91 XLT Explorer project.

[jeffpcell]

Howdy, I've recently acquired a 91 Explorer XLT that has been sitting for about 9 months. The previous owner said that it needed a fuel pump and wanted it gone. After towing it home and doing some reaserch I dove head first into it, but now I'm coming up for air and looking for any advice available that will help me. I did get it to run one time, but i had to ground out the fuel pump test lead to do it. The fuel pump doesn't prime with the key on, nor does the check engine light come on. I checked out the relays, tried jumpering past them and still nothing. After consulting a chiltons and haynes manual I gained a decent understanding of the PCM and wiring harness (thanks in large part to members of this forum) Pin 20 is a good ground at all times, 40&60 on the other hand are grounded only with the key off, when you turn the key on 40&60 bring back 12 volts. My idea of the problem is that either a sensor has gone bad, shorting the +12V line to the ground or a wire in the main engine harness is doing the same. Does anyone have an idea on what might be a common failure in this cenario? I've done quite a bit of homework already, but a fresh perspective can always be a good thing. Thanks Ya'll!

 

[ghosttrain2]

Make sure the pcm ground at the battery is clean and tight,and does not look like a science project.Turning the key on grounds the eec relay and provides power to the pcm. The pcm grounds the fuel pump relay, and provides power to the fuel pump after going through the inertia switch. A good clean ground is critical. You should be able to check for power starting at the fuel pump relay,and then the inertia switch.

 

[BrooklynBay]

Welcome to this forum! I've moved your thread into the stock 91-94 section. The computer is not turning on. The fuel pump can't turn on unless the computer turns on first.

 

[jeffpcell]

I know the computer isn't turning on. My haynes manual shows the ground wire that feeds the pcm relay and pins 40&60 as being the same line. The PCM relay is turning on, and i know it is because with the PCM disconnected I get 12 volts at pin 1 all the time, and at 37 & 57 with the key on. Just for the giggles of it I took the battery cables off, cleaned all of the connections and put it back together, including the ground to the body. The fact that my two ground pins at pin 40 & 60 bring back 12V as soon as you turn the key on would almost invariably mean that the wiring harness is FUBAR, wouldn't it?

 

[jeffpcell]

One more quick thing, about the ground line that feeds the pcm and relay, does anyone know where it should actually be bolted to the frame or body or where ever?

 

[92exp4x4]

Ok, from what i could find your circuts are working as they should. Please read carefully. I tried to be as straight-foward as i could.

Pin 20 is the PCM case ground. Probably for magnetic and radio interference. This is one wire not spliced into the ground circut. This wire along with two pink/orange wires bolt to the body at the top of and behind the PCM bracket, its tough to see.

Pin 1 is keep alive power and will have batt voltage to the PCM at all times. This circut also puts power to the PCM relay contacts.

Closing the relay by turning the key to on or start, puts voltage to pins 37 and 57. These are listed as vehicle power in the book. This circut also powers the various engine sensors, injectors, relays and solenoids in the auto tranny.

Pins 40 and 60 are power grounds. Key on should show voltage at these pins. These circuts are spliced in the harness and are one in the same. This circut is the negative side of the PCM relay coil as well as the PCM and these ground straight to the battery terminal. Seeing voltage at these pins with key on; on the connecter side of the PCM harness, means that the PCM relay is good and the harness is too.

Pin 22 is the ground side of the fuel pump relay coil. It is a light blue/orange wire that splices at the diagnostic connector then runs to the PCM. Check continuity from the diag connector and pin 22 at the PCM, if good then your PCM is most likely bad. You still could have a volt drop in the wire, so inspect it for cuts or damage to rule it out completly. The PCM controls the ground to the fuel pump relay to run the fuel pump. It looks like continuity from pin 1 to pin 22, with key on, will give you the same result.

I hope this helps, let us know what you find.

 

[jeffpcell]

one question tho, if the line between pin 22 and the fuel pump test lead at the test connector is bad, would the computer be likely to shut down and not run the check engine light in the on position?