there minding its own business. It has a female two hole connector and a red or brown solid wire (hard to tell because wires are faded) that has 11 volts coming from the ignition in the on position. The other wire in the same connector is red/yellow stripe or brown and yellow stripe , again both faded. I couldn't find the mate to the wire.

The wires feed into the same harness most of the wires that lead to the fuse box and the other end of the harness leads to the firewall and then disappears in the direction of the steering wheel area inside the car. But not sure.

I did hook up the wire to the main relay using test clips and the good news is that it worked, it did trip pin 86 to pin 30.

Looks like it is back to the schematics to see the source of those free wires. I am afraid to use it without knowing its original source.

Any thoughts?
demetk
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Re: Power Supply to Main relay

Post by demetk »

If you have one it's between the main fuse box and coolant expansion tank.

Image

Any green wire should be ignition on output. Where's your diagnostic connector? On the engine? It has a green wire.

That wire you found if it's red/white then is it always hot or only with ignition? If the later then I would use it. You only need 150ma to power up the main relay.
///M
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Joined: Apr 17, 2009 11:21 PM

Re: Power Supply to Main relay

Post by ///M »

As Demetk has said the diagnostic connector would be an easy place to pick up the ignition switched 12v+
https://www.mye28.com/viewtopic.php?t=151988

Looking at the ETM for 1986 528e the ignition feed is from the switch to C2 connector of the OBC relay box pin3. This is through the relay to C2 connector pin 4, this is the Green/Red wire.
This wire goes to C235. This connector is a single pin connector located according to the ETM under left hand of dash, near on board computer. That connector C235 then connects to the C103
Once you find the connector you have the complete circuit and testing each section will highlight the problem. There is a resistance in the circuit which combined with the high resistance in the main relay means that the relay does not operate.
You can try your expensive new tool to help trace the wire.
Like Bmer
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Location: California

Re: Power Supply to Main relay

Post by Like Bmer »

Demet, I think you are a Genius!!

I finally found that auxiliary fuse box that you were speaking of, my apologies I had no idea it was an auxiliary fuse box because it was next to the main fuse box and has fuses attached to it. I originally thought it was being used just like all the other fuses and relays next to it.

However, after taking another look at the aux box based on your message I ran some tests and the green wire entering the aux fuse box is from the ignition on position (11 volts) and there is no wire exiting the aux fuse box on the other side. Therefore I do not believe the aux fuse is being used and should be perfect to use for a power supply to the main relay.

I am surprised that BMW did not mark this box as an aux fuse box, I could not find it listed anywhere. Would have been nice if they would have put a label on the cover of the box. I am sure it may be labeled in the schematics somewhere but I didn’t run across it there either but of course I am sure I missed it.

Going forward I believe my best approach is to cut the current green wire leading to the main relay and attach it to a new wire coming from the Aux relay. I will use a butt connector to connect the two wires.
demetk
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Re: Power Supply to Main relay

Post by demetk »

That should work perfectly. You won't have the code function on the OBC anymore though. Neither do I.
Like Bmer
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Re: Power Supply to Main relay

Post by Like Bmer »

Like Bmer wrote: Dec 19, 2021 3:20 PM Demet, I think you are a Genius!!

I finally found that auxiliary fuse box that you were speaking of, my apologies I had no idea it was an auxiliary fuse box because it was next to the main fuse box and has fuses attached to it. I originally thought it was being used just like all the other fuses and relays next to it.

However, after taking another look at the aux box based on your message I ran some tests and the green wire entering the aux fuse box is from the ignition on position (11 volts) and there is no wire exiting the aux fuse box on the other side. Therefore I do not believe the aux fuse is being used and should be perfect to use for a power supply to the main relay.

I am surprised that BMW did not mark this box as an aux fuse box, I could not find it listed anywhere. Would have been nice if they would have put a label on the cover of the box. I am sure it may be labeled in the schematics somewhere but I didn’t run across it there either but of course I am sure I missed it.

Going forward I believe my best approach is to cut the current green wire leading to the main relay and attach it to a new wire coming from the Aux relay. I will use a butt connector to connect the two wires.
Update:

Connected wire coming from Aux relay to Main Relay and Car has been running great for about a week.

Special thanks again to Demet for telling me about the Aux Relay, had no idea it even existed.
BenTomm17
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Location: New Jersey

Re: Power Supply to Main relay

Post by BenTomm17 »

Instead of cutting the green wire leading to the main relay why not just attach the new wire from the aux fuse box the to existing green wire leading to the main relay. You can use one of those crimp connector thingies which allow you to attach a wire to an existing wire without actually cutting it. That way you can keep the OBC code function.
tn535i
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Re: Power Supply to Main relay

Post by tn535i »

I missed this but honestly I would say the best thing to do (however you probably have to remove the instrument cluster to find it) is remove the add-on harness for the OBC code relay box altogether, unplug those wires and plug the two green wires back together as if it had no OBC relay box. It simply adds a few more places for poor connections and who uses the feature anyway.
Like Bmer
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Joined: Nov 29, 2021 4:36 PM
Location: California

Re: Power Supply to Main relay

Post by Like Bmer »

Thanks for all of the suggestions I really appreciate it.

There are probably numerous approaches to fixing this problem, but after doing a lot of research on numerous forums I found the suggestion made by Demetk on this forum to be nothing short of brilliant, it was the easiest and most logical approach.

Attaching a new wire from the Auxiliary fuse box to the green wire on the Main relay took 10 minutes and worked perfectly. By using this method I was able to avoid any other bad wires that may currently exist or any that may go bad in the future. Those wires are over 36 years old.

Car has been up and running every since I made the repair.

Thanks again to everyone for the suggestions.
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