Trany Overheating Going Downhill - Chevy and GMC Duramax Diesel Forum
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post #1 of 13 (permalink) Old 07-15-2018, 03:24 PM Thread Starter
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Trany Overheating Going Downhill

Just completed a 5500 mile trip out west in the mountains with my 9K lb 5th wheeler and had two events where the trany overheated (red zone) only when going downhill in 1st gear on a 10 percent grade. Had to pull over both times to let it cool. 2nd gear 8% grades no problems. Up hill no problems either as long as I didn't push it to hard. The trany was the only problem, the engine was at normal temp. So why did the trany overheat going downhill? The engine revs was near the red line ~3900. Was in in tow/haul mode.

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post #2 of 13 (permalink) Old 07-15-2018, 06:57 PM
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Might be time for fresh fluid or a trans cooler


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post #3 of 13 (permalink) Old 07-15-2018, 08:43 PM Thread Starter
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Originally Posted by 07Dmaxx93 View Post
Might be time for fresh fluid or a trans cooler


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Only 10,000 on new fluid and new trany cooler.

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post #4 of 13 (permalink) Old 07-16-2018, 08:14 AM
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two thoughts come to mind.

1. By being in 1st gear and 3800 RPM the heat generated by the transmission spinning so fast internally coupled with the slow speed which decreases the airflow through the cooling stack was limiting the ability of the stack to cool your TF.

2. When was the last time you cleaned that cooling stack? If it is plugged with bugs, etc, that too can severely limit the ability of the cooling stack to cool the fluids. This seems less likely to me becasue the rest of the fluids were normal you said

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post #5 of 13 (permalink) Old 07-16-2018, 08:38 AM Thread Starter
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Originally Posted by 407driver View Post
two thoughts come to mind.

1. By being in 1st gear and 3800 RPM the heat generated by the transmission spinning so fast internally coupled with the slow speed which decreases the airflow through the cooling stack was limiting the ability of the stack to cool your TF.

2. When was the last time you cleaned that cooling stack? If it is plugged with bugs, etc, that too can severely limit the ability of the cooling stack to cool the fluids. This seems less likely to me becasue the rest of the fluids were normal you said
I never overheated the engine so I believe cooling stack is OK. The vehicle speed was about 25 mph and so that reduced the air flow some what.

Maybe the turbo brake I have heard about could help as maybe that would have kept me in second gear with torque converter locked.

The switch backs limited my speed to 25 max so maybe a electric fan would have helped.

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post #6 of 13 (permalink) Old 07-16-2018, 08:48 AM
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I never overheated the engine so I believe cooling stack is OK. The vehicle speed was about 25 mph and so that reduced the air flow some what.

Maybe the turbo brake I have heard about could help as maybe that would have kept me in second gear with torque converter locked.

The switch backs limited my speed to 25 max so maybe a electric fan would have helped.

Cinno
The bad news is that your LBZ does not have a Turbo Brake from the factory. The good news is that it is in the logic of your ECM. Here is a link to a thread I started when I put it in my last truck. I was not the one who discovered this however so in the first paragraph I gave credit and a link to the original thread.

https://www.duramaxforum.com/forum/0...lmm-works.html

I think you need to read both his thread completely and my thread completely if you wish to do this. I HIGHLY recommend it. For less than $20 you will not believe what a HUGE difference the TB makes going down hill. I was on some 7% grades day before yesterday. NEVER touched the brakes, the TB did it all and held whatever speed I had cruise set at.

Good Luck. If you have questions, I will be happy to assist, as I did with a guy in the thread I started.

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post #7 of 13 (permalink) Old 07-16-2018, 11:13 AM
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Quote:
Originally Posted by cinno View Post
Just completed a 5500 mile trip out west in the mountains with my 9K lb 5th wheeler and had two events where the trany overheated (red zone) only when going downhill in 1st gear on a 10 percent grade. Had to pull over both times to let it cool. 2nd gear 8% grades no problems. Up hill no problems either as long as I didn't push it to hard. The trany was the only problem, the engine was at normal temp. So why did the trany overheat going downhill? The engine revs was near the red line ~3900. Was in in tow/haul mode.

Cinno
I'm speculating here, but I believe the torque converter will be unlocked in 1st gear, even in T/H mode. With the TC unlocked, there will be viscous heating due to the difference in speed between the input and output shafts of the TC. At 3900 RPM there will be a significant amount of heat addition to the transmission fluid and the low vehicle speed means limited heat dissipation. Maybe put it into manual mode and keep it out of 1st gear and use the service brake a bit more.
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post #8 of 13 (permalink) Old 07-16-2018, 12:29 PM
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Using first gear (bad choice) you eliminated the option of the trans design to help control some braking.
Wrong wording but most should be able to finger what I said out.
Yes, the towhaul works in 2nd up, not first gear.

Was you using towhaul (recommended) or just in drive ?
It might just be me, but I would never red line or go above 3000 - 3200.
Just asking for future trouble IMO.

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post #9 of 13 (permalink) Old 07-16-2018, 01:53 PM
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Quote:
Originally Posted by jdwarren View Post
I'm speculating here, but I believe the torque converter will be unlocked in 1st gear, even in T/H mode. With the TC unlocked, there will be viscous heating due to the difference in speed between the input and output shafts of the TC. At 3900 RPM there will be a significant amount of heat addition to the transmission fluid and the low vehicle speed means limited heat dissipation. Maybe put it into manual mode and keep it out of 1st gear and use the service brake a bit more.
This is exactly what I was thinking, but you beat me to it.

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post #10 of 13 (permalink) Old 07-16-2018, 06:21 PM Thread Starter
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Originally Posted by D_R_C View Post
Using first gear (bad choice) you eliminated the option of the trans design to help control some braking.
Wrong wording but most should be able to finger what I said out.
Yes, the towhaul works in 2nd up, not first gear.

Was you using towhaul (recommended) or just in drive ?
It might just be me, but I would never red line or go above 3000 - 3200.
Just asking for future trouble IMO.
Yes tow/haul. I think using first gear is the only real choice as riding my brakes down a 8 mile 10% grade may have faded the brakes. The speed limit on the switch backs was 25 MPH, and 1st was the only gear that would reduce the speed without brakes given the weight of my rig. Using braking and 2nd gear engine braking may have done it but that would have been experimenting which would have been to dangerous, if it resulted in the loss of braking do to fade. Stopping and letting it cool was my only choice.

This topic was started so I could confirm what I always suspected and that is the torque converter does not lock up in 1st gear. Given that fact I believe my choices for future extreme mountain towing are; increase trany cooling with aux cooler with electric fan (in series with stock cooler) , and turbo braking. I'll look into these mods before next years towing adventures.

cinno

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