So my 91 F250 with a 351w lost spark the other day while I was driving. Ran through the normal checks everything seemed to point to the pip or icm so I just ordered a brand new distributor and installed it and still no spark. So I checked the old coil for spark and if I isolate it I can get it to generate spark no problem, so the coil is functioning fine. I then disconnected the PCM and I got spark so I assumed that the PCM was bad and pulled it out only to find that it was from a 94 not a 91. Went to the local pick n pull and found a wrecked 94 with a 351w and the same pcm in it as mine, swapped it and still no spark. Im starting to think I might have an issue with my wiring harness or shorted injector possibly. Anybody else have any ideas? Also the truck more than likely has a 94 engine and trans in it as it is custom built and supercharged as well so im fairly certain that the 94 PCM is what I needed for a replacement.

 

Sounds like you got a bad PCM from the yard.
Try again.

 

The original pcm showed zero signs of failure or cold solder joints and the new one came from a wrecked truck so I have to assume it is fine too. It was also the only 94 in the yard so no others to try.

 

Also when I first crank the engine I get spark and then as soon as I let off of the key I get one last spark.

 

Im getting ready to start working on this issue again finally and just want to verify that the pcm is the only thing that could be causing this issue and not a shorted wire or injector? I dont want to spend $200+ on a pcm only to find the issue to be something else. If there is other stuff to be checked prior to ordering the pcm then I want to check .

 

Unplug the PCM Computer to take it out of the game. Then see if it has spark all the time it cranks. If it has spark now it would point to the PCM computer. If it loses spark during cranking it is something else.

If it still does not have spark with the PCM unplugged check that the PCM Power Relay is making (click) when the key is turned on/start positions and dose not drop back out during cranking. If it does then you have a bad Ignition switch or Power Relay or the wiring under it.

 

Its been awhile since ive had time to mess with this truck but I will be towing it to my house tomorrow so I can get to work on it. I know that if I unplug the PCM I get spark from the coil consistantly. I have tried a junkyward PCM and it did not cure my problem. I just want to make sure there is nothing more I can check that could create this issue before spending $250 on ordering a PCM. Could a wire be shorted somewhere? Are there any other possibilities? thanks for any response.

 

Next thing would be what PCM can I order for my truck? The truck is a 1991 but the PCM that was in it was from a 1994. Has a 351 and auto trans

 

I found a guy locally with a 90 f150 with a 351 and auto trans for sale for cheap that I could buy and use for troubleshooting if a 90 will interchange with mine.

 

A 1990 and 1991 are very close to the same if not the same as long as you have the same engine and transmission.

Make sure you have power going to pins 1, 37 and 57 with the key on. Also make sure you have a good ground on pins #40 & 60 of the Computer.

Make sure that the small black wire with a green stripe has a good ground at the NEG post of the battery. Note this wire may be on a stud near the battery with another black wire running to the NEG post. This is the computers ground wire going to pins #40 & #60.

Also make sure you have a good body ground from the NEG post also, we are not talking about a frame ground.

 

The problem is that you have no way of knowing if the PCM in the jy truck was any good. It could have been a tired old truck that was junked because it would no longer run because the PCM had failed. Not saying that's the case, just saying that swapping a jy PCM is no way of verifying that it isn't the problem unless you can put the jy PCM into a known good truck and it works fine.

 

thanks subford I will look into those wires tomorrow. As for the truck at the junkyard it was wrecked, tboned to be exact, and that is why it looked like it was there as it was no longer driveable.

 

Good point. If it as junked 'cause it was wrecked, it was probably driving at the time. Therefore it is a valid assumption that the PCM is good.

 

Just checked and no 12v on any of those pins. Grounds are good though. So now what to check?

 

Deleted original E4OD question.

Check your fusible links. There are a bunch of fusible links that connect to one side of the starter solenoid. You'll be looking for a 20 gauge fusible link, which if I remember correctly is light blue in color. It will be connected to a yellow wire on the other end of it. (A fusible link looks just like a length of wire that has been butt spliced to the rest of the harness with a ring terminal on the other end if it where it connects to the starter solenoid.) It sounds, based upon you description, that the fusible link has blown. Look for discolored, burned, puffy looking, or soft and squishy feeling fusible link. If you find on, replace it. Of course, if you find one that begs the question, why did it blow in the first place?