Fuel Pump relay / PCM issue

[bmxr123]

I would first like to say that I have been lurking for a while and have gained an invaluable amount of knowledge from this site already. I thank all of you that take the time to show us "newbies" the ropes.

So I acquired this 93 4.0L 4x4 ranger from a guy that said he wasn't getting power to the fuel pump.
After installing a battery i was able to do some troubleshooting and these are my findings:

I can jump the fuse pump relay contacts on the fuse box(i believe they are labeled 87 and 30 on the underside of the relay) and the fuel pump give a healthy "whine". Seeing as how the relays were all replaced by the previous owner,I figured maybe the ground contact on the fuel pump relay port was bad so i ran a new ground wire from the existing "ground" port for the fuel pump relay. Now when i turn the ignition to "on" i get power to the fuel pump but it just STAYS ON. It does not do the typical 2-3 second prime. It just stays on. I can get the truck started like this but of course fuel pressure seems a bit high as the motor is bogging and backfiring/running way rich. I have read a few things about the PCM on these bad boys and from what i gather the PCM chooses when to ground the relay and kick the pump on..If this is true, then my "always-grounded-fuel pump relay" will not work out for me. So now, i have a couple of questions:


1. From what i gather from old posts, most of these issues point to the PCM. Is there any way to be CERTAIN it is the PCM before i spent a boatload of money on this part? I read of one guy's success in replacing the capacitors on his 4.0L explorer's PCM. I might try this as Advanced Auto Parts just quoted me $247.00 for a remanufactured 4x4 PCM!!

2. My fuel filler hose is looking a bit sketchy. If it has any cracks near where it meets the tank, would this cause the fuel pump to run all day, trying to reach the required psi, or is a running-all-the-time fuel pump a definite indication of an electrical issue? When the fuel pump runs, i DO hear a "slurping" sound in the tank that i can't quite identify. I do know that on alot of systems a "static" fuel pump can run ALL THE TIME if its trying in vain to reach a spec fuel pressure level. Was just wondering if and when the pcm goes from "prime" to "static".

3. I don't have a fuel pressure gauge and plan on buying one when i have the money. In the meantime, i will let the fuel pump run a bit and press the shrader valve at the fuel rail and i get fresh gas pouring out but it doesn't SHOOT out at all. Is this indication of below-acceptable PSI at the rail? lol I know..roll your eyes if you must.


Thanks in advance for any insight or advice you can give!

 

[Earl43P]

Welcome to The Ranger Station!

Send me a PM (include your e-mail address) and I can send you 1993 schematics.

Yes on #3. Should spray out pretty forcefully, at 30 psi or so.

To PM, click on my username.

 

[RonD]

The fuel pump relay has 12v at two contacts, when key is on, yes the PCM grounds the relay to send 12v to the inertia switch which passes the 12v to the fuel pump.

A loose or bad ground at the PCM could cause the relay not to operate.

The PCM turns on the relay for a few seconds when key is turned on, and then turns on the relay intermittently based on RPM and speed.

If you look here:
http://www.therangerstation.com/tech_library/EDiagrams/index.htm

Then click on "3.0/4.0 Engine controls 1 of 3" a page will open showing the wiring and the pin on the PCM that grounds the fuel relay, and the wire color.
Pin 22
You could test if this pin is grounding when key is first turned on, if it is then wire to relay is the issue.
If pin is not grounding then most likely a PCM issue, if all grounds are tight, I would look for one at a recycler.

 

[bmxr123]

Thanks for the great info guys!


Two new developments:

1. This "vaccuum" line was broken. It seems to lead straight to the fuel rail, where a line to the fuel pressure regulator comes from! Not sure if this is unrelated.Probably have a few existing issues here but its nice to know i knocked one out today.



2. Upon removing and inspecting the PCM I found there was oxidation on almost EVERY one of the pins on the PCM..I traced it from what appears to be some water in the main harness-to-pcm shroud! Looks like water had at one time ran down the wires and into the pcm ports!yikes!..I hope nothing shorted. i cleaned up everything on the pcm plugg and pcm itself as best i could with some QD electronic cleaner and a brush.

 

[RonD]

Low voltage Grounds and wire connections in general don't work around corrosion very well, good find.

The fuel pressure regulator(FPR) works on a vacuum, at idle vacuum is highest so FPR diaphragm is held open and fuel is sent back to the fuel tank via the return line, as RPMs increases vacuum decreases so diaphragm closes to match vacuum.
When there is no vacuum, engine off, FPR is closed to keep some pressure in the system.
Fuel pump in the tank has a check valve that prevents back flow.
If working properly the FPR should maintain 30-40psi of pressure.

The diaphragm in the FPR can get a leak as it gets older, smell that vacuum line for gas smell, if you smell gas then replace FPR.

