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Old 07-17-2016, 07:21 PM   #1
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Transmission Temp Way Up

We just returned from a week long trip with our '25 Flying Cloud through the mountains of NM. This was our 5th trip towing with our '13 6.2 Denali with 3.42 rear axle. Two of the sites took us to some fairly steep high country mountain climbs where the transmission temps exceeded 240f. Only once did the transmission warning go off. During normal highway driving the transmission temp was in the 205-215 range.

Needless to say, we are very concerned about the Denali as a reliable tow vehicle. My first thought is to increase the size of the transmission cooler so we can stay with our Denali. Ambient temperature during this tow was in the 100 degree range, so that probably had some bearing on the transmission temp.

I've read numerous threads where people are towing with similar vehicles, but can't tell if they are staying on fairly level roads or climbing steep hills. We really love high country travel and hate to think that we'd have to consider a different tow vehicle. A diesel tow vehicle would give us better towing torque, but I'm not quite ready to pull the trigger on another expense. I'd really appreciate some input from people with tow vehicles in the Rocky Mountain region who are pulling an AS in the '25 range.
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Old 07-17-2016, 07:42 PM   #2
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I'd check the gauge first. I've had issues with my aftermarket transmission temperature gauge and it had to do with a poor connection. The gauge works from resistance so when the connection is going bad the temperature shows hot. Renew the connection and all is good. I've had a couple of real scares turn out to be nothing more than a bad connection. Your situation may not, probably is not, the same but it's worth considering.
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Old 07-17-2016, 08:20 PM   #3
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The '13 Denali does not have a transmission temp gauge on the dash. I took it to the dealer and he showed me how to toggle through the "info" button to get the digital read out for transmission temperature, so I'm sure it is accurate. I was also surprised to not see transmission temp alarm until it had reached in excess of 240. There is nothing in the owners manual that discusses normal vs. high transmission temp range. I'm pretty sure those high temp ranges are not good on the transmission, but are they OK for short periods of time?
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Old 07-17-2016, 08:44 PM   #4
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Transmission Temp Way Up

I think your initial comments are right on target. You were towing a heavy load on some good hills, it was very hot outside, and you should investigate a larger/better transmission cooler.

Do you know if your Denali has a separate cooler already, or does it rely on the radiator for cooling?

I think I would lean towards a fan-assisted cooler. That way when you are going relatively slow up hills, the fans will provide airflow across the cooling fins.

Heat kills automatic transmissions. Be sure to have your transmission serviced soon.


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Old 07-17-2016, 08:44 PM   #5
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With today's synthetic oils 240 is ok for short periods, but I would still look into a good aftermarket oil cooler for it. You may want to get your filter and fluid changed if its been more than 30k as well.
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Old 07-17-2016, 09:18 PM   #6
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GMC temp numbers

Quote:
Originally Posted by abqdor View Post
We just returned from a week long trip with our '25 Flying Cloud through the mountains of NM. This was our 5th trip towing with our '13 6.2 Denali with 3.42 rear axle. Two of the sites took us to some fairly steep high country mountain climbs where the transmission temps exceeded 240f. Only once did the transmission warning go off. During normal highway driving the transmission temp was in the 205-215 range.

Needless to say, we are very concerned about the Denali as a reliable tow vehicle. My first thought is to increase the size of the transmission cooler so we can stay with our Denali. Ambient temperature during this tow was in the 100 degree range, so that probably had some bearing on the transmission temp.

I've read numerous threads where people are towing with similar vehicles, but can't tell if they are staying on fairly level roads or climbing steep hills. We really love high country travel and hate to think that we'd have to consider a different tow vehicle. A diesel tow vehicle would give us better towing torque, but I'm not quite ready to pull the trigger on another expense. I'd really appreciate some input from people with tow vehicles in the Rocky Mountain region who are pulling an AS in the '25 range.
I have a similar tow vehicle: 2010 Yukon Denali (short wheelbase), with the 6.2L engine, 6 speed tranny, 3.42 rear end, AWD. I see higher temps than I would like when doing big climbs in Colorado and points west. I have not ever had it give me a warning message on the dash.

