Fuel pump/starting problem for the electricians and experts

[stevebuk]

I'd be wary of resisitance measurements when the DME is powered - I'd be inclined to measure the voltage between battery positive and pin 1 of the DME you should get 12 volts if the DME is switching.

A switch could be a short term measure but I would be concerned about the fuel pump running all the time, some DMEs switch off the fuel pump when the car is travelling down hill with the throttle closed and also after impact to stop the fuel pumping but not sure if the 5 series does this but I thought it did.

[Bellicose Right Winger]

The coasting w/closed throttle fuel cutoff is usually done via fuel injectors. I'm not aware of any mfrs that shut off fuel pump to do this.

The "impact switch" mechanism that shuts off fuel pump in an accident should be looked at. It sure seems odd though that there would be multiple problems, that is a bad CPS and something else.

Paul Shovestul

.......... some DMEs switch off the fuel pump when the car is travelling down hill with the throttle closed and also after impact to stop the fuel pumping but not sure if the 5 series does this but I thought it did.

[stevebuk]

Paul

Thanks for clearing up my confusion.

Steve

[azale]

I'd be wary of resisitance measurements when the DME is powered - I'd be inclined to measure the voltage between battery positive and pin 1 of the DME you should get 12 volts if the DME is switching.

I can test pin and battery positive tonight but I'll do you one better: I found 12 volts between fuel pump relay socket 86 and socket 85 (which directly connects to DME pin 1) while the DME was plugged in and powered. But I think the reason the relay still wasn't switching was the amperage.

The "impact switch" mechanism that shuts off fuel pump in an accident should be looked at. It sure seems odd though that there would be multiple problems, that is a bad CPS and something else.

I would like to hear more about the impact switch or crash control module but I don't think bahnstormer wanted to scan 500 more pages of wiring diagrams, and the diagrams I saw showed it working mainly for hazard lights/relays.

It does seem odd that there would be multiple problems but it's not impossible either. I could very easily have ruined something in all my electrical testing or sensor replacement. I am still baffled as to why the fuel pump wouldn't even run after I put the key in the 'run' position.

My friend Jerry Rigger says I could jumper a small ground wire to the fuel pump relay socket 85 wire to get the relay to switch when the key is in 'run.' Furthermore I could take out all the slack in the wire and attach a 5 oz weight to the center. If the car were in an accident of significant magnitude, the weight would pull the wire and disengage the fuel pump: mechanical fuel pump safety feature.

Whaddya think?

[Javier]

and car starts and run fine, + to FP relay 86 is OK and, and relay itself is OK, my only guess is that DME output transistor is damaged (or the contact at pin one of DME got bent or some how damaged in the connecting / un-connecting work you have been doing lately). The only sensor I know of being capable of shutting down the pump is the CPS, and it acts as a permissive for the pump, the engine is required to run in order to keep pump working, after the initial prime due to the key turn, when it would run briefly with the engine stopped. If it does not do that, sorry, DME output at pin 1 should be checked.

Javier

[Bellicose Right Winger]

Unless the crash control module is also a permissive for fuel pump operation, although it's connection to the DME isn't obvious. It's connected to the GM and the LKM.

Impact switch is under rear seat on left side.

If 12v was accidentally applied to pin 1 of DME, via fuel pump relay socket, and cranking followed, this would certainly fry the output transistor.

Paul Shovestul

and car starts and run fine, + to FP relay 86 is OK and, and relay itself is OK, my only guess is that DME output transistor is damaged (or the contact at pin one of DME got bent or some how damaged in the connecting / un-connecting work you have been doing lately). The only sensor I know of being capable of shutting down the pump is the CPS, and it acts as a permissive for the pump, the engine is required to run in order to keep pump working, after the initial prime due to the key turn, when it would run briefly with the engine stopped. If it does not do that, sorry, DME output at pin 1 should be checked.

Javier

[azale]

250V 5 amp fuses for my dad's multimeter: $6
DME/Main relay with shipping: $17
Crankshaft position sensor two day shipping from AZ: $105

Successfully starting my car after 4 weeks of troubleshooting and repair but having to get out of the car immediately afterward so when I wet my pants I wouldn't ruin my leather seats: Priceless.

I pulled the DME and looked in vain for burn marks or signs of damage. I replaced it and turned the key but the pumps still didn't run. Resigned that I would not be able to get a DME any time soon, I tested if my jerry rigging job would trip the relay and it would. I then ran a wire from fuel pump relay socket 85 to a ground. When I cranked the engine I flooded it. So I promptly removed my hack job and cranked until the engine worked out the extra gas. I assumed the engine would eventually die but it never did. So I tested for voltage at the fuel pump fuse: 12v, no complaints. I let it run for a little while, double-checked that my hack job was removed, and shut of the engine again. Then she started right back up and ran like a dream. I heard a disturbing noise for the first mile but then I pulled the hand brake and cleaned the drums. What a relief.

I assume when I pulled the DME it reset??? I'll let you know if it brakes down tomorrow.

You guys have been great. I bet you were hoping this thread would end about 30 posts ago but I think the ending makes it all worth it.

Please allow me to at least thank everyone who chimed in: ryan roopnarine, 632regal, DanDombrowski, Garlic Breath, genphreak, SSR2, Bellicose Right Winger and especially lowell, Bill R., Javier, and stevebuk

I know you guys are going to help me enjoy this car for a long time; you already have.

[stevebuk]

Great news!

Really glad you have it running and can enjoy driving it.

[Javier]

Javier