2002 Suburban 8.1 2500 Trans question

[jlrmfc21]

Hi all,

I have a 2002 Suburban 2500 with the 8.1 and 4.10 gears. I've had it 8 years now and it has 130k miles on it. When towing my 8k lb Whitehawk 29bh, my transmission will go through the gears smoothly, but then when it shifts into 4th gear (overdrive), it shifts really hard, almost jolting the car. It doesn't always do this, but 7/10 times it will and I can feel it pretty good. It doesn't do it at all when I'm not towing. I tow in tow/haul mode and usually run around 65 mph. Sometimes when it does this, it will then set off the "service engine" light. I've towed over 5k miles this year and have never had an issue.

Can anyone tell me if this is normal? What could the issue be?

Thanks
Justin

[ALJO]

It isn't normal and should be serviced. Talk to a transmission specialist.

[oldmanAZ]

It isn't normal.

Possible issues...
1 Have you checked the transmission fluid level?
2 Have you ever had the transmission fluid changed?
3 If 'overdrive' is actually 'torque converter lockup', it could be a solenoid, a control module issue, or the lockup clutch.

[PlayersZ28]

It should be a 4L80 (MT1 on your RPO sticker) which is a 4spd with overdrive 4th. The lockup is just that, not a gear.

Get it scanned for trans codes. Think there's a code that shows for TC issues. Good trans shop should be able to help.

[DOUBLETROUBLE]

Just for the sake of trying don't put it in overdrive. Try towing in 3. I don't know how the tow/haul mode is supposed to work, my '99 burban don't have it so when I tow anything heavy I put the selector in 3rd gear. (mine is also a k1500 so I have the 4L60 trans). Does you truck show the trans temps on the display? If so, watch those for a correlation between high temps and odd behavior.