Transmission overheating

[dsm1995gst]

I am the first person I know of to have a stuck closed trans thermostat. You’re doesn’t sound like that.
 
I also towed and even raced once while towing 4klbs (before I knew my trans thermostat was stuck closed)
 
-My temperature never increased anywhere close to as fast as yours
-it never got above 230, that’s towing 3k lbs through the city of philadelphia for an hour
 
So here’s how to tell if your thermostat is stuck.
 
Thermostat opens at 190-200. when your trans temp is like 140-160 range, pop your hood, and feel the drivers side trans cooler line in the front of the truck. Then feel the passenger side.
 
If your thermostat is stuck closed, your driver side line will be about as hot as your trans temp says it is. Your passenger side line will be ambient temp until the trans reaches 190-200. Once it reaches target temp the flow gates should open and the passenger side should be nearly as hot as the driver side.
 
if you have trouble located the lines, you can follow them right out of the transmission. It’s just easier to check them at the trans cooler since you don’t need to get under the truck
 
the techs had never dealt with this and were stumped. I had to basically figure this out on my own and verify the thermostat wasn’t operating and then tell them to change it.
 
Oh and btw you need to change all your trans fluid out it’s burnt.
 

Thanks for the info.

Truck has been in the shop for one full day so far.


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[truckguy82]

4 minutes ago, dsm1995gst said:

Thanks for the info.

Truck has been in the shop for one full day so far.


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I’d be surprised if they find the problem

 

well actually they might because you actually overheated the trans and set off a code and now they have a step by step guide to resolve it.

 

not sure the trans thermostat is in that guide though

Edited July 23, 2019 by truckguy82

[Racindave]

On 7/20/2019 at 6:56 AM, dsm1995gst said:

No brakes, good idea though


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Bad wheel bearing on trailer? Just wondering....

[HeySkippyDog]

Could it still be the thermostat if it doesn’t do it when not pulling the boat?

We ended up unhooking the trailer (and swapping trucks) and my truck drove home without the boat in the 195 degree range.

Still too high with no load.

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[dsm1995gst]

I am the first person I know of to have a stuck closed trans thermostat. You’re doesn’t sound like that.

 

I also towed and even raced once while towing 4klbs (before I knew my trans thermostat was stuck closed)

 

-My temperature never increased anywhere close to as fast as yours

-it never got above 230, that’s towing 3k lbs through the city of philadelphia for an hour

 

So here’s how to tell if your thermostat is stuck.

 

Thermostat opens at 190-200. when your trans temp is like 140-160 range, pop your hood, and feel the drivers side trans cooler line in the front of the truck. Then feel the passenger side.

 

If your thermostat is stuck closed, your driver side line will be about as hot as your trans temp says it is. Your passenger side line will be ambient temp until the trans reaches 190-200. Once it reaches target temp the flow gates should open and the passenger side should be nearly as hot as the driver side.

 

if you have trouble located the lines, you can follow them right out of the transmission. It’s just easier to check them at the trans cooler since you don’t need to get under the truck

 

the techs had never dealt with this and were stumped. I had to basically figure this out on my own and verify the thermostat wasn’t operating and then tell them to change it.

 

Oh and btw you need to change all your trans fluid out it’s burnt.

 

Do you have any pictures or diagrams by chance? The dealer still hasn’t fixed this and actually have somehow made it worse (now transmission overheats in town when not pulling anything).

 

edit - the strange thing is it went over 250 but the alarm didn’t go off this time

[truckguy82]

2 hours ago, dsm1995gst said:

Do you have any pictures or diagrams by chance? The dealer still hasn’t fixed this and actually have somehow made it worse (now transmission overheats in town when not pulling anything).

 

edit - the strange thing is it went over 250 but the alarm didn’t go off this time

Nope, I just open the hood and i forget which lines they are now.

 

Look under the truck, there are two metal lines coming out of the trans. It’s very obvious, you don’t even need to get under the truck completely. No jacking involved. You’ll traxk them down in a minute. Just follow them to the trans cooler. You figure out which ones you need to feel.

 

if passenger side is ambient temp you have an easy fix

 

if it’s similar temp to driver side you have problems

Edited August 2, 2019 by truckguy82

[UnlimitedMatt]

On 8/2/2019 at 3:10 PM, dsm1995gst said:

Do you have any pictures or diagrams by chance? The dealer still hasn’t fixed this and actually have somehow made it worse (now transmission overheats in town when not pulling anything).

 

edit - the strange thing is it went over 250 but the alarm didn’t go off this time

This thread has the transmission line diagram.  I used it when installing my remote atf filter.  Good luck!

