Fuel pump won’t prime

[Pix3L8]

Ok guys here’s my issue, driving the bronco today and she died on me. turned the key and got a battery light, my abs light, no check engine light and fuel pump wouldn’t prime. Let it sit for a minutes and boom it came on, surged and sputtered then died. Tried again about 5 minutes later and it fired right up (keep in mind when the pump would prime the ce light would come on when the key was in run) . So I get home and start testing, I swapped the relay, checked grounds, reset inertia switch just in case etc...nothing. So I cleaned my garage come back out and it fires right up again.

I’ve had this problem in the past and it came down to a broken wire in the obd1 access plug. I have 2 questions, can I “jump” the FP to test it and make sure it’s not just busted, and 2 what are your thoughts on the check engine light coming on like normal when the FP primes but other times when it won’t prime I get no light at all. Truck still cranks regardless.

The biggest thing is that I got it home, but ide like to figure this out. I’m not an expert with an multimeter but with a little guidance on which wires to test/jump etc I should be able to work thought the FP circuit (89 Bii)

 

[rusty ol ranger]

You can "hot" the pump. I ran rusty #1 that way for years.

The ECM controls the fuel pump prime/running. Im wondering if your ECM is on its way out.

When rusty #1 wouldnt run and i just hotwired the fuel pump, years later i tried tHE ECM in rusty #2 and the fuel pump wouldnt run, and pulled a code for the fuel pump.

Try scanning it.

 

[PetroleumJunkie412]

 

[Pix3L8]

Well when it would prime “occasionally” I scanned it, got code 95. In my manual that corresponds to fuel pump secondary circuit fault. Now I can’t even scan it because my CE light won’t even come on

 

[rusty ol ranger]

Well when it would prime “occasionally” I scanned it, got code 95. In my manual that corresponds to fuel pump secondary circuit fault. Now I can’t even scan it because my CE light won’t even come on

Code 95 is what i got. I believe it means the fuel pump circuit in the ECM is fried.

 

[rusty ol ranger]

I suppose it could be fuel pump harness wireing, but that wouldnt explain the check engine issue.

Possibly a faulty ignition switch? That could possibly cause what your describing.

But i would almost bet ECM.

@PetroleumJunkie412

 

[PetroleumJunkie412]

The CEL failure is reeeeeealy suspicious of ecm as well. The CEL circuit runs through the ecm.

If you're positive it's not a relay, fuse, or effed up wire, pull your ecm and look to see if one of the electrolytic capacitors crapped itself all over your main board.

 

[Pix3L8]

I suppose it could be fuel pump harness wireing, but that wouldnt explain the check engine issue.

Possibly a faulty ignition switch? That could possibly cause what your describing.

But i would almost bet ECM.

@PetroleumJunkie412

I have a few ecms and I did swap another in..same deal. Mind ya both of these ecms worked yesterday. So say it's not the ecm how would I properly test that FP circuit? It very well could be the ecm but I want to test everything before I drop 200 buckaroos for a new one

 

[Paulos]

Code 95 means the ECM is not sensing voltage on the load side of the fuel pump relay.

Here's the diagram for bypassing the fuel pump relay.

The wire coming off the relay to pin 8 of the ECM is the wire that tells the ECM there is no fuel pump secondary voltage (load side of the fuel pump relay).

I don't trust the inertia switches. I had one that didn't like cold weather. It's worked fine since I bypassed it several years ago.

 

[Pix3L8]

Code 95 means the ECM is not sensing voltage on the load side of the fuel pump relay.

Here's the diagram for bypassing the fuel pump relay.

View attachment 39394
The wire coming off the relay to pin 8 of the ECM is the wire that tells the ECM there is no fuel pump secondary voltage (load side of the fuel pump relay).

View attachment 39395

I don't trust the inertia switches. I had one that didn't like cold weather. It's worked fine since I bypassed it several years ago.

so if I ground the FP test point I should hear the pump kick on?

 

[Paulos]

so if I ground the FP test point I should hear the pump kick on?

Yes.

 

[franklin2]

You have a intermittent problem. So that makes it difficult to find right there. What I would do is get a light bulb and some long wire, tie the wires to the bulb and then put the wires to the bulb in a strategic place so you can monitor the voltage there. For instance the fuel pump relay location. Put one of the bulb wires on a good ground, and put the other bulb wire to the wire that leaves the relay and feeds the pumps. Then just drive it. If you have a problem again, you can look at the light. If it is lit up, but the pump is not running, you know the problem is downstream and possibly even the pump itself. If it's not lit, then you know the relay may not be on, or it may not have voltage to it, you could then move your testlight apparatus upstream.

 

[Pix3L8]

Gentlemen, we have liftoff! Traced it back to a bad EEC relay

 

[Pix3L8]

I went thought the diagram Paulos posted and I had voltage where I should except the EEC relay. It was so crusty I figured my multimeter wasn't making contact so I ignored it. I had a spare FP relay that I replaced the busted EEC relay with and it works, as far as I can tell using a FP relay shouldn't hurt anything. Inertia switch, fuses, everything was fine and of course, it was the easiest and simplest thing that I overlooked.

 

[rusty ol ranger]

Nah the ECM and fuel pump relays interchange. Glad you figured it out!