Downshifting problem?

2013 GMC SIERRA
100,000 MILES • 4.3L • V6 • 2WD • AUTOMATIC
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BTSIMS
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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?
Feb 28, 2024 at 3:33 PM
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JACOBANDNICKOLAS
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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

Feb 28, 2024 at 8:33 PM
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BTSIMS
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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.
Apr 11, 2024 at 12:26 PM
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JACOBANDNICKOLAS
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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
Apr 11, 2024 at 7:36 PM