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Discussion Starter #1
A couple of times over the past couple of years, I've lost overdrive and the air conditioner stopped working. At those times, the transmission was also pretty hot (280 degrees by the infrared heat gun I used to shoot the pan). Once was while I was climbing a 14,000 mountain in Colorado, another was while pulling a camper through Tallahasse in bumper to bumper traffic when hurricane Micheal was about to hit. The light on the OD button on the dash would come on and I lost OD no matter if I hit the button or not. The AC also stopped. The fan would run, but the compressor turned off. The trans temp light on my dash did not come on.

If I get moving down the road and get to speed, the AC will come back on and the OD will work again. I've only had the OD problem a couple of times. The AC problem happens any time it gets good and hot, like pulling a camper for a few hundred miles or idling in a hot parking lot. Is this an electrical problem, or it is a safety feature, some sort of "limp mode" to keep temps down (although OD would help temps come down instead of raising them)?

I took the truck on a 4000 mile trip with no problems on the interstate, but back in town stop and go on a hot day, it does this. Especially the AC.

Any ideas?
 

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PCM kicks you out of overdrive if the trans gets too hot.

I'd imagine it shuts the AC off if the engine gets too hot, not sure though
 

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Sounds like you have a cooling problem that needs to be addressed. The cooling systems in these trucks are pretty robust when they work properly.

280* pan temp you've essentially cooked the trans. You should make sure it is not slipping due to burnt clutches as that will only compound the problem. Then its time to start saving for the eventual rebuild.


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What was the engine temperature gauge at when the tranny was overheating?
 

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Sounds like you have a cooling problem that needs to be addressed. The cooling systems in these trucks are pretty robust when they work properly.

280* pan temp you've essentially cooked the trans. You should make sure it is not slipping due to burnt clutches as that will only compound the problem. Then its time to start saving for the eventual rebuild.


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This is false. While 280 degrees is damn hot, it's not a guaranteed failure by any stretch. You would have to be over 325 degrees to start cooking the tranny immediately. At 280, the fluid is deteriorating at an accelerated pace, but transmission failure charts show the fluid should last well over 5,000 miles at that temp.
 

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This is false. While 280 degrees is damn hot, it's not a guaranteed failure by any stretch. You would have to be over 325 degrees to start cooking the tranny immediately. At 280, the fluid is deteriorating at an accelerated pace, but transmission failure charts show the fluid should last well over 5,000 miles at that temp.
The fluid coming out of the converter would be well over 325 degrees with pan temp at 280. Considering the temp probe on the RH is on the hot side and it trips at 280 degrees... it's well over where it should be. Lockup clutch disks won't last long at that kind of fluid temp. Destroy the disks then its a rebuild. Not to mention the wear that is caused by fluid that has lost a good portion of its lubricating qualities.

Pretty sure any manual you read on DEXIII will recommend at the very least to replace the fluid ASAP. Even then, just like when overheating an engine there is never any guarantee the damage hasn't already been done.
 

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Discussion Starter #7
The tranny was freshly rebuilt with a new billet TC and Amsoil in it. I pulled over at the first sign of trouble and it was around 50 degrees outside when that happened. The engine temp never climbed over the normal range. I've never seen the engine temps get even close to the high side, always around 200-210 if I remember right. The transmission shop deleted the heat exchanger for the transmission with the assurance that they do it all the time and have never had any issues with it. That is probably contributing to the higher trans temps, I just wasn't sure if the trans temps were responsible for the OD kicking out or not (and the AC). I can see the AC turning off to help keep some heat away from the tranny cooler up front maybe, but not sure if it's designed to do that or not.

My transmission guy says he thinks I need to replace my fan clutch. Maybe, it's original with 275k on it. It has resistance when I spin it but it probably isn't up to spec anymore. He builds a lot of performance transmissions and says the exchanger delete has been successful for him every time. Both times it got hot enough to turn off OD, it was 45 mph or less and shifting up and down a lot. I suppose I could try that and keep an eye on temps, but I'm debating if electric fans might be the way to go.
 

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Discussion Starter #8
After some research, it looks like there is an upgraded fan clutch and fan for this truck. I think I'm going to put that on. Even though my temp gauge doesn't climb, it probably isn't cooling the AC condenser, intercooler, and trans cooler like it should. The more I think about it, it's been a long time since I heard that loud fan sound that it used to make on occasion when it was warmed up and working hard.
 

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The tranny was freshly rebuilt with a new billet TC and Amsoil in it. I pulled over at the first sign of trouble and it was around 50 degrees outside when that happened. The engine temp never climbed over the normal range. I've never seen the engine temps get even close to the high side, always around 200-210 if I remember right. The transmission shop deleted the heat exchanger for the transmission with the assurance that they do it all the time and have never had any issues with it. That is probably contributing to the higher trans temps, I just wasn't sure if the trans temps were responsible for the OD kicking out or not (and the AC). I can see the AC turning off to help keep some heat away from the tranny cooler up front maybe, but not sure if it's designed to do that or not.

My transmission guy says he thinks I need to replace my fan clutch. Maybe, it's original with 275k on it. It has resistance when I spin it but it probably isn't up to spec anymore. He builds a lot of performance transmissions and says the exchanger delete has been successful for him every time. Both times it got hot enough to turn off OD, it was 45 mph or less and shifting up and down a lot. I suppose I could try that and keep an eye on temps, but I'm debating if electric fans might be the way to go.
After some research, it looks like there is an upgraded fan clutch and fan for this truck. I think I'm going to put that on. Even though my temp gauge doesn't climb, it probably isn't cooling the AC condenser, intercooler, and trans cooler like it should. The more I think about it, it's been a long time since I heard that loud fan sound that it used to make on occasion when it was warmed up and working hard.
If it's overheating to those temps at 45 mph, a fan clutch is not your only problem. Something is not right. Also, I question a transmission shop who just arbitrarily deletes the tranny heat exchanger. What was their justification for that?

