Bad running after manual swap

[JPVortex]

I didn't say it was a rotor problem I was explaining a symptom, not a problem

Oh gotcha, I just went back and reread that; my bad I didn’t read that right.

 

[JPVortex]

Really though Im at the point of just relocating the coil to where its happy lol. Im going to try adding that ground and wiggling the wires some more, aswell as hard wiring it to the battery for testing, but past that if its happy with the coil on the firewall, ill just let it be happy there.

 

[JPVortex]

I didn't say it was a rotor problem I was explaining a symptom, not a problem

Any chance the tach signal wire could create any problems? On the gray connector, white which goes up to the cluster. Pretty sure it's just an output wire that goes to the cluster, but just making sure.

 

[1998_K1500_Sub]

Im going to try adding that ground and wiggling the wires some more, aswell as hard wiring it to the battery for testing, but past that if its happy with the coil on the firewall, ill just let it be happy there.

I'm reminded of this:

Patient: "Doctor, it hurts when I do this"

Doctor: "Then don't do that"

IMHO that's just hiding a problem and hoping it doesn't come back to bite you in the @ss.

Any chance the tach signal wire could create any problems? On the gray connector, white which goes up to the cluster. Pretty sure it's just an output wire that goes to the cluster, but just making sure.

Well, in normal operation it's an "output" to the tach, and it's "driven" by the ignition control module... but if something was amiss on the tach wire it could cause a problem. If prior owner did some funky stuff and that wire was left bare and somehow exposed to a possible ground point (firewall, etc.), a momentary ground of that wire would create a momentary spark from the coil, and repeated, erratic grounding of that wire would cause repeated, erratic sparks from the coil.

 

[JPVortex]

I'm reminded of this:

Patient: "Doctor, it hurts when I do this"

Doctor: "Then don't do that"

IMHO that's just hiding a problem and hoping it doesn't come back to bite you in the @ss.



Well, in normal operation it's an "output" to the tach, and it's "driven" by the ignition control module... but if something was amiss on the tach wire it could cause a problem. If prior owner did some funky stuff and that wire was left bare and somehow exposed to a possible ground point (firewall, etc.), a momentary ground of that wire would create a momentary spark from the coil, and repeated, erratic grounding of that wire would cause repeated, erratic sparks from the coil.

Side note from the post first, I wish I knew how to quote your different parts of the response, would be a lot more organized lol, but,


Yeah I agree with that analogy for sure. It may just be the only option I’m left with. It’s still at the shop, but I’m getting ready to pull it out of there, they have had no communication with me and I have no clue what they’ve done or are doing with my truck.

That’s good to know for testing with the tach wire. If direct wiring the coil doesn’t do anything, then I’ll temporarily cut the tach output wire to take that out of the equation.

 

[JPVortex]

Well another update here. Shop wasn't calling me so I drove down there and talked to them face to face, reason for no call is there is basically no update. Problem still persists. They've replaced 2 ground straps and added a missing one, truck still has the coil problem. They've wiggled all the wires and get it to act eratically by doing that, seems to only happen when the coil is next to the intake or dizzy.

They do suspect an electrical issue, and they told me theyre going to be hooking it up to a computer to hopefully get a better clue.

Not the update that I wanted to hear, but its something none-the-less.

Just worried they won't figure it out and ill be screwed.

 

[JPVortex]

@1998_K1500_Sub do you think there’s any chance a bad PCM could be causing my problems? I’m just starting to second guess the wiring issue; because when me and the shop wiggled the wires it did nothing at all to make the problem worse, which I would expect it to.

I don’t know how a bad PCM would explain it running bad with the coil in a certain spot though. It’s just all so weird.

 

[1998_K1500_Sub]

do you think there’s any chance a bad PCM could be causing my problems?

Well, you could always try another ECU. The last time I checked (well, it’s been a while) they were cheap and plentiful. I have one in the basement in fact for the 1995s.

They’re pretty easy to swap, it’s right there in the glove box.

I’m just starting to second guess the wiring issue; because when me and the shop wiggled the wires it did nothing at all to make the problem worse, which I would expect it to.

But… everything we’ve seen (referring to your video that you posted earlier, with the exception of what you just told us about the experience at the shop, which I highlighted above) points to an electrical problem that’s related to the wiring / connectors between the coil and ignition control module / tach. I would focus there.

