Fuel pump not running despite good pump, relay, and electrical. Any ideas?

[T0x1x]

Hi all,

Long time lurker on this forum, first time poster. We've been slowly fixing up a 1987 325is, using a lot of insight from previous posts on this forum, but this is the first time we've been a little stumped on a problem.

We recently reinstalled the gas tank and some of the wiring. After double checking everything, we went to start the car (we had to jump it since the battery had died in the time the car was down for the project). The engine cranked, but would not start. We measured voltage across the pins on the fuel pump, and there was nothing. So, we went into the engine bay to check out the fuse and relay.

The fuse was fine, but the relay didn't seem to be triggering when we tried to start the car. We jumped the relay pins and we could hear the fuel pump click on/off, so that's all fine. Now, here's where it gets a little confusing. The switched connection of the relay is getting >12 volts when we measure across it, and the signal to relay is about 11.6V (when attempting to start the car). A little low, but not unreasonably so. This does not trigger the relay, so we assumed it was a bad relay. We tried a different relay in the socket, but that didn't work either. So we tried to put 12V to the switch in the relay (where signal would go to) - they both clicked on. We looked through the electrical diagrams and multiple electrical troubleshooting trees for this area, none of which addressed this particular situation.

So, we have good relays, a good fuel pump, and apparently a good connection to the relay that has the correct voltage across the switch AND signal (a tad low) going to the relay. Could the slightly low signal be causing this? Any ideas on what other items could cause this issue?

Thanks for your help!

[ETurdy]

Here's the applicable wiring diagram: https://ww2-secure.justanswer.com/up...325_wiring.PNG

My first thought is to try directly grounding pin 85 on the relay. If the relay is able to switch the fuel pump on with a known good ground, I would expect there to be a bad ground in the current wiring.

Does anyone know where to check for potential bad grounds in the relay signal? Looks like it gets routed through the DME.

[e30sh]

The early cars trigger pin 86+ coil fuel pump relay through the on board computer box. The relay inside the OBC box does go bad on occasion. If you don't have a spare OBC box, you can bypass the complete OBC harness by plugging directly into the C104 connector like the non OBC cars did.

[ETurdy]

Where are the OBC relay box and C104 connector physically located?

[e30sh]

OBC is interior firewall above ABS computer, sorta right behind the headlight switch.

C104 is 3 wire plug on DME harness above glovebox

Good luck.

[ETurdy]

Thanks for the help!

I believe I have located C104 (3 wires, black/blue, green, and white) coming from the motronic unit above the glovebox. I looked through a few other forum posts, and it looks like there should be a similar 3 pin connector to swap into C104, but I can't find anything like this in the car. Is this a sign that this car doesn't have OBC, or that I'm dense?

[e30sh]

If it's an 87 325iS it should have an OBC.

Follow the existing harness that's plugged into C104. That's the OBC harness. Look for the other plug that's plugged into the OBC harness that will plug into
the DME harness C104.

Maybe someone has a pic handy of the early OBC harness.

[ETurdy]

https://drive.google.com/file/d/0B4yNVhnzvEYWZE1YQzFCV2VINUU/view?usp=drivesdk

The above photo (assuming the link works) is the wiring above the dash in my car

I tracked the wiring from the 3 pin connector to the driver's (LHD) side of the car until it goes over the steering column, but I can't tell where it goes from there. Should I keep looking over there it should I have found the other connector on the passenger side?

[ETurdy]

Also I meant to say this in my first post, but T0x1x and I are working on the same car together

[e30sh]

Your pic is correct...3 wire con in middle is C104.

If you don't have the ETK, download it here.

http://wedophones.com/Manuals/BMW/

Here's a snap shot of the two different versions, OBC vs 6 button.

[ETurdy]

Great diagrams!

Oh, that link is so helpful! I have now downloaded the 1987 3 series manual. It looks like page 1360-5 is the correct diagram for this car (2.5i engine). This also seems to match the physical wiring in the car.

So, today when I removed the fuel relay and turned the key to the RUN position, I measured ~12.5 volts between pins 85 and 86 in the fuel relay socket (0 volts with the key OFF). That should indicate that the main relay is triggering correctly since that's what connects pin 86 to power, correct?

As stated in the initial post, we have verified that if we power the relay externally, it will properly connect pins 30 and 87 and turn on the fuel pump.

So, we have 12.5 volts across 85 and 86, and a working relay, but the relay doesn't switch when we turn the key. Does this indicate not enough current through 85 and 86? Too much resistance in the circuit maybe?

[e30sh]

Pull the FPR. Place the positive lead of your meter in pin 86 socket. Ground the neg lead. I always use an alligator clip and hook it to the strut top nut.

Pin 86 socket should be battery voltage ~12.6 in both run and start. Pin 85 socket is ground so with the meter still grounded to the strut tower, prob it for ohms or beep test to verify a good ground.

The relay box is a know issue for a no start in the early cars. If the voltage test passes, it may be a current issue. I would bypass the OBC harness like the above diagram or see if you can locate a known good OBC relay box.

[ETurdy]

I'll have to double check the continuity to ground tomorrow. What's the max ohms I should see for a proper ground connection?

I still can't find the OBC relay box, I'll need to take another look up behind the abs module. But is the idea essentially that that I unplug C235 and C104 and then connect those together, bypassing the relay box?

Sorry this is taking me a while to digest, but thank you for your help! This is my first foray into auto electrical...

[e30sh]

Less than 1 ohm or if you pass the beep test, you should be fine.

See page 7000-5 figure 5 for OBC box location.

C103 to C104 is the path without OBC

There is a ground block right above the brake pedal. Check to make sure the nut is tight. BMW uses solid brown wire color for all grounds.

[jeffnhiscars]

Im imaging aliens who went to "invade earth" school to learn English before they attack then reading this thread, throwing up their arms in complete confusion and going home