How to drop the fuel tank and replace the fuel pump.

[GumbyRock]

Hey guys, I'm having a similar problem...
I'm new at this, but I just got done replacing my fuel pump on a '91 Sport 4x4. The task itself is pretty straight forward, I was able to do it and I actually had to borrow almost all of the tools (just to show how often I do this stuff...) Only problem is, the pump doesn't run when I turn the ignition. I've replaced the relay, and I tested the pump directly, and it ran. I probed the harness at the tank with a test light, and the yellow wire (fuel gauge?) is the only one that would light up (dim though). Where do I go from here?

 

[James909]

Assuming that you've checked all fuses, I would make sure that the inertia switch hasn't been tripped. It's probably a good idea to check for continuity across it as well. Also make sure that the CEL comes on with the ignition (PCM getting power). And you are correct in that the yellow/white wire at the fuel pump is for the fuel gauge.

 

[GumbyRock]

Thanks for your response, James.
I did check the fuses, and I found the inertia switch under the glovebox in the passenger kick panel but am not sure if it is tripped, or how to tell. Again, I'm new at this, but what is the ''CEL'' and how can I check that?

 

[James909]

Sorry if I was kind of ambiguous in my other post, I didn't have much time. To make sure the inertia switch isn't tripped just check to see if the button on the top is pushed in. Now would also be a good time to check the switch with a test light or multimeter. Have a friend turn the ignition switch from off to run while you check for +12V at the switch. There should be +12V at both the Dark Green/Yellow wire (input to switch) and the Pink/Black wire (output from switch) for 2-3 seconds after you turn the ignition on. CEL = Check Engine Light. When you first turn the ignition on make sure that the check engine light comes on for a few seconds. Let me know what you find.

 

[GumbyRock]

Right. Thanks James, here's what I got...
The button on the inertia switch is depressed and there is no power going to or coming from the unit. Also, the check engine light does not turn on with the ignition. I've seen some other threads that indicate this may be the EEC Relay, and that's an easy replacement, so I feel that is the next logical step.
Any input is greatly appreciated.

 

[James909]

Yeah, I agree with replacing the EEC relay. I'd start there.

 

[GumbyRock]

No luck...
I'm not sure which relay is the EEC relay (one is brown, the other gray), but I tried the new one in both places with no improvement. I was, however, able to acquire a multimeter and record some data... So, here goes:

The voltage at my battery is a little low, but the truck has sat for over a year.
At battery: 11.4v

With the ignition off:
Brown Relay: 11.4v @ Yellow/Black only (mostly yellow, black stripe wire)
Gray Relay: none
Green(fuel pump) Relay: 11.4v @ Black/Yellow only ( mostly black, yellow stripe wire)

With ignition on:
Brown: 10.9v @ Red/Green, Red, Yellow/Black
Gray: 10.9v @ All (Purple, Red, Green, Black)
Green: 10.9v @ Yellow/Green, Red, Black/Yellow (The only set of wires without power
are Green/Yellow)

At fuel pump wire harness with ignition on:
Black->Yellow: 7.1v
Orange->Yellow: 7.1v
Red wire dead...

Still no CEL with ignition on. I'm not sure where to go from here, and all I want is to drive the explorer again. ANY advice will be greatly appreciated.

 

[DeRocha]

You won't be able to test for 12v at the fuel pump unless you're very quick! The EEC will only power the fuel pump when:
The Ignition key is first turned to run (only for 2 secs)
Or when the Tach reads greater than 0 rpm.

The good news is the ~7v reading is normal when the fuel pump is OFF. Since the inertia switch leads directly to the fuel pump you can test for 12V right there. The positive end of the meter goes to either wire on the inertia switch the other to ground. Turn the ignition from Off to Run and you should see it jump up to 12v for 2 secs and then back to ~7v.

 

[DeRocha]

GumbyRock, I know you said you replaced the fuel pump, but what exactly is the problem you have??

 

[GumbyRock]

Sorry for not giving any background, but it's a long story...

Awhile back, I went out to my truck and found it wouldn't start. I was with a friend, who's brother happened to be a mechanic. At the time, there was no sound of the fuel pump priming while I turned the ignition, so he suggested that one of us lay beneath the truck and bump the fuel tank in an effort to get it to power on. It worked.

Everything seemed fine for quite some time, when an unrelated incident occurred.
My A/C compressor seized and destroyed my serpentine belt while on my way to work.
Got the Explorer towed back to my house the next day and within a couple days replaced the serpentine belt, bypassing the A/C (who needs it, anyway).

After completing this repair, the Explorer stayed in my garage a little while, but would start readily. Until it didn't. My battery died and it would not recharge with jumper cables, so I figured I'd get a new battery and all would be well.
Needless to say, the new battery did not help the truck to start, and thus began this journey...

Truck turned over when cranking but just wouldn't start. So I checked and measured spark plugs, and that's when I noticed there was no fuel getting to the plugs and when I finally took the time to notice, I realized the fuel pump was not running at all at start up.

So, I replaced the fuel pump a couple weeks ago and have been trying to troubleshoot this problem all the while... But I'm having no luck.

Thanks James909 and DeRocha for taking your time to help out a fellow explorer guy.

 

[James909]

It seems to me that the PCM is not powering on. Based on your tests, it seems like the EEC relay (brown relay) is working correctly but the fuel pump relay isn't being triggered to close. With the ignition on try grounding the light blue/orange wire in the self-test connector. The fuel pump should turn on. Also I would just like to confirm that the PCM is not turning on to rule out a burned out CEL bulb. With the ignition on using the multimeter back probe the throttle position sensor. Positive lead to the brown/white wire, negative lead to the gray/red wire. The meter should show about 5 volts if the PCM is getting power. At this point I'm thinking there's a bad ground somewhere or possibly the PCM is bad. Let me know what you find.

 

[GumbyRock]

Thanks again, James. Sorry for the late response, but here we are...

The fuel pump powers on when grounding the self-test connector with the ignition on.
The TPS does not get any power with the ignition on.
I'm having trouble determining how to be sure if the PCM (ECU or ECM?) is even getting any power, and whether or not it works at all..?
Spending another $50-$150 on a ''maybe'' is getting OLD... But, when this is all said and done, It'll totally be worth it.

 

[GumbyRock]

There are four wires that feed power to the PCM (I don't have a wiring diagram in front of me, so I don't remember pin assignments -- should be able to get that from a wiring diagram): a) one that is "hot at all times" (yellow I think) that feeds power for the PCM's memory when the key is off and b) three others (should be red) that should come from the EEC relay and should have power whenever the key is on.]

Gonna give this a try just to see if any power actually makes it to the PCM/ECU.
Also, I opened the unit to check for damage (burns, smells, etc) and the chip 'IC5' looks like it might be fried...

 

[GumbyRock]

So... After all of this frustration and troubleshooting, I finally figured it out. Thanks for pointing me in the right direction James.
It was the ECU all along. I probed the harness (it has 60 pins...) and found that, indeed, the yellow wire is always hot, and there are three more that get power with the ignition on. And since I've replaced/tested everything from the fuel pump up to the ECU, and verified that there was power to the unit, I decided that replacing it was the next best plan. So, a brief search lead me to eBay and $35 + $10 shipping (and two weeks of waiting) got me an exact replacement. I have, just minutes ago, installed the replacement, and... SUCCESS! My fuel pump has never sounded so good! Thanks again for all of your help and I can't wait to rock the Explorer again!!!

 

[James909]

I'm glad to hear you got it figured out. Sorry I never answered your other post I have just been really busy with school lately.