Transmission thunk at start up?

EDL · July 2, 2018

So, 2 weeks ownership of my 2018 (8-speed with 6.2L engine) and almost 700 miles on it, I get in this morning to head to work, put in the key, crank it up and about 2 seconds after start there is this thunk from the transmission and the truck actually jerked a very slight amount. Hmmm. I've been in and out of the truck a few times today and it's done it two more times, but didn't in three other start ups.

Anyone ever experience this?

Mileguru · July 2, 2018

A lot of us have had strange transmission clunks/lunges at odd times with the 8-speeds. For yours, try giving it 30 seconds to idle while in park before you turn it off. I've only had the startup clunk once and I think it was because I was driving it hard and abruptly parked it and turned it off.

newdude · July 2, 2018

Have a brief read below:

Quote

#08-07-30-026E: Information on Slight Vehicle Movement in Park at Start Up After Cold Soak - (Dec 12, 2017)

2018 and Prior GM Passenger Cars and Light Duty Trucks

Equipped with Automatic Transmission

Some vehicles may rock or move slightly forward or rearward while in Park at start up after cold soak. This condition may be accompanied by a clunk noise. This is a slight movement that is more often seen visually, rather than felt, when viewed from the outside and using the auto-start feature, if equipped.

This slight movement is due to the residual transmission fluid in the clutch packs that creates a partial apply on start up. This movement is stopped by the parking pawl and/or transmission fluid exhausting completely from the clutch packs.

This condition is normal and no repairs should be attempted. This can be verified by comparing with another identically built vehicle under the same cold conditions.

Please communicate to the customer this condition is a normal operating characteristic of the vehicle. This slight movement has no adverse affects on the transmission. To prevent this condition from occurring, the parking brake should be utilized as per the Owner's Manual recommendations anytime the vehicle is parked. Please share this information with the customer including a copy of this message.

EDL · July 2, 2018

They're kidding, right? This is normal? Scary.

And, it doesn't just do it after sitting and cooling. I parked this afternoon, went into the office for about 20 mins came out and it did it again (and did it worse than the other times). It's 91 degrees here today, so it definitely wasn't cooled off.

I'll try letting idle for 30 seconds or so next time before I shut it off and see what that does.

Still, if they do this, why do they not do it from the very first time you run the thing, but only show up later?