I don’t know how a bad PCM would explain it running bad with the coil in a certain spot though. It’s just all so weird.

Agreed.

I have a thought.

On the '94 models, the timing is set by disconnecting the wire from the ECU to the ignition control module and then twisting the dizzy to get 0deg with a timing light.

AFAIK, ECU uses that wire to send a timing signal to the ignition control module, in normal operation. When it's disconnected, the ignition control module is on its own.

At low RPM, with the wire disconnected, the module operates at "0" advance and, if the RPM is increased above a certain threshold, the module advances the timing by 20deg.

The take-away from this is that, AFAIK, you can remove the ECU as a factor by disconnecting that wire; all spark generation is determined solely by other factors, e.g., reluctor wheel, ignition control module, ignition coil, and associated wiring.

If you can demonstrate the tach / ignition problem like you did in your earlier video with the ECU wire disconnected, I think you can likely disregard the ECU as a factor.

I hope somebody else reads this and keeps me honest

 

[JPVortex]

Well, you could always try another ECU. The last time I checked (well, it’s been a while) they were cheap and plentiful. I have one in the basement in fact for the 1995s.

They’re pretty easy to swap, it’s right there in the glove box.



But… everything we’ve seen (referring to your video that you posted earlier, with the exception of what you just told us about the experience at the shop, which I highlighted above) points to an electrical problem that’s related to the wiring / connectors between the coil and ignition control module / tach. I would focus there.



Agreed.

I have a thought.

On the '94 models, the timing is set by disconnecting the wire from the ECU to the ignition control module and then twisting the dizzy to get 0deg with a timing light.

AFAIK, ECU uses that wire to send a timing signal to the ignition control module, in normal operation. When it's disconnected, the ignition control module is on its own.

At low RPM, with the wire disconnected, the module operates at "0" advance and, if the RPM is increased above a certain threshold, the module advances the timing by 20deg.

The take-away from this is that, AFAIK, you can remove the ECU as a factor by disconnecting that wire; all spark generation is determined solely by other factors, e.g., reluctor wheel, ignition control module, ignition coil, and associated wiring.

If you can demonstrate the tach / ignition problem like you did in your earlier video with the ECU wire disconnected, I think you can likely disregard the ECU as a factor.

I hope somebody else reads this and keeps me honest

Yeah the timing connector under the dash does go to the ECU.

Can confirm it still does it with that unplugged, was still happening when I reset the initial timing after I replaced the dizzy.

However, on the ICM the 4 wire connector does all go back to the PCM, which is what made me start questioning the PCM, maybe it’s starting to get intermittent loss of signal through a part of the board or something? Just a thought. They are cheap at the junkyard like $30-40. Can rule out those wires aswell because the issue occurs when I move the coil and not when I move the 4 wires on the ICM(which doesn’t touch coil at all)

Local junkyard near me does have a couple of these trucks with the correct ECU(at least they did 2 weeks ago when I went looking for a manual PROM).

Still do think it’s the wires though, I think it’s either going to be the 12v ign switched wire or the tach output wire(a short of some kind that’s grounding out somewhere).

Can pretty much rule out the 2 wire connector that goes from the back of the ICM to the coil, as I got a junkyard connector and plugged it in both sides and it’s still doing the issue(was doing it before I replaced aswell).

Only other thing that was suspected was a ground issue, but the shop covered that. It was missing a couple grounds and one was loose, but they added the missing ones and redid the loose one, no change.


This is absolutely the strangest problem I’ve ever had on a vehicle I’ve worked on, and it’s even stumping the mechanic.

 

[1998_K1500_Sub]

... on the ICM the 4 wire connector does all go back to the PCM, which is what made me start questioning the PCM, maybe it’s starting to get intermittent loss of signal through a part of the board or something?

Those are AFAIK signal outputs from the ICM to the ECU, so I doubt the ECU could affect the ignition through those connections... doubt, provided the "timing" wire was also disconnected.

Can pretty much rule out the 2 wire connector that goes from the back of the ICM to the coil, as I got a junkyard connector and plugged it in both sides and it’s still doing the issue(was doing it before I replaced aswell).

Remind me: Have you absolved the ICM? And you're confident the ICM's ground through the dizzy, hold-down clamp and engine block is soild?

Regardless, you really need a guy with an oscilloscope to pay you a visit... someone who knows how to use it and also knows what he's doing. This would go oh-so-much-faster.