 

 

[dsm1995gst]

I’d be surprised if they find the problem

This guy predicted it. Multiple trips to the dealer in the last month and still not fixed.

[truckguy82]

You have to feel the lines. When your transmission is hot, like 200 plus, you have to feel your trans lines.

 

if one is hot and one is much cooler, problem has been solved, your thermostat.

 

if they are both hot, good luck go under your truck and feel the if you want

Edited August 15, 2019 by truckguy82

[2018GMC]

Get one of those cheap (under $20) infrared temp sensor guns. Point it at the transmission pan and the cooler lines. Does the temp on the gun match the temp in the cluster?

 

Could be bad temp sensor in transmission, low on fluid, trans cooler line clogged, thermostat, etc. 

 

We have 2014 duramax in the family. Suddenly read 120psi oil pressure. Dealer we had been going to wouldn’t replace sending unit and suggested we change oil and that GM didn’t specify a max oil pressure only a minimum. After arguing and getting nowhere, went to another dealer who properly diagnosed and replaced sending unit. 

 

Contacted gm GM customer care and informed them about original dealer. They offered a free oil change for their mistake. 

[300 Blackout]

Below is a picture of the thermostat housing disconnected from the trans. The bottom port is the output from your transmission to your trans cooler. The top port is the return from the cooler and back into the trans.

I let my truck sit overnight and the trans fluid was 80*F when I disconnected my top line from the thermostat housing (thermostat still connected to the truck) to flush out some fluid during a complete fluid and filter replacement. Even though the fluid was at ambient temperature, the return line flowed trans fluid.

My truck had 75k miles and I decided to replace the trans cooler lines (which includes a new thermostat and housing) as part of my service interval. Before installing the new lines, I opened up the thermostat housing, flipped the thermostat upside down per some directions I found online and reinstalled everything. I cleaned the pan, replaced the filter, fluid and pan gasket.

My trans temps have dropped by 20 to 30 degrees. I’m averaging about 160 while cruising on the highway in 80 degree weather.

Now, if my truck was within the warranty period like yours is, I would take video and pictures of my trans temp readings along with pictures of the driving conditions, outside temperature reading on the dash and of the vehicle to show that nothing is being towed.

I would then bring the truck back to the dealer with a written explanation of the problem and would attach the pictures along with copies of the previous work orders when they couldn’t find or fix the problem.

Then, I’d reach out to GM customer care and provide them with everything you provided the dealer and ask them to assist in making sure the problem is resolved promptly.

I’m not saying the dealer is incompetent or that you should be rude. It’s possible they couldn’t reproduce the issue. Just remove any legitimate excuse for them not fixing the problem and get some oversight involved that doesn’t fall under the chain of command of the dealer. Good luck.

[dsm1995gst]

Thanks for the info. Unfortunately, at this point, I’ve had the truck at the dealership three times and it currently is still sitting there with the transmission taken apart waiting on parts yet again.

I have no idea if they have checked the items y’all are referring to but I will ask.

[dsm1995gst]

.

Then, I’d reach out to GM customer care and provide them with everything you provided the dealer and ask them to assist in making sure the problem is resolved promptly.

I’m not saying the dealer is incompetent or that you should be rude. It’s possible they couldn’t reproduce the issue. Just remove any legitimate excuse for them not fixing the problem and get some oversight involved that doesn’t fall under the chain of command of the dealer. Good luck.

Yea I have an escalated case open with Customer Care now as this has become a bigger issue since at this point I have a 2018 truck that I’ve been without for about 20 days.


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[TXGREEK]

So I just got a new boat (only 17ft fiberglass) and was on the way to put it in the water for the first time and my 2018 GMC Z71 with 10k miles apparently can’t handle that, as within about 15 miles on the interstate I get a warning saying my transmission is overheating and I need to pull over and idle.

 

After a couple of minutes I was finally able to navigate through the control panel it said the trans temp was 258 degrees.

 

I was driving in tow mode.

 

After it cooled down to 210 degrees I tried again without tow mode and it was back to 241 degrees before I got to the next exit about 2 miles away.

 

Needless to say I’m pissed off.

 

Not sure if this would work for you but I recently had my transmission flushed and added the newer GM transmission fluid and shocked to find the transmission temperature was at least 20 degrees lower than before the flush and with only 20k miles on it. Good luck!

 

 

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[BigBadSierra]

Three times back to the dealer sorry I would be barking at them to replace the whole transmission by now.

Funny how dealers are my dealer would have probably went after GM to replace is on tear down # 2.