DME - fuel pump - crank position sensor - need help troubleshooting

[ratherman]

Got a '91 850i - have had it for many years (my dad and I both own a 850 and we have had (3) 750's at once - so we are fairly familiar with the v12's). But...

Need some help troubleshooting...


1.) Determined that one of the fuel pumps is not running (affecting the driver's side). Jumped the front fuel relay and it will run - so fuel pump is fine.
2.) Tested & swapped fuel relays - they appear fine
3.) Checked main DME relays - they test fine

Here is my question:

What, besides the CPS / crank position sensors can tell the DME to turn off the fuel pump (on other words, not trigger the relay)?

The CPS's both show ~480ohms

Will anything on the ignitions side (coil, cam CIS sensor, etc.)? also tell the DME to shutdown the fuel pump?

How often do the DME's go bad - should I try to swap them?

Car is somewhat a virgin - so most everything should be original - ~135k miles.

Any help is appreciated!

Thanks,

John R.

[Omega man 1969]

Yes swap the DME around and see if the pump that isn't running starts to run. The DME monitors cylinders 6 and 12 for spark and shuts down fuel to protect the cat if no spark is detected. It also monitors the primary side of the ignition coils so you can also swap the coils to test.

[cartoonz]

Will anything on the ignitions side (coil, cam CIS sensor, etc.)? also tell the DME to shutdown the fuel pump?...

Yes

Yes swap the DME around and see if the pump that isn't running starts to run. The DME monitors cylinders 6 and 12 for spark and shuts down fuel to protect the cat if no spark is detected. It also monitors the primary side of the ignition coils so you can also swap the coils to test.

just so he's clear on this... if you swap the DME's and the problem does not switch sides, it isn't the DME. That's likely going to be what you find with that exercise, but it is good to rule it out.

Next up, check the ignition components as Omega has indicated. Also, the connectors for both the CPS sensors and the Spark wire sensors to the main loom sometimes get a bit dicey, the crack/corrode/break/etc. Make sure to remember which plug goes where, it isn't exactly intuitive

[ratherman]

OK, spent the whole day on this - a couple of questions:

1.) The front fuel pump relay - does that correspond to cylinders 1-6 (passenger side)?
2.) We checked the crank position sensor via pins 67 & 68 on the DME's connectors - we get 2.5 VAC on both cranking the motor over - does that mean they are both OK? Can someone else please confirm those pins for me?

A.) We swapped the DME's the same fuel pump doesn't work - so that isn't the problem.
B.) Both sides of the motor have spark
C.) We tried to replace the CIS cam sensor on both banks with another complete spark plug cable

And yet the DME will still not tell the fuel pump relay to open

Any other thoughts??

Thanks,

John R.

[chinito03]

Try swapping the plug wires with pulse sensor...

Sent from my iPad using BF.com

[ratherman]

Sorry - we already tried swapping out the pulse sensor with another that we have...is there a more definitive way to test them via the pins on the DME cable while turning the car over - should they also be 2.5 VAC like the crank sensors?

Thanks,

John R.

[Omega man 1969]

I'm not sure what A/C voltage you should have but if it's the same reading as the side that's working then it should be fine. If you bridge the pins of the fuel pump relay socket to keep it running does the car run normally?

[ratherman]

Regarding bridging the pins on the fuel pump relay - thanks, I had just thought of that also - will try that in a little while...

John R.

[chinito03]

Sorry - we already tried swapping out the pulse sensor with another that we have...is there a more definitive way to test them via the pins on the DME cable while turning the car over - should they also be 2.5 VAC like the crank sensors?

Thanks,

John R.

what i meant is the spark plug wires... because what happened to my car few months back is that one of my fuel pump would stop working and my car would rn n a LHM. was able to pin point the problem when i swapped the spark plug wires..
Hope you get you 8 running.

[ratherman]

Problem appears to be the main relay - even though I had previously tested them

I never tried to swap them - so I tried that - and it started right up on and ran fine for about 2 minutes and then dropped a bank - tapped the one relay on the top with the back of a screwdriver and car completely died. Pulled the relay from bank 1-6 - shook it hard and put it back in - car started up and ran fine for about 5 minutes.

I got two on order...

Thanks - will keep everyone updated...

John R.