2Late

Hi, new here and this site was recommended to me by a friend!

A bit of history, I have been restoring a 1991 F150 4X4 from bumper to bumper and while the body is now at long last looking great I have a perplexing electrical issue and am about at the end of my rope as the old saying goes. I can not get the PCM to power up, here is what I have done so far,

I have determined that the PCM relay activates and supplies power to the fuel pump relay via the red wire and also supplies power to pins #37 and #57 on the PCM while pin #1 has power via the yellow hot-at-all-times wire for the keep alive memory function, all these tests check out good. I also have good grounding on pins #20, #40 and #60 so the PCM should work???? Am I missing anything?

I know the original computer had two bad capacitors so I replaced it with a rebuilt unit but it made no change, I still have no spark and no fuel pump relay signal via the bl/o wire on the relay. After (again) checking all the pins in the wiring harness PCM connector to determine that I had power at all the proper pins, good grounding at all the grounding points/pins and no shorting at any of the other connector pins I determined the rebuilt PCM must also be bad so I returned it for another one but still no change!

Any ideas on what I may be overlooking? I notice in the pin-out schematic for the PCM there is a wire from the PCM to the neutral safety switch (A40D automatic trans) but the engine cranks ok it just won't start. If the engine starter engages ok this switch "should" be good, or not???

BTW, before posting I looked around the site and it seems to be refreshingly different than a couple of other places I have frequented, I think I am going to like this place!

Chris_1

iTrader: (1)

If the engine cranks from the key the neutral safety switch is fine, you would think.
There are a couple grounds to check out - I've got to go with the 92-96 because that's what I'm sure on, but it should be the same; check the ground wires on the rad saddle in front of the washer bottle and the Rf fender just ahead of the battery.
Also all the other ones of course, but those in particular.
I'm not sure if you have a relay/fuse box under the hood but if so those can be taken apart (clips) so you can turn the whole thing over and check for bad connections on the underside of the relay plugs. Wherever your relays are, the backside should be reasonably accessible so you can check if power is getting to the relays.

88xlt

You say "I can not get the PCM to power up". Clarify something, does everything else work normal except that the ignition will not fire and the fuel pump(s) don't run briefly when you turn the ignition on?

2Late

Ok I finally found the problem while doing a last check before buying a complete underhood wiring harness from a salvage yard. Up until this point I had been checking all the pins in the connector for power at all the right points, grounding at all the right pins and had even checked between the power supply pins and the ground pins with a test light made from a brake light bulb burning both filaments to make sure the voltage wasn't breaking down under a load, EVERYTHING indicated the computer should function properly. As a last ditch effort I reconnected all the sensors etc, reconnected the battery and turned the key "on", then I started to check every single sensor, the ignition, injectors, etc. Almost immediately I found I was getting a dead ground on the reference voltage wire for the sensors, I found this while checking the MAP sensor but all of them in turn showed the same thing. So I started disconnecting them one at a time until I disconnected the Throttle Position sensor and the ground disappeared. The odd thing, and the reason I didn't find this when checking the PCM connector sockets is that when checked there it shows 180+/- oms resistance rather than a dead ground???????

Not sure why that reference voltage pin showed only some resistance but no matter what it was the problem! The wiring is all good and the problem is the TPS is shorted inside which caused a dead ground to the PCM output for the reference voltage circuit which in turn triggered a complete PCM shutdown. Who would have thought a TPS malfunction would cause not only the fuel pump circuit to fail but also the loss of ignition spark and fuel injector function but it did. Fortunately no harm was done to the computer and as soon as I swap out that faulty TPS I should be up and running since everything else is working normally now.

It's been a real pain finding this but at least it led me to this site which is a real plus!

Thanks guys