I was concerned about the temperatures I was seeing so I emailed GM's customer service dept. and they sent me back some interesting details.

One item was a chart that showed the conditions that active the cooling fans on either low or high. I interpret these numbers as being temps where brief exposure is not serious. They listed four things that drive the fan speeds: engine coolant temp, engine oil temp, tranny fluid temp, and air conditioner compressor pressure. Here are the temperature triggers:

Engine oil: low speed at 277, high speed at 288.
Tranny fluid: low speed at 253, high speed at 257
Engine coolant: low speed at 226, high speed at 234.

The email from GM said that the tranny temp warning message comes at 270, and says "Abnormal Trans damage starts to occur when the message is displayed."

When I'm doing a big climb with my 25FB, like I-70 to the Eisenhower tunnel, I see the engine temp gauge run up to 235, then drop back to 210, then repeat. The tranny temp readout usually climbs steadily, topping out at 240-something at the summit (I've seen 248 but not 250).

After my last long trip, I notice minor oil leakage from my front differential. I had a mechanic check it out. He found the fluid smelled burnt. He thinks it overheated and pushed some oil out. The center differential fluid also smelled bad, so I had both changed. Then I went on a shorter trip last month, and had a little more leakage.

I've been tempted to upgrade my tranny cooler, but haven't done so. My suggestion to you is to have all your fluids checked and changed if needed, and plan on changing them often.

You could write to GM (socialmedia@gm.com) and ask them about the temp limits for your vehicle. If they are different from what they told me for my 2010, I'd love to know that.
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Old 07-18-2016, 05:48 AM   #7
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If you pay attention to the DIC you shouldn't have a problem. Under extreme conditions, Limp Mode will engage.

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Old 07-18-2016, 07:14 AM   #8
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I have a 2007 Denali. We have a 27 ft. Airstream. If I tow with it not in tow mode the trans. heats up,when I put it back in tow mode it goes back down,The transmission is constantly shifting down on the slightest grade. I bought a 2015 F150 Platinum with the 3.5 Ecoboost,It tows much better than the Denali and performs great.



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Old 07-18-2016, 11:58 AM   #9
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Another thing that can increase trans temps while hauling up hill is if your tranny is constantly shifting between gears. If so choose the lower shift number of the range, slow down and stay in that gear.
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Old 07-18-2016, 12:39 PM   #10
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If your tranny Temps get to 240, something inside will fry. Why not spend $150-200 and install an additional tranny cooler and save $3000 for a new tyranny. For you folks that think it's OK to have those high temps, I predict a tranny failure at the least opportune moment. If I were you, I would get a complete tranny service and new fluid.
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Old 07-18-2016, 12:46 PM   #11
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What about changing the 3.42 gears to something better for pulling a trailer? Plus an additional transmission cooler.
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Old 07-18-2016, 12:49 PM   #12
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You should be thinking, "Auxillary transmission cooler." For starters, check with your Chevy/GMC parts dept.

Also, check brand-name after-market kits as well as independent transmission shops.

Frankly, the person to talk to should be an independent, "old school," tranny guy - someone familiar with RV applications.

Q: Did you "cook" your ATF?? Did you give it the "sniff" test??

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Old 07-18-2016, 01:07 PM   #13
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I had transmission temp up around 240 on my F-150 on a 6-mile 8% grade in NC. I was going slow in lower gear with high RPMs. If that was your situation I would suggest an auxiliary "active" cooler, i.e. one with a fan. I pretty sure my problem was insufficient air flow over the built-in HD (but passive) cooler. My cooler was low mounted, not in the direct flow of the radiator fan, behind the AC evaporator and the radiator.

I chose to change trucks but was tempted to try an active cooler.

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Old 07-18-2016, 01:18 PM   #14
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We have a 2012 27 foot and were towing with a Tahoe with the same engine and gears. It had a trans cooler and we were seeing temps like that up here in the Northwest. And, it was SLOW going up mountain passes.

We actually drove back down to our home state of Texas, sold the Tahoe and picked up a diesel 2500 and that performs much better, obviously.
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