Downshifting problem?

BTSIMS

MEMBER
2013 GMC SIERRA
4.3L
V6
2WD
AUTOMATIC
100,000 MILES

When I have to slow to turn onto another street, or for a stoplight that turns green before I fully stop, or in stop-and-go traffic, I get a "clunk" from the rear as I begin to re-accelerate. This has been happening for several years and doesn't seem to be getting worse, but it's bothersome.

It does not happen when the tow-assist button on the gearshift is engaged. That leads me to believe it's a problem with downshifting, and something is causing the transmission to be "stuck" between gears until I press the accelerator. The U-joints appear to be okay, best I can tell.

Any ideas why this happens, and only when the tow-assist isn't engaged?

3 Replies

JACOBANDNICKOLAS

MECHANIC
109,747 POSTS

Hi,

If you are hearing the clunk from the rear, my first suspect is a universal joint or there could be excessive play in the rear differential. At 100K miles, I wouldn't expect it to be the differential, but I do recommend checking the fluid level and condition. If it hasn't been serviced, you may want to consider doing it.

Take a look at this link that explains how to test a universal joint. Again, this is my first suspect, and I feel it is worth the check.

https://www.2carpros.com/articles/how-u-joints-work

Let me know. Also, since you don't hear it when in the tow/haul setting, it could simply be because the transmission is designed to have different shift points when in this mode.

Let me know what you find,

Joe

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Wednesday, February 28th, 2024 AT 8:33 PM

BTSIMS

MEMBER
2 POSTS

I had a mechanic look it over, and he can't figure out the issue. It's definitely not a U-joint, since the problem has been around for literally years, and doesn't happen with my tow assist engaged. I thought after some more research that it might have been a shift solenoid problem, since it seems like the transmission doesn't fully shift to the lowest gear until I begin to re-accelerate after slowing down. He suggested a rebuild or replacement, but I can't justify a $4,000 transmission on a $9,000 truck. The problem hasn't gotten worse over the years, and I doubt the transmission will go out on me. It's just that the "clunk" is annoying, and I think it will keep potential buyers away if I decide to sell the truck.

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Thursday, April 11th, 2024 AT 12:26 PM

JACOBANDNICKOLAS

MECHANIC
109,747 POSTS

Hi,

I agree that will bother a potential buyer. You mentioned the sound comes from the rear. If it hasn't been checked yet, I recommend checking the fluid in the differential to see if there is evidence of excessive debris or metal shavings. The problem could be coming from the differential itself.

here is a link that explains how to check the fluid. Take a look through it and let me know if you have questions.

https://www.2carpros.com/articles/differential-gear-oil-fluid-check

Take care,

joe

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Thursday, April 11th, 2024 AT 7:36 PM