I would send the truck right back to the shop and say "whoooooahhhh, this tranny is really overheating." Make them deal with it, and I would want the exchanger put back on.

The fan clutch didn't all of a sudden go bad right after the tranny rebuild. That's a bunch of BS. It's probably a combination of the deleted exchanger and a problem with the air to liquid cooler and a possible obstruction in the system. They may not have flushed all the lines and everything correctly.

When you look into the "exchanger delete," the guys who do it are adding aftermarket coolers. The ones who didn't reported their trannies running 60 degrees or more hotter.
 

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Agreed with brownbear on this one. The heat exchanger is the primary way to removing heat from the fluid.



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Same O/D shutoff issue happened in my truck after I had the trans built, though it was throwing a false positive. The transmission isn't working as hard in 3rd than 4th so the PCM tells it to take a breather when it reaches a certain temp. I wasn't running my A/C when this happened so I can't comment whether or not it too was overrode.

At a minimum, it sounds like you should flush your trans fluid. I would also install a trans temp gauge.
 

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Discussion Starter #12
Thanks for the input. His reason for the delete was that it was old, had a lot of miles on it, and if it leaked, it would ruin the new transmission and it was not worth the risk. He said he would rather put a new one on or a cooler on if needed. It has around 275k miles on it, maintained pretty well, so should I reconnect the old one, put a cooler on, or put a new one on?

I'm still interested in the AC issue... is there some sort of safety feature that is turning it off or do I have some other problem to hunt?
 

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I have heard of heat exchangers going bad and filling the transmission with coolant and ruining them.

My transmission would overheat alarmingly fast without it during commercial operation. Now my transmission will just barely get to 240* hot line (around 190-200 pan temp) climbing a bit hill with a heavy load.

If your coolant/transmission exchanger looks ok and coolant was mantained you should be ok.
 

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I'm skeptical on the factory heat exchanger being prone to failure. I haven't heard of this and to the best of my knowledge there are plenty of trucks out there with 2-3x the mileage of your truck still running the original. It's really a pretty simple part, just an enclosed housing with hose connections for the trans fluid and coolant. No moving parts or anything like that to worry about.

At a minimum, I think you need an secondary transmission cooler if you choose to ditch the exchanger. But, at that point, why not just reinstall the exchanger since you already have it?

As for the A/C, this will require some more digging. It wouldn't surprise me if the trans overheating O/D lockout also cut out the A/C, just to reduce overall heat from the vehicle, but I'm really not sure.
 

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I have heard of heat exchangers going bad and filling the transmission with coolant and ruining them.
This is scary. Is there any rhyme or reason to why they go bad? Would improper maintenance to the trans fluid and/or coolant contribute? Replacing it wouldn't be such a big deal, except for the Cummins part I've found is upwards of $900 which seems INSANE.

Cheeze, you might consider looking into Mishimoto's aftermarket trans cooler. It mounts at the factory cooler location in front of the intercooler. It is a beefy unit with double the fin surface area and fluid capacity over stock. I have one in my truck and it works great. I'm not suggesting it as a substitute for a secondary cooler or the heat exchanger, but it's not terribly expensive for the amount of extra cooling ability you get. Part # MMTC-RAM-94SL
 

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I have heard of heat exchangers going bad and filling the transmission with coolant and ruining them.

My transmission would overheat alarmingly fast without it during commercial operation. Now my transmission will just barely get to 240* hot line (around 190-200 pan temp) climbing a bit hill with a heavy load.

If your coolant/transmission exchanger looks ok and coolant was mantained you should be ok.
I have heard of this with other brand vehicles, but never these trucks. The only failures with contamination I think I've ever read about are the reverse - transmission fluid in the cooling system.
 

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I have heard of heat exchangers going bad and filling the transmission with coolant and ruining them.

My transmission would overheat alarmingly fast without it during commercial operation. Now my transmission will just barely get to 240* hot line (around 190-200 pan temp) climbing a bit hill with a heavy load.

If your coolant/transmission exchanger looks ok and coolant was mantained you should be ok.
I have heard of this with other brand vehicles, but never these trucks. The only failures with contamination I think I've ever read about are the reverse - transmission fluid in the cooling system.
Most cooling systems have about 9-16 psi in them, I think the pressure in the cooler lines for the transmission is about 20-40psi
I don't know what would happen when the truck is shut off, I think the cooling system will hold pressure and leak into the transmission.
My heat exchanger is off D89/Tony's truck because mine was missing for unknown reasons.

I would guess It has over 300,000 miles on it .

Unless the cooling system is in piss poor condition I would imagine it would last forever...
 

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I have a buddy that his heat exchanger went bad and filled his trans with coolant. I had to remove mine when I put compounds on it so I used a derale 15870 under the bed of the truck. I rarely turned the fan on and when I did, it would cool the fluid down quick.
 

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Discussion Starter #19
It's reconnected to the heat exchanger now. I still plan to put a new fan clutch on it because the compressor is cycling a lot and it gets warm when it's idling on a hot day, high side pressure goes up like there is not enough airflow across the condenser. Hopefully this will = problems solved.

Thank you all for your input.
 

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It's reconnected to the heat exchanger now. I still plan to put a new fan clutch on it because the compressor is cycling a lot and it gets warm when it's idling on a hot day, high side pressure goes up like there is not enough airflow across the condenser. Hopefully this will = problems solved.
Thank you all for your input.
Is everything clean and unobstructed?
 
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