8 speed suddenly went out. No forward or reverse gears

[kgkern01]

Of course this happened exactly 1 year after the warranty expired.  I accelerated and then it just lost forward gear like I had put it in neutral.  No forward, reverse, and doesn’t even feel like I’m switching it into Drive or Reverse when it’s running.  Before the warranty went out I had taken it in regarding the delayed cold engagement issue, but the dealership said they could reproduce it of course .  So, any suggestions or ideas what could have happened to just lose all forward and reverse gears in this 8-speed?

[YYC-SIERRA]

Sorry for possibly offending you, but have you checked the trans fluid level? 

 

 

[kgkern01]

No I didn’t, but I had the dealership do a flush last summer.

[flyingfool]

check for any known TSB's 8 speeds have tech  issues in software and i'm sure a pro shop knows whats up.  call Precision Transmission in TX  he might might know where to point you to. he does quality work but i think he's only into older transmission. 806) 372-2210

 

also a tuner might be able to reflash it , and do a recal on it and purge cycle. this might work, GM dealer might be able to do this too

Edited November 26, 2020 by flyingfool

[kgkern01]

I don’t think a tune or reflash is going to make drive/reverse start working again though, it has to be a physical issue to cause it to go out like that.

[YYC-SIERRA]

3 hours ago, kgkern01 said:

No I didn’t, but I had the dealership do a flush last summer.

Fire it up, put in neutral, and check the fluid level, is it normal?

[kgkern01]

14 minutes ago, YYC-SIERRA said:

Fire it up, put in neutral, and check the fluid level, is it normal?

Since there is no dipstick on the 8-speed, and the fill plug is in the oil pan, I just easily check the fluid.  However I’ve had zero leaks in this truck, and underneath it has no oil at all, very clean, I’d assume it isn’t a low fluid level.  The transmission temp when it went out was not hot either, maybe 190?

[mjm-1957]

Even if you unplugged the wiring harness you would still have reverse and one forward gear as a failsafe. That 8 speed transmission is a huge piece of ****** and has been trouble from the start in 2014. I am the transmission guy at a Buick- GMC dealer and can't believe that GM released this trash and can't come up with any fixes for it. I spend most of my time trying to fix the unfixable, harsh downshifts, flares on up shifts, delayed engagement and of course the TCC shudder.Absolute garbage, they should make a retrofit kit to slip a 4L80 in instead.

[YYC-SIERRA]

Oh ok, sorry didn't realize 8-speeds don't have a dipstick. I have a 6-speed.

Cheers

[kgkern01]

4 minutes ago, mjm-1957 said:

Even if you unplugged the wiring harness you would still have reverse and one forward gear as a failsafe. That 8 speed transmission is a huge piece of ****** and has been trouble from the start in 2014. I am the transmission guy at a Buick- GMC dealer and can't believe that GM released this trash and can't come up with any fixes for it. I spend most of my time trying to fix the unfixable, harsh downshifts, flares on up shifts, delayed engagement and of course the TCC shudder.Absolute garbage, they should make a retrofit kit to slip a 4L80 in instead.

If you’re the transmission guy at a GM dealer, any suggestions or ideas as to what could have happened?  I had always gotten a slight whine when I would accelerate hard too, other than the above issue of the delayed cold engagement.  I just want to get an idea of what could have gone out and what I’m looking at for a repair?  Thanks

[mjm-1957]

I would be looking for a loss of hydraulic pressure like a pump failure or a catastrophic mechanical failure.Any noises while running in gear and not moving?

[kgkern01]

12 minutes ago, mjm-1957 said:

I would be looking for a loss of hydraulic pressure like a pump failure or a catastrophic mechanical failure.Any noises while running in gear and not moving?

No noises, when I put it in gear it does nothing, sounds exactly like neutral.

[mjm-1957]

Does the shift quadrant follow the shifter movements and is there any check engine light on?

[mjm-1957]

You of course slid under there and made sure the shift cable and lever are still in one piece and attached I am assuming?

[JimCost2014]

45 minutes ago, kgkern01 said:

Since there is no dipstick on the 8-speed, and the fill plug is in the oil pan, I just easily check the fluid.  However I’ve had zero leaks in this truck, and underneath it has no oil at all, very clean, I’d assume it isn’t a low fluid level.  The transmission temp when it went out was not hot either, maybe 190?

CEL on?