Trans won't stay cool

I've been having issues with keeping my trans temps down on my 2000 f350. The truck has 183,000 and have around 50k on the rebuilt trans with a billet torque converter and am running a PHP hydra. While driving in the highway at around 70-75 my trans will get up to 180 after about a hour and keeps slowly going up to the point where I stop and let it cool down. I installed a 6.0 trans cooler on it just recently and thought it may solve the problem but it has not.

Could it be the radiator fan clutch not engaging and causing not enough air flow?


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How hot is it getting? I don't worry about 180... Lol

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It gets up over 210* not pulling anything.


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Clogged cooler?

Or sounds like TC Is failing.

My transmission guy said it had good flow and he flushed the cooler out and the trans.


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Sounds like clutches slipping or TC failing to me. Do you hear the fan clutch engage?

I'll hear the fan only when I first start the truck in the morning for the first 300ft or so. After that I don't hear it again even on warmer days. I checked the resistance on the fan when the motor was cold and after a few hours on the highway and it had the same resistance as it did while cold.


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Might want to think about a new fan clutch then. I hear mine come on in the summer time after about twenty minutes, it sounds like a 747 under my hood for about fifteen to twenty seconds. After that it will come on for a few seconds every ten to thirty minutes, depending on driving conditions.

When I had the trans rebuilt my trans guy made it so it would shift firmer. Now when I loaded the tunes onto the my Hydra I picked the non modified valve body tunes. Would that make a difference as far as slippage?


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smitty4605 said:

Could it be the radiator fan clutch not engaging and causing not enough air flow?

Click to expand...

Not likely. You don't need the fan at 70 MPH. Ram air is forcing more air over the coolers than the fan can move.

Check the coolers for debris blocking the airflow. Then you need to run a flow test.

Get two containers, at least a gallon each. Also get a helper to start and stop the engine. Warm up the trans by driving at least 10 miles.

Remove the trans cooler line from the rear of the trans. Point the line into container #1. Start the engine. When the flow is steady move the line to container #2 for exactly 15 seconds, then move it back to container #1 and shut the engine off.

If there is at least a quart in container #2 it passes. If fluid was spraying (not dribbling) out of the trans where you removed the line, it fails the test.

If it fails for either reason you need to rebuild the cooler bypass. It is inside the tube between the two cooler ports. A rebuild kit is very cheap, and it's easy to install.

He took out the bypass because he has seen too many of them fail. We are checking the flow again tomorrow.


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Could it be reading hot because of where I have my sending unit for the temperature gauge? I have it installed in the inspection port on the driver side of the transmission


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smitty4605 said:

Could it be reading hot because of where I have my sending unit for the temperature gauge? I have it installed in the inspection port on the driver side of the transmission


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Click to expand...

No. I have my sensor there and I only ever see a max of 140 while towing. Typically around town I see 100-130 degrees.

Just got my truck back from the trans shop. He said it flowed about 1 1/2 quarts in 15sec.


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smitty4605 said:

He took out the bypass because he has seen too many of them fail. We are checking the flow again tomorrow.

Click to expand...

That's a very bad idea.

If the bypass fails the trans will get hot. But if the cooling circuit gets restricted and you don't have a bypass your first indication that there is a problem is that the transmission is destroyed. If you have a bypass and get a restriction you'll notice temperatures increase and you can fix it before destroying your transmission. But to each his own, if you like it better this way it doesn't affect me.

smitty4605 said:

Could it be reading hot because of where I have my sending unit for the temperature gauge? I have it installed in the inspection port on the driver side of the transmission

Click to expand...

No, that's the best spot to put the sender.

smitty4605 said:

Just got my truck back from the trans shop. He said it flowed about 1 1/2 quarts in 15sec.

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That's plenty of flow.

That points to two possibilities. One is not enough air over the coolers, and that would come from debris blocking the airflow. I've seen grass, twigs, leaves, paper, etc, get between the cooler and radiator and really restrict flow.

The second possibility is that the converter isn't locking, so the trans makes a lot more heat. You'll need a scan tool that can look at data to diagnose this one.

Mark Kovalsky said:

That's plenty of flow.



That points to two possibilities. One is not enough air over the coolers, and that would come from debris blocking the airflow. I've seen grass, twigs, leaves, paper, etc, get between the cooler and radiator and really restrict flow.



The second possibility is that the converter isn't locking, so the trans makes a lot more heat. You'll need a scan tool that can look at data to diagnose this one.

Click to expand...

He hooked his scanner up and checked everything. He can't find anything. Everything was working the way it should be.

Does it matter what tune I'm in? Like a performance tune or a daily driver tune?


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Scanner won't see that rats nest in between the coolers on the front of the truck.

Arisley said:

Scanner won't see that rats nest in between the coolers on the front of the truck.

Click to expand...

Lol this is true. I did check to see if there was anything blocking it. And didn't find anything other than a couple of moths and June bugs.

Did you look in between the coolers. You have the radiator, ac, intercooler and trans cooler all in a row there. A blockage anywhere will reduce flow.