96 GMC K2500 Suburban Restoration (with a couple of mods)

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South VA

K2500 454 Long Roof
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We just got back home from the camping trip, and I am truly stumped.

On the way back, I started driving with the gear selector in 3, as I was on a low-speed road and didn’t want to put it in D.

It wouldn’t shift out of second.

I drove a few miles farther and found a place to safely pull off, shut it off for a moment, then started it back up. I put it D, got back on the road, and it shifted normally. Splendid! I thought ah ha! - an electrical problem!

Ten or so miles later, while in OD I depressed the accelerator to make it downshift, as I was only going 55 mph and going up a hill.

It seems to downshift to second, then third. At a stop sign I quickly shut it off and started it, put it in D, and it stayed in 3rd gear for the last 20 miles of the trip. Fortunately we were on a two-lane road with a 55 mph speed limit. I kept it at ~2500 rpm, which is about 55, and got home.

I start wondering if I’ve somehow toasted the transmission.

FWIW, the charging system seems to be working fine. The gauge stayed right around 14v the whole time, with and without headlights and wipers on. And when it shifted, it did so smoothly.

My GF (who is not very mechanically inclined, but observant) noted that this failure to upshift only happens when we are towing the camper. The hard shifting, on the other hand, occurred both with and without a trailer. Thinking back, she is correct, and now I have to conclude that the trailer may be a significant variable.

The first thing I’m going to do is go for a walk, even if I have to put a raincoat on. Because I think better during and after a walk. When I’ve come down a bit from the trip and unhooked the camper, I‘ll start doing what I can do. First thing is to check the negative battery terminal to see if it has somehow managed to loosen up.

Then I’ll go for a drive sans camper and see how the Subdivision plays. And I’ll do so with the voltage gauge installed that @Road Trip suggested (it arrived during our absence).

Depending upon the results of the above, I’ll call the transmission guy, who has been recommended by three different knowledgable sources, tomorrow morning. Fine. I have a plan, of sorts.

My GF again floated the idea that maybe we need a different tow vehicle; that selling the Subdivision just might be the thing to do. I replied that we’re not yet to that point; it’s a vehicle with a finite number of components, only a very small subset of which is likely causing this problem. In other words, it’s solvable.

She actually seemed to take this response rather well, and didn’t press the matter.

However, we both agree that there are no more trips until this issue is solved. Our next trip is scheduled for the last week in September, to the mountains. We had hoped to squeeze one more flatlands trip between now and then. Perhaps that’s not realistic.

Looking over this thread, I’m feeling a bit foolish, to tell the truth. It has to be a bit like watching a tennis match from the sidelines - back and forth, back and forth. Fixed, not fixed; fixed, not fixed.

Whatever. Whining won’t help. I really need to sort this thing out. Sooner than later. And could really use some help doing so.

But first I’m going for a walk.
 

South VA

K2500 454 Long Roof
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So here’s what I did:

Plugged the digital voltmeter into one of the power ports in the dash. Verified that it’s within a few tenths of a volt of the Klein DMM, at least with the engine off.

Voltages seem to vary normally.

* 12.4 v while the key is off.

* 14.4 v once started and running.

* Dropped to 13.8 v on the way back from town (10 miles away), presumably because the battery was becoming fully charged.

Throughout the drive, the Subdivision shifted normally. Not even a hint of a problem.

Next I called the highly-recommended transmission guy, and described the symptoms, again.

Again, he said it sounds like faulty shift solenoids.

However, he said that being intermittent sounds like an electrical issue, and it “could be anything.” He doesn’t know because he doesn’t get into electrical diagnosis. He just rebuilds transmissions and services them.

He said he could replace the shift solenoids, but would make no guarantee that the problem would be fixed. Great.

So now I’m back to wondering if I should bother replacing them. That is, if the electrical issue - whatever is causing the shift solenoids to behave the way they do - will do the same thing to the new solenoids. I don’t know.

I’m also wondering why it fails to upshift only while it’s towing the camper, which is less than 6,000 lbs GVW. Odd.

I’m not sure where to go from here. I’m hoping that someone in this forum is a 4L80 E nerd and can point me in the right direction.

Edit: No stored codes, either.
 
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OutlawDrifter

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I had some hard shifting issues that were tied to the TPS sensor on the throttle body, I would check that connection and sensor.

Also crawl under and look at the plug and wires for the trans whre it connects, could have some more rodent issues there as well.
 

Stumpy

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Probably not much help, but I recently had a 4L80E rebuilt, by a very experienced guy. He said he always replaces shift solenoids when doing a rebuild, because they are so unpredictable. He changed them on mine. He also replaced the internal wiring harness. Either of those two things could be your issue, and would be hard to nail down, just going by the conversation I had.
 

OutlawDrifter

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Probably not much help, but I recently had a 4L80E rebuilt, by a very experienced guy. He said he always replaces shift solenoids when doing a rebuild, because they are so unpredictable. He changed them on mine. He also replaced the internal wiring harness. Either of those two things could be your issue, and would be hard to nail down, just going by the conversation I had.

Good point Stumpy, the internal harness can be replaced just by dropping the pan South VA.
 

South VA

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I had some hard shifting issues that were tied to the TPS sensor on the throttle body, I would check that connection and sensor.

Also crawl under and look at the plug and wires for the trans whre it connects, could have some more rodent issues there as well.
I’ll look up the TPS testing procedure and a give it a try.

I have spent quite a bit of time underneath looking at wiring and other components, but am thinking it’s worth checking the transmission wiring again. Especially for rodent damage.

Thanks!
 

South VA

K2500 454 Long Roof
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Probably not much help, but I recently had a 4L80E rebuilt, by a very experienced guy. He said he always replaces shift solenoids when doing a rebuild, because they are so unpredictable. He changed them on mine. He also replaced the internal wiring harness. Either of those two things could be your issue, and would be hard to nail down, just going by the conversation I had.
So you’re saying it might be worth doing the solenoids and the internal wiring harness. Sounds reasonable.

Thanks!
 

Orpedcrow

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He doesn’t know because he doesn’t get into electrical diagnosis. He just rebuilds transmissions and services them.
That’s aggravating. This isn’t 1973 anymore. The 4Lxxe is a modern transmission and one of the most common. If you’re going to be a transmission guy you should stay up to date.
 

South VA

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That’s aggravating. This isn’t 1973 anymore. The 4Lxxe is a modern transmission and one of the most common. If you’re going to be a transmission guy you should stay up to date.
I agree. And right or wrong, I decided to take it in to the Chevy/GMC dealer on Monday for a diagnosis. They ought to have what’s needed to figure this out. At least I hope so.
 
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BNielsen

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I've had similar symptoms for some time, no OD; the truck will fall flat on it's face at speeds higher than 50 MPH/over 2200 RPMs, there's no codes with a simple code reader. But when I plug in my HPTuners it shows a Transmission Performance Circuit 1 code and looking at the live data my TQ converter would go nuts causing the engine to advance and decrease timing.

I've pretty much narrow it down to the transmission harness being bad. I would go the route of simply replacing the harness but I'm going to be biting the bullet and having my transmission rebuilt and beefed up ahead of the mildly built 454 that I'm collecting parts for.

There's a multitude of sellers on Ebay and Amazon that sell "tune-up" kits with AC Delco parts that include the harness, new solenoids, and a neutral safety switch I believe. Might be a good place to start?
 
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