Fuel Pump Power Problem

[Scott B.]

As I am nearing the end of the V8 conversion project, I powered up the computer to test the wiring. (I have not started the engine, yet.)

I am using a Ron Davis engine wiring harness - to that, I have adapted the truck's chassis harness. IOW, I am using my original fuel pump circuitry (power relay in the PCM, inertia switch, etc.)

I am using an A9P computer, 1991 vintage.

When you turn the key on, the fuel pump cycles normally (on for a moment, then shuts off when pressure is reached.) The problem is, when you turn the key off, the fuel pump turns on. You have to pull the relay to shut off the pump. Replacing the relay does not turn the pump back on.

I swapped the relay with a new aftermarket one - no change in behavior.

Any thoughts?

 

[allnpt0]

Sounds like a bad ground somewhere. Make sure all of the grounds are hooked up properly to the PCM.

Pete

 

[Scott B.]

Fixed it!

It turns out I tapped the wrong PCM "ON" lead for the new wiring harness.

Apparently, there was some current flowing in the reverse direction, after shutting off, that was keeping the FP relay engaged.

I switched to the correct lead, now it works as it should.

All I need to do now is build a bracket to mount the throttle cable, and I am ready to fire up this engine!

 

[RangerSVT]

There is a fuel pump relay feed, and there is a fuel pump relay on lead. Basically, one sends the ground to turn the relay on, and the other tells the ecu that the relay is sending power to the pump. I bet you got em backwards....

SVT

 

[Scott B.]

There is a fuel pump relay feed, and there is a fuel pump relay on lead. Basically, one sends the ground to turn the relay on, and the other tells the ecu that the relay is sending power to the pump. I bet you got em backwards....

SVT

That was actually my first thought. But not exactly where the problem was.

I needed a signal that was +12 at ON and START. I thought it best to tap into the PCM relay (as opposed to the fuel pump relay.) As it turned out, the same signal goes to both relays.

I used circuit 361 - which sends +12 to the fuel pump relay, the PCM relay, and several other devices.

Looking back, it was a simple problem. Just took some thinking to see it...