Overheating Transmission (2 Viewers)

[RoguesPierre]

I've been getting A/T temp lights. 1st time was shortly after my timing belt was done. I pulled over & found the Rad reservoir was empty. Let it idle as manual says to do for temp lite, added coolant & waited for light to go out. Continued on to an off-road trip and no issues. Few weeks later similar trip no lite until late in the trip, then went on a couple more times after cooling. Took it in asked shop change drive train fluids & to look at it. Don't think they looked at it but did change the fluids. Drove it around town no issues a just went on a trip & it overheated again. I can't feel fluid flowing in the rubber lines leading to or from A/T cooler. Also the lines don't feel hot after driving. What temp does the light go off at? What could cause the fluid not to flow through the cooler. In a 12 mi city drive the temp can get to 240 (no A/T light). I just started monitoring A/T temp so not sure what temp it is when light goes on. I did pop one of the A/T cooler lines off, there was fluid in it didn't gush out.

 

[GVegasG]

Check transmission fluid lever perhaps in case shop did not add enough.
Did you burp it properly after TB/WP change?

 

[RoguesPierre]

Check transmission fluid lever perhaps in case shop did not add enough.
Did you burp it properly after TB/WP change?

Tks I would have hoped the shop did that, I'll look for instructions here.

 

[awesomeissquid]

If the transmission lines are not getting hot it is possible there is a blockage in the lines preventing fluid to flow. One other member had a shipping plug that got sucked into a line. You would need to have each line pulled off and checked to ensure it does not have restrictions. Next would need to ensure the radiator transmission section does not have a blockage as well as the external transmission cooler. I would also second checking the transmission fluid level. Make sure to check it when it is hot.

 

[Mike NXP]

I hope it's just fluid related and no damage has been done. Now that you are monitoring temps get the transmission fluid up to 150-180. I think there is a forum on here with the exact temp, maybe 185 before checking trans fluid.

Otherwise, do those lines get removed when doing timing belt job? Any chance someone hooked them up backwards or did something dumb? I don't have much experience in this category but start simple with a fluid check like others have mentioned and report back. I don't think I've ever seen my coolant or trans temps go over 200F

 

[Eyedaho]

You said your coolant reservoir was low, but did you also check your radiator level with a cold engine and top that off if needed? If not then I would suspect you may have air trapped in your radiator, which may not cause your transmission temperature issue on its own, but won't help and isn't good.

 

[Malleus]

I've been getting A/T temp lights. 1st time was shortly after my timing belt was done. I pulled over & found the Rad reservoir was empty. Let it idle as manual says to do for temp lite, added coolant & waited for light to go out. Continued on to an off-road trip and no issues. Few weeks later similar trip no lite until late in the trip, then went on a couple more times after cooling. Took it in asked shop change drive train fluids & to look at it. Don't think they looked at it but did change the fluids. Drove it around town no issues a just went on a trip & it overheated again. I can't feel fluid flowing in the rubber lines leading to or from A/T cooler. Also the lines don't feel hot after driving. What temp does the light go off at? What could cause the fluid not to flow through the cooler. In a 12 mi city drive the temp can get to 240 (no A/T light). I just started monitoring A/T temp so not sure what temp it is when light goes on. I did pop one of the A/T cooler lines off, there was fluid in it didn't gush out.

First, what year? There were two very different transmission in the 100 series.

Second, I seriously doubt the shop changed the transmission fluid. They likely just changed the fluid in the pan. Not really a problem, because this likely is not the cause of the overheating problem, but if they charged more than $50, you were screwed.

Third, fluid will only weep out of the cooler lines, unless the engine is running. The engine drives the transmission pump.

If you have to (I doubt this is the problem):
You can remove the cooler lines from the transmission and flush the cooling circuit. Use kerosene and a drill operated pump. You'll need two containers, one for the clean kerosene and one for the dirty oil/solvent. It doesn't matter which way you flush it, but a case can be made for backflushing if there is a blockage somewhere. Once the kerosene is flowing clean, you'll need to fill the cooler circuit with ATF. Use the same procedure.

My bet would be that your radiator needs to be thoroughly cleaned. The radiator provides the bulk of the cooling for the transmission. You can verify that this is the problem if you can cool the transmission fluid by washing the radiator with cold water next time it overheats. Unfortunately Toyota made this process very difficult (because they undoubtedly didn't expect these problems) by not providing a transmission oil temperature gauge (which is a crime because there are two sensors in the oil) and the light doesn't illuminate until it's really too late (the 80 series engines have the same problem).

 

[RoguesPierre]

First, what year? There were two very different transmission in the 100 series.

Second, I seriously doubt the shop changed the transmission fluid. They likely just changed the fluid in the pan. Not really a problem, because this likely is not the cause of the overheating problem, but if they charged more than $50, you were screwed.

Third, fluid will only weep out of the cooler lines, unless the engine is running. The engine drives the transmission pump.

