Computer Replacement:

[Alan_nc]

94, 3.0, Auto, 2 wd

I have always had to run my fuel pump with a seperate switch. Works fine but I really would like it to come on with the ignition switch.Thought it might be a computer problem.

Fellow doing a V8 swap with the exact same year and drive train offered to give me his computer. Finally got around to doing the swap today.
I had read the details of how to do it and looked at a couple of Utube vidios. This model has the computer on the firewall under the brake master cylinder.
Had to remove the cruise control unit - just for clearence.
My truck had a factory cutout in the fender well so I was able to get a socket with extension on the lower computer mounting bolt.
Other bolts came out very easily.
Because of the wireing harness the computer itself was a bit of a pain to remove. I was afraid to move the wires to much for fear of breaking something.
Putting the new unit in went much quicker - guess I knew what to expect at this point.
Took maybe 30 to 45 minutes for the total job.

So thats the good news.
Bad news is it didn't fix anything.
The truck still runs very well....but I have a seperate switch to turn on my fuel pump.

 

[Shadowrider6661]

Why don't you just bypass your switch and connect the wiring directly to your ignition fuse panel ? It's the simplest solution, it's also what I would do for the short term. Chasing bad wiring is a pain, but its something that needs to be done.

 

[racsan]

well the good news is you can leave the keys in it all the time and it not be stolen.

There should be a fuel pump relay spot in the underhood fuse block. Thats where I sourced my “key on” battery + for my electric fuel pump.

 

[RonD]

Where did you high jack the wiring for fuel pump switch?


Fuel pump gets 12volt power from the fuel pump fuse and then the relay in the engine fuse box

Battery(+)----FP fuse----------FP relay-------------inertia switch---------------------------fuel pump-----ground

The fuel pump relay is controlled by the computer via it "coil" ground side
Fuel pump relays "coil" gets 12volts with key on from the EEC Relay
Computer grounds the "coil" for 2 seconds only when it boots up, then nothing until engine starts
You can cycle key off and on again to get 2 more seconds, repeat as much as you want

Key on 12v-------EEC Relay closed-----------(+)FP relay coil(-)------(OBD1)--------computer pin 22


On the main wiring harness near engine fuse box you will find the OBD1 connector
Look at the drawing here: https://www.therangerstation.com/tech_library/OBD_I.shtml

It shows the Fuel Pump slot in OBD1 connector
This slot is a pass thru from pin 22 on computer to FP relay coil, so if you GROUND is slot, then turn on the key, the Fuel Pump relay should activate and pass 12volts to the inertia switch and fuel pump

FP relay coil(-)----------------OBD1 slot---------------------computer pin 22


Bad capacitor in computer can cause fuel pump issue, under $5 to replace all 3 caps in these computers

 

[black_demon69]

 

[Alan_nc]

My switch is wired across the fuel relay in the black box (in the 94 the box comes off the drivers fender just behind the battery). The way it's wired I still only get the 2 seconds on when I turn on the ignition key (when I have my switch in the on position). Then normal action of the fuel pump once the engine starts.
Honestly guys I think I'll leave it the way it is rather than tackle the wireing harness. With the wires as old as they are I could cause a lot more problems than I'm trying to fix.

 

[RonD]

From description your switch is sending 12volt to FP relay's coil, and computer ground(pin 22) is working fine
(the 2second run time is from computer pin 22 grounding the relay)

Most likely the underside of the fuse box(black box) has some corrosion which can get worst and effect other systems over time, or could be just that one slot in FP relay's base/plug in
The FP Relay is not getting 12v from the EEC(PCM) relay with key on

The RED wire in above diagram that runs from S134(splice) to C186(on FP relay's base)