With fuel pump running and no vacuum at FPR, pressure would have been quite high.
Another good find

 

[bmxr123]

okay so i swapped my junk pcm with 2wd 4.0L AT PCM. I have read that 2wd and 4wd PCMs will swap as long as they are both automatic trans.

After the swap, I'm still getting a weird 1.65 volts to the "keyed on" side of the fuel pump relay. This, of course is not enough to kick the relay or the pump on.


So maybe fuse box corrosion where the fuel pump fuse and EEC fuse is? This truck was noticeably submerged in water at one time, and there seems to be a bit of corrosion on the underside of the "power distribution box"(fuse box). I feel like i have covered all other bases...My main concern now is that it seems to part of the harness. So am i really looking at an entire harness swap? geez

Im at a loss.

 

[RonD]

Fuel pump relay should have 4 wires
1. Tan/green or Blue/orange is the PCM control wire, only a ground for a few seconds after key is turned on, from pin22 on PCM.
2. Red is power usually from from the EEC power relay(key on)
3. Brown or Green/orange is wire to inertia switch (then fuel pump)
4. Yellow or Black/yellow is fuel pump power from fuel pump fuse(key on)

Which wire is only 1v?
2. or 4.

3. also hooks to PCM(pin 8), to monitor when pump is on.

 

[bmxr123]

Fuel pump relay should have 4 wires
1. Tan/green or Blue/orange is the PCM control wire, only a ground for a few seconds after key is turned on, from pin22 on PCM.
2. Red is power usually from from the EEC power relay(key on)
3. Brown or Green/orange is wire to inertia switch (then fuel pump)
4. Yellow or Black/yellow is fuel pump power from fuel pump fuse(key on)

Which wire is only 1v?
2. or 4.

3. also hooks to PCM(pin 8), to monitor when pump is on.

-The Red wire(your #2) is the one showing 1.65 volts when keyed on..It shows nothing with key off.
-I get 12 volts from the black/yellow(your #4) (off or key on...doesnt matter)


Thanks for your help!

 

[RonD]

I would say the Red wire now has high resistance, I know duh, lol, internal corrosion or corrosion at the other end, it should be a short wire, either connected to the EEC relay or a common 12v bus connected to a relay that is turned on by the key.

1.6v key on and then 0v key off means the relay the Red wire is connected to is working, but it's open/close contacts could be corroded or it's IN voltage may be 1.6v.
But this relay would also power other devices so you should notice other issues if the IN voltage on this relay or it's contacts were out of spec, it's not just the fuel pump relay's power supply.
So I think it will boil down to the Red wire having damage and you just need to trace and replace that one wire.
It will be in the power distribution box so not in the harness.

 

[bmxr123]

That red wire that is getting 1.92v is coming straight from the pcm, which i just replaced. I don't get it.

I have also noticed that the EEC fuse block on one side is getting that same 1.92v, and the eec relay as a result is only getting 1.92v.

 

[Earl43P]

No, grasshopper.
That red wire puts power TO the PCM.

The source of that red wire is the PCM relay, when you turn the key to Run.

If you get 1.92v at the EEC fuse, that ain't 12v, replace that fuse and clean up its holder contacts.

Might want to disconnect the battery while you mess with that.

 

[RonD]

If you look at this diagram
http://www.therangerstation.com/tech_library/EDiagrams/files/Diagram_Enginecontrols4_0_3_0_1of3.JPG

You will see the EEC power relay (left side)
It is powered by a fuse(yellow wire), and is turned on by the key(red/green wire)
The red wire coming from the EEC relay powers many devices, including the fuel pump relay, MAF and two points on the PCM.
The red wire from the EEC relay should be 12v, as said above check the fuse first, "looks good" isn't a check test it or replace it

 

[bmxr123]

As soon as i replaced that 30amp EEC maxi fuse I checked it with a multi-meter and realized i was getting a full 12v at both sides, the EEC relay, AND sub-sequentially the fuel pump relay as well! I was so excited when i keyed it on and heard that fuel pump prime! Of course, she fired right up and all is good!

Thank you guys SO MUCH for the help! I could thank you all enough!! So happy right now that the hard work payed off!

Plus, i know the procedure for fuel pump troubleshooting on an eec-IV like the back of my hand!!



Now onto the next issue..A power steering pump whine like no other! lol..I didn't even care when i heard it as i pulled out of the driveway! I was laughing hysterically out of pure joy that i was actually driving this beast as it switched gears like the day it came off the floor! She has some power too!

 

[RonD]

Good work


A "quiet" Ford power steering pump, if you can find one let me know the part number

But I know what you mean, louder than "normal"

 

[bmxr123]

It was low on power steering fluid..The pump definitely got quieter after i filled it...One more thing..How can i tell if my 4x4 selector is working or not? The indicator lights next to "4x4" and "low range" are apparently not working? i can't tell if im in 4wd or 2wd..lol..Ill be looking that up now..