Reverse Clunk - 8L45

[buster]

Since I've had the truck, it has always shifted very rough when going from R to D, especially when sitting overnight. It's a pretty noticeable clunk. Once the truck has been driven a while, it happens less often. There are times it won't happen at all once warmed up. However, it always occurs on the first shift of the day. I usually back into the driveway to avoid it.

I have read on a few other forums and groups this is typical with many of the GM 8 speed transmissions (8l45). It's been hard to find any specific causes though as most chalk it up to being a typical GM product LOL. This is not related to the shudder in any way. Completely different issue, if you want to call it that. I thought of taking it to the dealer just so they have it on record. I can't see it being good for longevity of the transmission.

I had the pan/filter changed at 25k miles. No other issues. Shifting is very smooth.

I believe I did read one comment where an individual suggested it has something to do with the fluid not draining back quick enough.

 

[ripcord]

Have you checked the U-joints?

 

[buster]

Have you checked the U-joints?

I have not.

 

[ripcord]

It might also be excessive play in the pinion gear.

 

[dbias]

Slip yoke

 

[buster]

I forgot to mention this. If I start it up and quickly put it into drive from reverse before the rpms drop (first start of the day), no clunk. Which is odd because I thought that would have been worse.

 

[ctechbob]

That's pretty much just the way some of them are.

"C. Poor Drive Quality - Harsh Shifts

62. An additional problem with the GM 8L transmission is poor drivequality. This is the umbrella term GM uses to describe transmission shift issues like harsh shifts, first shift of the day, garage shift, rough coast downs, surges,lurches, and jerks. PX137 (GM000494541). These have been described as “feelinglike being rear-ended,” PX573 (GM000053785), or a Donkey kick. Radecki Dep.at 265:4-267:1.

63. Unable to come up with a fix, GM often tells customers that theslipping, abnormal upshifts or downshifts are just “normal operation.” See, e.g.GM000046284. However, GM knows that harsh shifts dating back to 2015 have“improved but still not acceptable to the customer.” PX219 at 4. An “Overall......"

'Garage Shift' is going into reverse to back out of the garage and then going to first. Likely the only way to fix it is programming or hard parts.

If you're not out of warranty, or have an extended warranty, some of these TSB's might help:

79. GM early Technical Service Bulletins addresses garage shifts, delayed engagement and other shift quality issues. See, e.g., GM00002297 (firm garage shifts); GM000025412 (harsh shifts, slips, or flares); GM000039463 (delay edengagement); GM000002332 (Harsh 1-2, Harsh 3-1 Decel Downshift, Harsh Downshifts); GM000134618 (Information on Transmission Harsh 1-2 Shift Upon First Start Up/Shift of the Day); GM000002329 (Harsh Shift, Delayed Shift,Unwanted Downshift, Transmission Stuck in One Gear).

 

[GMBoy]

Did this happen after the 25k fluid service? If so did you use the proper Mobil 1 LV fluid?

 

[buster]

Did this happen after the 25k fluid service? If so did you use the proper Mobil 1 LV fluid?

No it's done it since day one.

 

[buster]

I forgot to mention this. If I start it up and quickly put it into drive from reverse before the rpms drop (first start of the day), no clunk. Which is odd because I thought that would have been worse.

Sorry from Park to Reverse, NOT R to D.

 

[clinebarger]

Try This

 

[atikovi]

No it's done it since day one.

And you haven't taken it back to the dealer within the first couple thousand miles? Lemon laws vary but usually 3 or 4 unsuccessful repair attempts qualify for a buyback if done in the first 12 months.

 

[clinebarger]

Initial fill volume issues are common on a lot of "Clutch to Clutch" automatics & considered normal.

 

[buster]

And you haven't taken it back to the dealer within the first couple thousand miles? Lemon laws vary but usually 3 or 4 unsuccessful repair attempts qualify for a buyback if done in the first 12 months.

No. I had them look at it once. Guy didn't think much of it. I can't remember exactly what he said. It's not severe and once warm, it usually won't happen again. It's only going from Park to Reverse (not R to D as I said in first post). It just jolts a bit more than it should.

 

[LDM]

Sounds similar to the hard 1-2 shift when cold, which I had on my 2018 Sierra. Had them perform the TSB for the transmission fluid shortly after I bought it and haven't had the issue since. I'd take it in and have them perform the TSB, it exchanges all the old fluid that would absorb moisture for the new fluid that won't. If that doesn't fix it then I'd start looking at other options.