If you have to (I doubt this is the problem):
You can remove the cooler lines from the transmission and flush the cooling circuit. Use kerosene and a drill operated pump. You'll need two containers, one for the clean kerosene and one for the dirty oil/solvent. It doesn't matter which way you flush it, but a case can be made for backflushing if there is a blockage somewhere. Once the kerosene is flowing clean, you'll need to fill the cooler circuit with ATF. Use the same procedure.

My bet would be that your radiator needs to be thoroughly cleaned. The radiator provides the bulk of the cooling for the transmission. You can verify that this is the problem if you can cool the transmission fluid by washing the radiator with cold water next time it overheats. Unfortunately Toyota made this process very difficult (because they undoubtedly didn't expect these problems) by not proividing a transmission oil temperature gauge (which is a crime because there are two sensors in the oil) and the light doesn't illuminate until it's really too late (the 80 series engines have the same problem).

Thanks, yes, it's an '06. The radiator is only a couple years old; I'll see if I tranny cools with cold water. I don't feel any pressure in the Trany cooling lines even when the engine is on, I imagine the rubber hose should feel pretty solid when the engine is on. I don't think the temp light comes on til its over 250, the temp got to 240 and no light.

 

[3_puppies]

AFAIR the trans uses a seperate cooler, it does not use the radiator.
there is no reason to disconnect the cooler lines while doing a timing belt, not saying it didn't happen but it is not in the way.

 

[Malleus]

AFAIR the trans uses a seperate cooler, it does not use the radiator.
there is no reason to disconnect the cooler lines while doing a timing belt, not saying it didn't happen but it is not in the way.

The transmission cooler circuit on the A343F (1998-2002) uses the lower tank in the radiator as a first stage cooler and the separate transmission cooler mounted in front of the radiator as a secondary cooler.

The A750F (2003-2007) uses a similar arrangement:

You can see the cooler lines from/to the transmission case connected to the radiator, shown in phantom lines, in both parts diagrams.

 

[Malleus]

Thanks, yes, it's an '06. The radiator is only a couple years old; I'll see if I tranny cools with cold water. I don't feel any pressure in the Trany cooling lines even when the engine is on, I imagine the rubber hose should feel pretty solid when the engine is on. I don't think the temp light comes on til its over 250, the temp got to 240 and no light.

You are correct about the temperature limits. I would be very suspect about any transmission cooler line I could "feel" pressure in when the fluid was moving. The pressure is very small and the rubber is very thick.

The only way to know if the pump works is to disconnect the return hose at the radiator and connect a separate tube to that nipple on the radiator. Put the other end in a clear(ish) container and start the engine. If the pump is working and the cooler circuit is clear of obstructions, the fluid will flow into the container. This takes 5 minutes to set up and break down and less than a minute to run the engine to verify. You won't lose enough fluid to make any real difference, if you don't run the engine for long.

This similar to the method for replacing the transmission fluid, the only difference is you use the other cooler line at the radiator for that. And filling a 750 is a PITA without the fill tube.

 

[RoguesPierre]

The transmission cooler circuit on the A343F (1998-2002) uses the lower tank in the radiator as a first stage cooler and the separate transmission cooler mounted in front of the radiator as a secondary cooler.
View attachment 3415924
The A750F (2003-2007) uses a similar arrangement:
View attachment 3415923

You can see the cooler lines from/to the transmission case connected to the radiator, shown in phantom lines, in both parts diagrams.

I tried burping the radiator as someone had suggested it didn't seem to help. I then pulled the undercarriage shield to trace the lines in the space I have I couldn't do a very good job but I was squeezing the rubber hose lines that go into the radiator. Not sure if I dislodged an air bubble or something but after that it seems, ok?
Finally had a chance to take it on a good run on the freeway & mountain roads. Temp stayed below 150. I haven't put the undercarriage shields back on so I don't know if that extra airflow is helping. Also outside temps were pretty low so I'm not 100% convinced it is resolved. I still need to check the fluid to see if it is burnt.

 

[Jack90210]

The OEM metal skid under the radiator actually helps with cooling by forcing more air through the radiator (instead of allowing it to flow around it), FWIW.

 

[flintknapper]

If the transmission lines are not getting hot it is possible there is a blockage in the lines preventing fluid to flow. One other member had a shipping plug that got sucked into a line. You would need to have each line pulled off and checked to ensure it does not have restrictions. Next would need to ensure the radiator transmission section does not have a blockage as well as the external transmission cooler. I would also second checking the transmission fluid level. Make sure to check it when it is hot.

That was me actually.

NEVER a problem with the transmission prior to that.

But clearly no flow to the radiator reservoir (temp gun and feeling by hand confirmed). Pulled the lines and I'll be damned.....a plastic piece (looked like some type of shipping plug) was lodged in the line.

 

[RoguesPierre]

So I finally found a mechanic who was willing to drive my rig around until it overheated. I think he actually took his family up to a local lake and used it as a daily driver while I was on vacay. He drained AT & added after market AT cooler. Seemed to help but after more testing over heated again.
Off hand I think he drained not flushed it, kept old fluid., it was blocking lines. Apparently ATF also cools through the radiator, he bypassed that with another 2nd cooler. BTW radiator was fairly new. No issues since then.