I have previously posted about ATF temperature spiking in my A340F transmission when I let go of the gas. I think I narrowed it down to a specific issue - the torque converter (TC).

In short, when I am driving at highway speeds (over 80-85km/h) or uphill for any extended period of time, ATF temp is generally fine: 150-200F, measured by a sensor in a t-fitting installed on the flex portion of the outlet pipe.

Once I release the gas to slow down or coast downhill, ATF temp quickly spikes by 50F or more, and if I don't rev back up quickly enough, the ATF light comes on (I believe that happens at around 275F). It happened to me twice this weekend: (1) I was climbing to a mountain pass and, once I reached the top, I had to slow on on the down slope because of slower traffic - temp spiked and ATF temp light came within a 30 seconds; and (2) driving at about 100 km/h on flat highway, had to suddenly brake because of traffic, again, had to pull over to cool it down. Otherwise, I managed temps by keeping it in O/D and lower gear to always have a load on the tranny.

I had the tranny looked at by a shop last week and they didn't find any issues. Flow rate checked out fine, as did the ATF fluid and the filter (this was before my recent drive through the mountains).

Here is what I think. As long as the transmission is under load, the Torque Converter (TC) is engaged. The TC uses ATF as a fluid coupling between the impeller and the turbine. As soon as I let go of the gas, the TC disengages, and the ATF from the TC is released into the housing. ATF inside the TC is generally the hottest ATF in the transmission - hence why the outlet pipe and the stock temp sensor are located right by the TC outlet. What seems to be happening is that the ATF inside the TC is way hotter than it should be, and that's what I see am seeing.

This is reinforced by the fact that, lately, I've felt like the truck has become quite sluggish, as if the transmission is not transferring enough power. Of course, if the TC is not transferring power (kinetic force) from then engine to the transmission, power is lost as heat.

So, has anyone had similar issues with their transmission or the TC? Any ideas? useful links? Can the TC be replaced by itself without replacing the entire transmission?

I did overheat my transmission a year ago when I had blocked flow, could it have damaged the TC?

Things that have checked out OK:
- temp sensor - temp reported correctly
- cooling loop - stock rad in series with an external cooler, plus a fan on the cooler. I also just serviced the fan clutch on the rad (please, no "get rid of the stock rad" comments, unless you think this is somehow causing the issue)
- ATF flow - the shop said flow was fine, to spec.

You can just replace the TC, but it'll almost be the same price as a used transmission, which you have to drop completely anyway.

My money's on the TC is either damaged from the heat or blocked itself. That isn't normal operation, for some reason that fluid is not flowing through the TC like it should.

I would get a second opinion from another transmission shop.

Worst case scenario, find a low mileage used unit and swap it in. It's not too bad of a job.

PS. you could try putting a new torque converter in it but it would really suck if it didn't fix it and you had to do all that work over again (I know that is obvious).

Alas, my truck is a Hilux Surf with 1KZ-TE engine. While it uses the same A340F A/T as the 4R, the torque converter is a different part number. I am guessing that it has slower stall speed and somewhat different torque conversion parameters because of the low RPM / high torque of the diesel engine.

Your sure the heat exchanger in the rad is not partially blocked?

There should be no difference in you coasting or you sitting at a stop. That temp increase may be heat soak in the trans with marginal flow through the heat exchanger.


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Yes, flow checked out fine through both the rad and the cooler.

You are right, there should be little difference between coasting or sitting at a stop. And that's the issue I have, temp spikes as soon as I am coasting or coming to a stop (see problem with stopping from 100km/h because of traffic). With flow being to spec, other than the TC disengaging and releasing really hot ATF, I can't see what else would be causing the issue.

How is your (Throttle Valve) kick down cable? Is it adjusted properly?

Weird, that's for sure.....


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There is a difference in torque transfer between the input side spinning faster or slower than the output side of the torque converter. They will have different torque multiplication/transfer characteristics...

-Charlie

But think about it.

The PO saying he is trail throttle (coasting) and his temps spike is beyond normal.


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So what was determined? I have a new to me 2000 4Runner. Hooked up a scanguage and was surprised to see the same thing - temp spikes when coasting. Not stoked about this.

Thanks!

What kind of spikes?
What's the normal temp?
What does it spike to?

external trans cooler? Radiator trans cooler in use?

I'd have to agree with it being the TC. Everything that you're describing is pointing to that.

Running around town, AT temps roughly 140-170, with minor variability. Ascending Hwy 2 Stevens Pass (Washington), saw 240. Began coasting down the descent and the temp spiked at 260! Was impressed the AT Temp light didn’t come on. Temps settled around 200. Truck was loaded with maybe 100 pounds of gear and myself. Didn’t detect any slippage/loss of power and no hard shifts. Weather was 95 and I was running the AC, but my coolant temp never went above 190.

The concerning symptom is driving on the highway; 60-70 mph I’ll see AT temp at 180, will take my foot of the gas, and AT temp will rise to 200, or more. Press the gas, and in 20 or so seconds, back to 180 degrees. Coolant temp at a Constance 186-190.

Truck is a 2000 limited. Stock tire size, moderate OME setup, and an ARB bumper. Roughly 190,000 miles. Installed a new spectra radiator and had the dealer flush/drain the transmission roughly two weeks ago. Had the original radiator prior with no issues. Have not bipassed the transcooler with an external cooler. No loss in transmission fluid.


Appreciate all and any insight!

The guys on here are very sharp when it comes to diagnostics and repair. I am not one of them, but I can read sign. I am pointing to the tranny flush.