I was driving my 98 F-150 the other day and it just quit running out of the blue.The engine turns over fine but will not fire.I hooked up the Genisys OBD scan tool that I have access too and it repeatedly tells me that the tool cannot communicate with the computer.I am certain that the tool is properly set up and connected with the correct chip installed.I have checked all fuses that seem to be connected to the ignition system and they are fine.I have also checked out the relays that seemingly could be related.I pulled the connector at the PCM and checked the pins that established power and grounds to the related systemsThe grounds all checked out fine,but I had no power to pin #71.By the way,the truck has the 4.2 engine.It seems as though no power is getting to the PCM so the PCM obviously sends no signal to the engine ignition system.There is power to the coil packs and their grounds check out fine.Which relay is the PCM relay and where is it located?My diagram does not identify a PCM relay.Also,I had been getting an O2 sensor code for a few miles.I just had not had time to change it.Could this be related?My truck will not run and I am at my wits end.It is my only source of transportation.Any ideas ,help or suggestions would be much appreciated.

P.S. Yes,it has and is getting fuel.

 

Hi and welcome to the forum
Sorry it's taken so long for a reply. Any luck figuring it out yet?
Power to pin 71 on the PCM comes from the PCM power relay which is in the underhood fuse box. It's also called the EEC power relay. It should be at the top of the box, opposite end of the 2 diodes. You can probably just swap it with another one of the relays to see if it works. The next relay in line is the horn relay. It would be a good one to try. Fuse 24 under the hood and 30 under dash both send power to the PCM relay. Check them both with a test light or voltmeter.
Pins 71 and 97 on the PCM both get power from the PCM relay. If you have power at one and not the other then you probably have a wiring problem.

 

Thanks for the info. No luck yet, but I have traced the problem back to fuse 24. This fuse was blown and continues to blow as soon as I turn the ignition. I had already tried switching the horn relay with the PCM relay, and the truck still would not start. Also, the relay that I took from the power control module position was placed where the horn relay used to be. I put the horn relay where the PCM used to be. Horn works fine, truck still will not start, and fuse 24 continues to blow. I also disconnected the wiring harness from the power control module and switched the ignition on. This also caused fuse 24 to blow. Plugged the PCM back in, replaced the fuse, turned the ignition again and fuse still blew. Would the relay cause the fuse to blow? If so, wonder why when I switched the relays it still blew? I'm thinkin wiring problem as well? Any more ideas would be greatly appreciated....

 

This is probably going to be pretty tough to figure out.
Fuse 24 sends power to the PCM relay and from there the power goes to numerous places. I think you've got a wire shorted to ground after the relay. The red wire from the relay goes to MAF sensor, Vapor Management Valve, Canister Vent Solenoid, Intake Mnaifold Runner Control, all the Injectors, the EGR Valve solenoid, the IAC, the IAT, ECT, O2's, etc etc.
I would probably start looking for a rubbed through wire where the harness goes down the back of the passenger's side of the engine, over the transmission to the O2 sensors and back near the Canister Vent Solenoid which is above and behind the rear diff. Also take a look at the wiring on the outside of the frame, behind the L/F wheel.
Good luck with it

 

Has a solution come about to this problem? I am having the same issue on a 97 and even the local ford mechanics can't figure it out. They have had my truck for little over 2 weeks now with no solution.

 

Hi Chad and welcome to the forum

In 2 weeks they should be able to figure it out
reaves have you made any progress with yours yet?

 

After 13 days at the Ford Dealer, they came up with an internally shorted o2 sensor on the passenger side. They replaced it with a new Bosch part and the truck starts and runs just fine. I also noticed that they replaced some wireloom running to a transmission sensor, I dont know if this had something to do with it or not.

 

Good to hear that it's finally fixed
They may have opened up the wiring to the transmission to look for a problem.