Transmission Overheats When Coasting? (1 Viewer)

[96TrailSurfer]

Help!! Transmission overheating problem has me stumped


1KZ-te with a340f tranny. External tans cooler in series with the radiator (radiator first, cooler second). Temp probe in the flex hose connecting the transmission outlet pipe to the rad inlet.


When I am driving on the highway under load (100-120kmh), tranny temp is around 150F. As soon as I start coasting or slowing down with my foot off the gas, it spikes to 210-250F. Wtf?


I initially thought that the problem was engine cooling, so replaced fan clutch oil with 10,000 cst. However, stand-alone coolant temp gauge shows a consistent 190-210F and there is no connection at all between the engine temp and tranny temp spikes (coolant temp probe is in the radiator outlet hose - top left corner of the rad).


I use synthetic ATF and it’s at the right “hot” level.


What am I missing here? If the problem was the rad core, wouldn’t I see engine temp problems?

 

[IanB]

The "cooler" integrated into the rad is moreso to help bring trans temps UP to operating temperature as the engine warms up, it's crap as far as a COOLER goes, as it's only going to bring the oil temp down to the coolant temp, so around 190-210F? In most cases your trans oil will be cooler than the engine coolant, so it's actually warming rather than cooling.

You don't say what year your vehicle is (username suggests a '96), but the 3rd gen 4Runner and Surfs suffer common rad failure that allows the coolant and trans fluid to mix, search "pink milkshake" for more info. The best solution is to bypass the factory in-rad "cooler" and replace with an external trans cooler to eliminate the risk of damage to your trans from a rad failure. Loop the trans cooler lines so if it does fail you don't lose all your coolant.

None of this addresses your question though, usually trans temps rise when the torque converter is slipping, coasting should see temps drop. I can't explain what you're seeing on your gauge. Can you post a pic of the sensor probe location?

 

[gerg]

It doesn't get cold engouh here to warrant a tranny heater. Id bypass and run the external too.

 

[GTSSportCoupe]

I recently saw a Prado LJ78 with a cooked transmission. This is the A343F which is generally a pretty tough transmission. The owner had disconnected the rad cooler and went only with an external cooler. I feel this is what caused the transmission failure. Don't under estimate the ability of the rad cooler to cool the transmission fluid under extreme circumstances like long hills with the torque converter out of lock up. Personally I've been running the rad cooler in series with an auxiliary cooler and it's worked out really well for all my towing etc. And I personally feel that running through the rad cooler to heat up the ATF is necessary even in our climate. The trans won't even lock-up the torque converter until it reaches a certain operating temp. The faster it reaches the proper operating temp the better.

As to what causes the temp spike when coasting, I really can't say. Try asking on the Hilux Surf forums. Toyota 4WD Surf Owners • Index page

 

[Douglas S]

I imagine the transmission relies on engine power to pump fluid. Coasting = lower engine rpm = reduced transmission fluid flow rate, fluid spends more time heating up in the trans so it's at a higher temp before it enters the cooler.

 

[96TrailSurfer]

I imagine the transmission relies on engine power to pump fluid. Coasting = lower engine rpm = reduced transmission fluid flow rate, fluid spends more time heating up in the trans so it's at a higher temp before it enters the cooler.

That's true, but doesn't really explain the issue. Less load means less heat generated, which is commensurate with less flow due to less load. It didn't do this before and got worse over time.

I am thinking blockage somewhere.

 

[roscoFJ73]

When I am driving on the highway under load (100-120kmh), tranny temp is around 150F. As soon as I start coasting or slowing down with my foot off the gas, it spikes to 210-250F. Wtf?

I don't know much about autos, but maybe its normal. If the trans fluid stops circulating when you are coasting, it heats up from the hot transmission components. When fluid starts moving through the trans ,you see temps drop.
A bit like the engine when you turn it off after long run