Got the computer at RockAuto. It's the right computer according to the calibration. It is acting the same as the old computer. I'm leaning toward the wiring, possibly because I don't want to think I was sent a defective rebuild.

If I knew where I could buy a brand new one, if they're even made anymore, I'd have gladly paid the excessive cost versus a rebuild that might be defective.

 

Ya know, the dealerships had these really cool diagnostic tools for figuring out this kind of thing, you'd connect it into the main harness and look at the blinkenlights & meter readings. I'me borrowing something kinda similar from Bill Vose for my Taurii stuff but it's for diagnosing other electronic engine control stuff and isn't really designed to be testing harnesses (though I guess it could be used for that).

 

So now we know throwing parts at it will not work
Are you saying the external high pressure fuel pump runs and the internal tank lift pump does not? Do you have dual tanks? Same result in both tanks?
I had this happen on my 86 as well. Sorry, never figured it out and I stripped the truck down for a rebuild.
What a job indeed. And it will not help one bit because you do not have an ECM wiring problem.

If the frame mounted fuel pump is running and the tank pump is not, you have a problem in the wiring between the fuel pump relay and the fuel pumps themselves. The ECM wiring harness does not include this wiring. I would look for a problem in the tank selector switch and associated wiring to the fuel pump relay and the tanks themselves.

 

And I just read this. There will be no voltage on this wire unless the ignition is on AND the computer is activating the fuel pump relay. To test with the engine not running, you must jumper the fuel pump test pins at the test connector. The pink wire with the black stripe is the same wire at a single tank pump and the external fuel pump.
I am surprised that it would run at all with the tank pump disconnected. Are we dealing with original Ford wiring and pumps without any modifications here?

 

Are you saying the external high pressure fuel pump runs and the internal tank lift pump does not? Do you have dual tanks? Same result in both tanks?

No, single tank.

 

I am surprised that it would run at all with the tank pump disconnected. Are we dealing with original Ford wiring and pumps without any modifications here?[/QUOTE]

Hard to say. I bought the thing about 7 years ago and it's registered as a "reconstructed" vehicle.

 

Do you have a 4 connector plug to your tank unit?

To be sure your meter is set up correctly (dc volts), measure your battery voltage (across Pos & Neg battery terminals), about 12vdc, about 14vdc with engine running.

So disconnect the connector, start your engine (Engine need to be running), measure dc voltage, about 20 Vdc range, positive meter lead to pink wire on the connector, negative meter connection to ground. You should read 10-14Vdc ?? (This is power for the motor)

It is handy to have alligator clips to attach to your meter leads, also its handy to have a black and red alligator clip lead set about 10' long (clips on each end), next time you are in your radio shack.

If you have a yellow wire at the connector you should have about 12 Vdc, with the negative test lead to ground? (This is power from the fuel gage)(ArdWrknTrk reports that the voltage may be as low as 5vdc) record your findings,

The other 2 wires/connectors are ground. But don't use them as your ground for those first two test. One of them is black and the other is ?? Orange?

While you are at it check the other two wires to see if there is any power, make a note if there is.

Test the two ground wires; (Turn your engine off) Scale Ohm's Rx100 or higher, test from each wire to Body ground, you should get full deflection of the needle. Connect between both ground wires and you should get a full deflection of the meter. If you read any resistance, make note of it.

Make a diagram, Pink wire to pin1, Yellow to pin 3, Black to pin ? Orange to pin ?

Do these checks and we will go from there. Can you hear the in-tank pump if it were running?

 

Have a Dr. Appointment first thing this morning but will do this later today. I get back to you. Thanks, Vince.

 

Yes the truck can run OK on just the frame pump. The only time you need the in tank pump is if you run out of fuel or go up steep hills or burn out starts.
I know of guys that have drove their trucks for years in Fla with the in tank boost pump not running. It is very flat land there I am told.

That said a place you might check to see if you may have a one tank jumper behind the dash that plugs into the place that the two tank switch goes and see if it is missing.
Newer trucks have this jumper with a one tank system but I do not know about a 1985 truck.

 

Not sure what you talking about unless it is the SUN machine.

There was the BOB (60 pin Brake Out Box) that plug into the harness between the Computer and the wiring harness but it has no blinking lights or a meter. I use mine a lot.


There was also the Monitor, I have one but very seldom use it.


I have the Recorder that goes with it and I use it even less.


All of these units go between the computer and the wiring harness.

 

Power for the fuel gauge comes from the instrument cluster voltage regulator.
This is a thermal switching regulator and should show a (buffered) ~5.2V according to Gary's testing.

Gauges are thermal indicating.
Yes, that's right..
They warm up and tension against the needle reduces to indicate fuel level (oil pressure or water temperature)
This provides a measure of damping because of the time needed to heat or cool.

If these gauges saw full battery voltage for any length of time they would be off scale and quickly fail.

 

I must admit it's a little hard to follow everything that's going on here, but I might have a little bit of information that may be of use.

I think I have almost the same truck as yours: 85 5.0 EFI 2wd. I think the only difference is that I have 2 fuel tanks and you have one.

I'm a little confused when you say you can't measure any voltage at your tank, but you say that you can hear the pump running. In my truck, I have 2 pumps; one is on the frame rail and there's another one in each tank. I don't want to offend you, but when you are checking voltages have you ensured the meter is set to DC and not AC? If it was accidentally set to AC (usually a little squiggle instead of a straight line) then you probably wouldn't measure anything.

Also, I recently replaced my original computer with a rock auto one and ... unfortunately, I'm getting exactly the same problem you report. It has only happened twice; once right after I put the computer in and (unfortunately) yesterday. Pump runs for a very long time. If i try to start the truck before the pump stops the truck doesn't start. This is purely a guess on my part, but it's like the injectors never work. The computer is stuck in some funky state. Just to be clear I never experienced this problem with my original computer, so I'm pretty sure my problem is not wiring. This would suggest your replacement computer could have the same problem as your old (but not original) computer.

We have to figure out why it only happens every once in a while.

 

Question. You said that the pump never stopped (in your quote above), but somehow you were able to get it from never stopping to the same long (approx 3 minute) run you were experiencing previously.

How did you get the pump to not run forever?

 

I believe he did not measure any voltage at the tank, was due to the key switch was turned to "ON", and in that state there would have only been voltage there for the 1 second during initial relay power-up. His truck will need to be running for him to measure the pump voltage at the tank.

Good point making sure the correct range is set on the meter.

 

I have no idea. It just stopped running all of a sudden and then the truck ran perfectly. When the pump doesn't stop running, it chugs and pours out tons of smoke.