6l90 Running hot in 4th

Matthew Barens · December 19, 2016

I have a 2010 GMC Sierra 2500HD with 6.0L and 6l90. I have 205k on it with 5k on a reman'd transmission (reman, not rebuilt). The old trans ran great except it was starting to miss 3rd gear on 2-3 shifts. Sometimes up to 500 rpms before it "found" 3rd.

Took it to a reputable shop and got a reman trans with 3yr 100k warranty. No questions about the quality of the labor, they're well recommended in the area.

First time pulling a load last night. Approx 8,000 lbs (3500lb enclosed trailer, 4500 lb car), ambient temp outside was -4F. Going down the flat highway 65mph in 4th gear I checked the trans temp after about 15 mins on the road. 230F and climbing fast. I wasn't on it hard, literally just cruising down the road. I used to keep the vehicle limited to 4th gear with old trans when pulling this combo, keeps RPMs about 3100 for a good cruise. I tried slowing down, temp still climbed. I shifted it to 5th, and temps started to drop (after maxing out at 250f). In 5th, the temps dropped and kept dropping till they hit 175f (which was normal for the old trans).

Am I being a vehicular hypochondriac? Something seems wrong with my 4th gear, like TC isnt' locking or the clutches are slipping. I can't think of a reason why shifting to a HIGHER gear would cool the trans down (same speed, 65mph, same flat terrain). Anyone else have a 6l90 that makes heat in 4th?

I called the trans guy and his answer "don't drive it in 4th, drive it in 5th". He also said 8k lbs is the max I should pull with it.

sdeeter19555 · December 19, 2016

I have a 2010 GMC Sierra 2500HD with 6.0L and 6l90. I have 205k on it with 5k on a reman'd transmission (reman, not rebuilt). The old trans ran great except it was starting to miss 3rd gear on 2-3 shifts. Sometimes up to 500 rpms before it "found" 3rd.

Took it to a reputable shop and got a reman trans with 3yr 100k warranty. No questions about the quality of the labor, they're well recommended in the area.

First time pulling a load last night. Approx 8,000 lbs (3500lb enclosed trailer, 4500 lb car), ambient temp outside was -4F. Going down the flat highway 65mph in 4th gear I checked the trans temp after about 15 mins on the road. 230F and climbing fast. I wasn't on it hard, literally just cruising down the road. I used to keep the vehicle limited to 4th gear with old trans when pulling this combo, keeps RPMs about 3100 for a good cruise. I tried slowing down, temp still climbed. I shifted it to 5th, and temps started to drop (after maxing out at 250f). In 5th, the temps dropped and kept dropping till they hit 175f (which was normal for the old trans).

Am I being a vehicular hypochondriac? Something seems wrong with my 4th gear, like TC isnt' locking or the clutches are slipping. I can't think of a reason why shifting to a HIGHER gear would cool the trans down (same speed, 65mph, same flat terrain). Anyone else have a 6l90 that makes heat in 4th?

I called the trans guy and his answer "don't drive it in 4th, drive it in 5th". He also said 8k lbs is the max I should pull with it.

Umm, if he told you 8k is the max, why are they rated at 9600 with 3.73s and 13k with 4.10s.

Was your coolant at a normal temp?

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Matthew Barens · December 19, 2016

Coolant was normal, just trans temp. I have 3.73 so I'm only 85% of capacity and not driving hard.

I was thinking I need a bigger trans cooler but it seems to be making an abnormal amount of heat in 4th. I'd prefer to fix the root cause not patch the problem.

newdude · December 19, 2016

Stop driving in 4th? Use D and tow/haul. Higher rpm's is where the heat build up is from.

Raptordude · December 19, 2016

I have the 4l80e, don't have experience with the 6l90 but 230-250 degrees with an outside temp of -4 is absurd. You shouldn't see that even if it were 90 degrees outside. I pull a small trailer and riding mower around in the summer in 90+ heat and I've never seen temps over 180

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Formulabruce · December 19, 2016

Gear ratios., Towing should be done at at least 1:1 in the trans. Above that ( lower gear) will cause too many RPM and heat, and Also you get OUT of the Torque band.

4th is 1.152

5th is .852

6th is .667

The torque curve of the 6.0L is broad and flat, with approximately 90 percent of peak torque available at ONLY 2,000 rpm.!!!

Excessive Heat and excessive fuel consumption exist above 3,000 RPM

ANY time Transmission temps go above 195 for any length of time you will start to ruin the clutch faces and cook seals.

Keep RPM below 3,000 and Tow in 5th, with Lock up in "tow mode".

Install a bigger trans cooler because now you have done some damage. Try to make sure no more happens

Bigger trans cooler isnt a big deal to install in these trucks as the grille comes off rather easily.

Good luck!

Matthew Barens · December 19, 2016

15H - Was in D and tow haul, it was holding 4th in that setup.

Formula - Thanks. I'll go with a bigger cooler and keep it as low in the gears as possible.

sdeeter19555 · December 21, 2016

I installed a Long 40k LPD cooler I got off Amazon.com, made some simple brackets out of aluminum bar stock, mounted the cooler on the passenger side of the radiator, and used the adapter nipple to tie into the existing cooler with a hose.

I'm basically running the OE cooler and the Long in tandem. At 30 degrees ambient, I barely get to 100 degrees on the cooler. Towing my camper (5400#s) on a 65 degree day, I barely get to 165 degrees even on hard pulls.

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sdeeter19555 · December 21, 2016

Whoops, I guess it's a 28k...Long Tru-Cool LPD Transmission Oil Cooler 4590 28,000 GVW.

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Formulabruce · December 22, 2016

Whoops, I guess it's a 28k...Long Tru-Cool LPD Transmission Oil Cooler 4590 28,000 GVW.

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This is the ticket!!

Here is my OLD Vs New

Matthew Barens · December 22, 2016

Thanks Sdeeter and Formula! I'm going to install that over christmas break. I put another 100 miles on yesterday in 30f weather and i'm having to baby the throttle way too much to keep it below 200f. I'll be adding more cooling capacity for sure.