Manager Forum

Has a p0351 history code. I have checked the brown wire back to the pcm. Of course it was good. Checked the green wire back to the transformer. Good. Ran the test where you unplug the coil, jump between the terminals with a test light. Test light flashed. Looked kinda dim, so I went to a good cylinder and it looked the same. Swapped coils with a good cylinder. Have found nothing out of the way. I have taken the coil out and held a plug in it while cranking the engine and get no spark. I can use the same coil and plug with a different coil wire and get a good spark. The lab scope picks up a really weak signal on that cylinder like it may be trying to work. I'm leaning toward a problem in the transistors (or whatever does the switching) in the PCM. Anybody agree with me? Am I overlooking something? I hate to replace this pcm and be wrong but I can't see a problem anywhere else. If it was on the other side, wouldn't there be a problem on 4 cylinders instead of 1? The diagram shows 4 coils (don't remember which ones) spliced together back to the fuse box and the transformer.

I do alot of coils on these vehicles and I mean alot and they are almost impossible to tell which one is bad. I would start with a coil or six.

351 usually means a driver or a wiring issue. If you can eliminate the wiring and you've swap-tested coils, I think you may be on the road to PCM-ville.

Dang i must be asleep I swore it said P0 301

The 351 wont come back, but it's got to be the pcm. I've been thru everythin else. I guess it just wont switch under load. I let yall know how it turns out. It belongs to a car lot, so I don't know if theyll go for it or not.

One thing that I would try before replacing the ecm is to disconnect all of the rest of the coils and see if the signal to this one improves. If it does, reconnect the other coils one at a time until your signal weakens. That would help isolate another coil that is robbing power from #1.
A shot in the dark.......

a&mauto wrote:One thing that I would try before replacing the ecm is to disconnect all of the rest of the coils and see if the signal to this one improves. If it does, reconnect the other coils one at a time until your signal weakens. That would help isolate another coil that is robbing power from #1.
A shot in the dark.......

That's a very good idea. Depending on how the drivers are wired internally, number 1 could be on the end of a feed circuit. Good thinking!!

You ain't gonna believe this, but I repaired the pcm myself. I took it apart and found the transistor for number 1. Of course the resistance was way different than the others so that confirmed the problem. I had an old PCM from a Ranger on the shelf. Took it apart and lo and behold, it had the same transistors. It was a royal PITA to get one out without destroying it. It was equally tough to get the bad one out of the Lincoln PCM without destroying it. Anyway, she's sittin over there purring like a kitten right now.

Holy Cow!! Great job man. I've repaired a few PCM's but that's really great. Thanks for the follow-up.

That IS cool. I'll put this in a future Forum Gazette issue!

It really surprised me that the pcm was causing this. I have suspected PCM's a few times in the past (when I was still a greenhorn) and jumped the gun and replaced them without doing all the diagnostics first. I have seen very few that actually had a problem. I mean, a pcm causing a dead miss? Sounds like an auto zone diagnosis.

PCM causing a miss? Ya bet. I seen that often over the last number of years. Not the place where I would start though but until proven innocent ha to be suspect otherwise. Hey. It's just a man made product.

Agreed! Thanks for the fix it post. Appreciate the input.

like all true tech's i love hearing about stories like this. following something all the way to the end and i know i personally would feel proud as heck that i did something like that. but as a shop owner i have to ask why? it was worked back to the p.c.m. diagnostic done, replace the unit. i know it sounds whacked. years ago that is what a tech did. repair the unit. in today's shops it's diagnosed and replaced.

Well, it was a kind of hurry up and wait job. It took a few days to get the approval to get a new PCM. Then I started doubting myself even though I had ruled out everything else. Thats when I decided to tear into it because I was pretty sure I could locate the problem inside. I had every intention of getting a new one after I verified the problem, but then I decided to try and fix it after I got a call asking if it would be ready that day. Then I thought about getting another job out the door and getting a check that day so I went for it. I saved the customer some money and time and made myself some money, time, experience, and boosted my reputation. Another shop owner came in while I was fixing it and said, "I don't see why you are doing that. I would have just ordered them a new one. It's not worth it." I don't understand that. I charged $200 for fixing the pcm. A reman was $375. I got two hours diagnostics and another .9 for r&r. You could tell that the car had been worked on before it cam here. It had some new coils (one of which was bad). I catch alot of criticism in my town for taking jobs that other shops have turned away or tried and gave up on because they couldn't fix. I usually make good money on those jobs and learn some good lessons. It also gets my name out there because the customer will go tell his friends. Sometimes I get trapped and lose money on a job but it's rare. I hate brakes and oil leaks.

Great job and story! Reminds me of when I fixed vehicles instead of people! If you love the job you will never work for your living.