Troubleshooting Fuel Pump Relay - Low power

[Timp]

A couple nights ago on my drive home, my '87 325e died - it was an instant cut-off, and it wouldn't respond to the pedal at all. I pulled over, and *luckily* another E30 owner pulled over to help me out. It turns out he's a mechanic as well, and knew exactly where to look. We determined it was a problem with the fuel pump(s) or it was an electrical problem. I got it towed home (I don't know what I'd do without AAA) and it sits in my driveway currently.

This morning I followed the steps in the Bentley manual to troubleshoot the pumps. Here's the rundown:

- Fuse 11 is good.
- I'm getting .2 volts to the reserve fuel pump.
- I'm getting just under 1 volt on my meter in the middle relay when I put the probes in socket 30 (+) and 85 (ground) with ignition ON.
- The fusible link in the trunk seemed to check out fine. My meter gave me a clean connection from both ends.
- The reading between 85 (-) and 86 (+) came out to be 10 volts when I tried to start the car.
- I swapped the Oxygen sensor relay and fuel pump relay (I don't have a jumper, and figured this would work) and I got the same exact results as above.

Any ideas?

[jlevie]

The DME only enables the fuel pump when it sees timing data from a rotating engine. So to check the pump's power circuits you have to do so while cranking or by jumpering the fuel pump relay socket.

Given the way the engine died, another possibility would be that the DME lost timing data from failure of the CPS. Check for spark when cranking the engine.

[Timp]

Alright, so I'm getting about 10 volts to both the reserve and main pump when I try to start the engine, and I am getting a spark from one of the plugs.

[jlevie]

Okay... That is too low a voltage and suggests a high resistance in the circuit. Replace the fuel pump fuse, just in case, and check the voltage at the fuse when cranking the engine. If the voltage is normal there and low at the pump the problem is in the wiring.

If the voltage is low at the fuse, release the fuel pump relay socket from the holder (so you can get to the bottom of the socket), and check the output pin (87) and input pin (30) while cranking.

[Timp]

Here's the results:

I didn't have an extra 15 amp fuse on me so I swapped it with a couple others and got the same results in the relay pins.

Fuse 11: 10.5 volts when cranking
Pins 87 and 30: 10.3 when cranking, 12 when ignition is ON.

Curious, no? I'm stumped as ever.

[jlevie]

Using the wiring diagrams from http://wedophones.com/BMWManualsLead.htm trace back from the fuel pump relay socket to find out where the high resistance is.

[tokky28]

Did you find the source of the problem? I am having similar issues with mine. I'm getting a low voltage reading (5v) at 30 to the fuel pump relay.

Thanks.