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2007 Suburban 2500 LS 6.0
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Discussion Starter · #1 ·
We recently needed to buy a reliable 4x4 and found a deal on a seeming great shape, high mileage but well cared for, 2007 Suburban K2500 LS 6.0.
Previous owner seems to have done well, caring for it mechanically. But it recently began a strange problem.
At highway speeds, without need, it decides to downshift from 4th to 3rd, then hold 3rd gear not wanting to upshift. If I bring it to a stop and put it in park and go again, it will shift 1-2-3, but not go to 4th. But if I turn off the ignition, and restart it, it will shift fine, until it decides to downshift again. Sometimes this can be several minutes. Other times in a few miles.
Having read about a similar problem on another 4L80E, here in these forums, being solved by an output sensor replacement, I thought I’d ask if anyone has a similar experience and might lead me in another direction?
 

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2000 Silverado Z71 4x4 5.3L 460k+ miles w/ GM rebuilt motor and trans
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1,424 Posts
Fluid color and level check out?
 

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2007 Suburban 2500 LS 6.0
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7 Posts
Discussion Starter · #3 ·
Yes. The fluid was old, but functional, prior to changing it out. Just changed out fluid and filter, in the past week, and still shifting hard at times, plus this downshifting problem.
 

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You may need to take to a transmission shop and get it scanned. It will be a $100 well spent.
I'm guessing the reason for a downshift from 4-3 is a rise in engine load calculated based on throttle opening, rpm and accelerator pedal position inputs to the TCM commanding the 4L80 e to downshift to 3rd.
 

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‘01 Silverado 2500HD 8.1/Allison 5sp xcab long bed
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3,747 Posts
Howdy and welcome, Sharkman.

What’s your fuel pressure?
 

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2007 Suburban 2500 LS 6.0
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7 Posts
Discussion Starter · #6 ·
In looking up the TCM, I’m trying to locate a diagram of its location, online. I see a TCM reset procedure. Could this reset help? I think that will be my next attempt, to resolve this.
It does seem to be temperature related. When the dash readout of Trans Temp reaches 230°, it will soon after, do the downshift. I just started noticing that, this morning.
I was hoping to leave on a long trip, tomorrow, (Oklahoma to Los Angeles) and after replacing the output shaft speed sensor, discover that the problem still exists. So it will delay my trip, until it can be diagnosed, and all shops are WAY backed up, around here. I’ll call a dealer, to see if it can be scanned, to at least diagnose it. We have no local “transmission shops”.
My scan tool is only getting one of two codes. Sometimes it pops up a report of minor slippage. Other times it says the transmission sensor has been illuminated. It never says both at the same time.
 

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2007 Suburban 2500 LS 6.0
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7 Posts
Discussion Starter · #7 ·
I don’t have a fuel pressure gauge, but it seems to have plenty of pressure. I can accelerate easily. Even when it downshifts. It definitely resets, when I cycle off the ignition, for 10-15 seconds. It’ll shift just fine, for a while, then does the 4-3 downshift, and while the transmission shifts 1-2-3 no issue, it won’t go in 4th, until I turn off the ignition for 10-15 seconds, when it will again shift all gears no problem, until it again downshifts from 4-3
 

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2007 Suburban 2500 LS 6.0
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7 Posts
Discussion Starter · #9 ·
I just returned from my 20 minute drive after doing that. Still doing it. I’m suspecting a TCM issue. Given that there is an output shaft sensor, is there an input shaft sensor?
 

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2007 Suburban 2500 LS 6.0
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7 Posts
Discussion Starter · #10 ·
One more question for y’all. If the transmission shop says the TCM is bad, where is it located? I’ve been trying to look that up, and some say below the driver side battery tray. Others say up under the master cylinder. If I do need to replace it, I’ll need to know how to access it.
 

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Here is how it looks at Rockauto.
Knowing how it looks will make it easier to locate.
Rectangle Font Parallel Screenshot Electric blue
 

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2007 Suburban 2500 LS 6.0
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7 Posts
Discussion Starter · #13 ·
Here is how it looks at Rockauto.
Knowing how it looks will make it easier to locate.
View attachment 170288
Yeah, I got that part of it. But, online, I’m seeing two possible locations for it. Under the driver side battery box, or below the master cylinder. I’m disabled, and limited on what all I can do, so I’m hoping to find the right location, to find it.
 
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