Toyota 4WD Surf Owners

A340F transmission on 1KZ-TE KZN185 surf. Transmission overheating and sluggish performance.

When I am driving at highway speeds (over 80-85km/h) or uphill for an 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, ATF temp light comes on. It happened to me twice this weekend: (1) after climbing to a mountain pass, had to slow down because of slower traffic on the way down - temp spiked and ATF temp light came within 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. In other words, as soon as temps started spiking, I managed to rev it up again.

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).

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 spill 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.

I think a symptom of the same problem is that 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? Any ideas? useful links?

While A340F is the same tranny as in the North American 4Runners, the TC part number is different - I am guessing because it mates to a diesel and thus has lower stall speed and torque transfer characteristics. Has anyone successfully installed a different TC (from a different Toyota or after market) on their 1KZ with A340F?

Things that have been checked out:
- ATF temp sensor - temp reported correctly
- ATF cooling loop - stock rad in series with an external cooler, plus a fan on the cooler (please, no "get rid of the stock rad" comments, unless you think this is somehow causing the issue)
- Stock radiator performance - no issues, coolant temp within normal range, I also just replaced oil in the fan clutch with 10,000 cst (175-185F under normal load, up to 205F on steep climbs)
- ATF flow - the shop said flow was fine, to spec
- Turbo - with boost controller, running up to 13.5 PSI (so not the cause of the sluggishness)
- EGT - measured pre-turbo, 800-900F on flat, keeping it below 1,100F on climbs