Transmission Clunks after accelerating from a stop (4T65E)

[engmate02]

I’ve recently been noticing a weird issue in my 2007 Lucerne where my transmission seems to kick or clunk into gear when accelerating from a stop. I have gotten no engine codes at all, I hooked up a computer to it and noticed that it sometimes likes to start in 2nd gear, then immediately shifts down to 1st when I start to accelerate. I believe this to be the source of the issue, however I don’t know how I would go about fixing that.

Interestingly, if I put the gear selector in 1 before I accelerate from a stop, it will never have this problem, which further increases my confidence in my theory. Does anyone know theses transmissions well enough to know what might be causing this and how to fix it?

(Last transmission fluid and filter change was 8k miles ago)

 

[02LTD]

What computer told you it’s starting out in second gear?

 

[engmate02]

What computer told you it’s starting out in second gear?

I have an Autel OBD2 scanner, I read the live data stream for transmission data. “Command Gear” would sometimes be 2 up until I started giving it gas, that’s when it switches to 1. Other than that the transmission shifts normally for every other gear.

 

[2007LucerneCXL]

When the vehicle is cold it shifts properly and then once it's warmed up from a full stop it has bang shift from a start? If so it may be the line pressure which is somewhat common and assume you have the 3.8L with the 4T65E.

As note the Lucerne 4T65E is a GM corporate transmission so any information from other GM models of the same era would apply. Best guessing would be the solenoid or a damaged pressure line, but there haven't been posts with the scan description so it may be something else.

Here's some sources that may help on troubleshooting.

Triple Edge Performance

tripleedgeperformance.com

https://m.facebook.com/story.php/?id=100067548755898&story_fbid=659139284556268

 

[engmate02]

When the vehicle is cold it shifts properly and then once it's warmed up from a full stop it has bang shift from a start? If so it may be the line pressure which is somewhat common and assume you have the 3.8L with the 4T65E.

As note the Lucerne 4T65E is a GM corporate transmission so any information from other GM models of the same era would apply. Best guessing would be the solenoid or a damaged pressure line, but there haven't been posts with the scan description so it may be something else.

Here's some sources that may help on troubleshooting.

Triple Edge Performance

tripleedgeperformance.com

https://m.facebook.com/story.php/?id=100067548755898&story_fbid=659139284556268

Yes, when everything is cold it takes off easier. It will sometimes clunk into gear sometimes it won’t, but when it does it won’t be nearly as harsh as it can get when the transmission is warm. This issue also only happens in drive, I’ve never had it do it in reverse.

I am able to completely remove the clunk by putting the gear selector into “1” before I take off, but I’d much rather fix the issue completely so I don’t have to fool around shifting. Also, thank you for all the information, it was a huge help.

 

[2007LucerneCXL]

A experience trans shop may be able to confirm on the exact problem and you can decide what action to take or go off the two sites and perform additional troubleshooting. On transmissions over a forum sometimes it's not a "one size fits all" solution and hopefully you can get it resolved.

 

[engmate02]

Update: I installed the TransGo shift kit and changed the fluid and filter, little if any metal found in the pan at all. This completely resolved the clunking problem.

Although I do have another issue that I’ve been experiencing for awhile now too. The transmission seems to shudder under heavy acceleration at lower speeds (10-30mph). But if I don’t give it more than 75% throttle it seems to never happen. Maybe a pressure sensor or torque converter clutch plate issue?

 

[engmate02]

UPDATE 2: The transmission has failed, I was at a stop sign and went to accelerate; the car clunked and wouldn’t accelerate anymore (only revved, like I was in neutral) I was able to get it to move by putting it in low range 1. Slips a lot in 1st gear and I don’t have 3rd or 4th anymore. 215,213 miles on the original. I had an 88k mile engine put in it recently so I had to towed to the shop and I’m going to see what the shop wants for a replacement transmission.