Engine Revs up and down at 35 to 40 mph

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Marky Dissod

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Without being able to look at the OE program for an '04 Denali 6.0L with 4L60E (very strongly doubt it's a 4L80E), my 1st guess:
it's vacillating between TCC Lock and TCC Unlock in 4th.

Instead of putting the shifter in 3rd, use the Tow / Haul mode button after shifting into 4th, after approximately 27MpH or so.
 

Mudsport96

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Without being able to look at the OE program for an '04 Denali 6.0L with 4L60E (very strongly doubt it's a 4L80E), my 1st guess:
it's vacillating between TCC Lock and TCC Unlock in 4th.

Instead of putting the shifter in 3rd, use the Tow / Haul mode button after shifting into 4th, after approximately 27MpH or so.
This is true. I am not sure if I have a Denali program I can look at on hand. But I do have several random ones. If I get time tonight or tomorrow after work I will see what I can find on the shift schedule tables.
 
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Alaxgoaly8

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All of rhe above is relevant. Have you considered that you may be in the 3-4 shift range and the trans doesn't know what gear to be in? 35-40 is the common range for OD.

I had a bus driver growing up who always drove in the shift range of the Allison trans at 40 mph - up/down.....up/down to the point I said something to her about the shift point. Some people are just dumb and not mechanical (not saying that about you).

Have you tried maybe going a little faster and see what happens? What about driving in 3rd for that speed and evaluate?
I appreciate all the responses... Going to try and answer a few of the questions including the one quoted...

1. I can accelerate out of the problem. It doesn't happen when trying to hold 55 or 65 on the highway for example.

2. Other than a sheet of paper about a warranty on a certified gm remanufactured transmission, I have no other paperwork from the previous owner. So quite possible it was never done. But that being said, outside of this issue, it shifts quite smooth. Goes into all gears easy from park (like Park to Reverse or Park to Drive). When shifting on the go, They're all smooth, so I'm inclined to think it was done, but fully realize it might not have been.

3. The throttle body was swapped out, and I was told it made a good difference with misfires (but not this particular issue), but when the maf sensor was replaced, the mechanic swapped the throttle body back to the original and said it was fine... so I'm assuming the throttle body was fine, but it's also a cheap part to replace, so maybe I'll do that as well.

4. Another interesting piece of info that I left out. I noticed someone mentioned the PCM... The mechanic wanted to get better info from the PCM, so he said he took it to a shop that could access it via hb tuners (the tokens for this truck were only $50)... But when they tried to access the pcm, they couldn't get in. They said it was "locked out"... so my mechanic assumes the previous owner may have either tuned it, but I asked the previous owner and he said "no". Probably lying. I purchased an pcm that was programmed to the vin, but the mechanic couldn't get the ignition key to program with the new ecu installed so he put the old one back in.
So I think I need to get the old pcm out or find someone who can get into it or replace it successfully.


It sounds like from a few responses on here, that a few people think the likely culprits could be the throttle position sensor, throttle body, TCC lockup (can that be replaced by dropping the trans pan or does the entire trans have to come out? Same question for the 3/4 clutch pack or solenoid?), fuel injector wiring harness, and/or PCM issues. I feel if it was the trans, I'd have the problem more often than just that specific scenario?

Thank you again for the responses.
 
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Alaxgoaly8

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Without being able to look at the OE program for an '04 Denali 6.0L with 4L60E (very strongly doubt it's a 4L80E), my 1st guess:
it's vacillating between TCC Lock and TCC Unlock in 4th.

Instead of putting the shifter in 3rd, use the Tow / Haul mode button after shifting into 4th, after approximately 27MpH or so
I'll try both. I'll try driving it in 3 and with tow/haul mode on... question with T/H... do I press the button after it shifts into 4th or right at start up? I'm familiar with T/H from my RV on my Ford V10, but never used it on this truck.
 

Mudsport96

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Okay so this is for a 6.0 4l65e vehicle with awd as far as I van tell from the tune. So either an awd Denali or a Silverado SS. Either tune should be close to the other as they are the same basic drivetrain.
I will post pictures, but from what I see in the tune, part throttle 3rd to 4th shift is 35 to 45 mph depending on throttle input ( 25 to 31 percent). Part throttle 4 to 3 downshift is 34 to 40 mph at 43 to 50 percent.
Converter lockup in 3rd is 43 mph at 6 to 25 percent throttle and in 4th 40 to 42 mph at 6 to 18 percent throttle.
Converter unlock in 3rd is 41mph from 0 to 62 percent throttle. And in 4th unlock is 37mph from 0 to 25 percent throttle. So it seems to me by the tune. The speed you are having this issue lays in the no man's land of the speed you are going in 4th. I would stick it in 3rd and see if it still happens.
 

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Mudsport96

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thank you for the information. If I keep it in 3rd and it doesn't do the same thing, then you're thinking it's the TCC lockup issue?
Shouldn't do it. And it really isn't an issue technically, it is just doing what the program is telling it too do.
If you could get a video of what you are experiencing, we could better diagnose this.
 

Marky Dissod

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... it really isn't an issue technically, it is just doing what the program is telling it too do.
If you could get a video of what you are experiencing, we could better diagnose this.
If it turns out it's just doing what the (GM OE) program is telling it to do, the solutions are:
Use Tow / Haul mode to avoid this behavior - assuming this might work
Have pcm properly reprogrammed - this WILL DEFINITELY ABSOLUTELY POSITIVELY work if the gripe is thoroughly understood by tuner
 

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