Help!!Transmission overheating problem has me stumped 😞

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?

Curious, any symptoms of actual overheating, or is this strictly off some type of gauge? Is the external trans cooler a new install?

The trans cooler has been there for a while, but the issue has gotten worse lately. No other symptoms - that said, what else would I see from overheating?

I did actually overheat the transmission last year to the point that the ATf temp light came on and I lost ATF through breather holes. But that was due to a blocked line. I flushed out the old ATF, replaced the filter, and put in new synthetic fluid. There were no shavings in the pan so I figured I didn’t do any damage.

What line was blocked?


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Smell and noise (gears)... What you said from before, an ATF temp light too.

Was thinking a circulation problem possibly.

Whatever was blocked before may be blocked again. If it is, something is happening and keeps plugging things up.

What are you using to read tranny temps? My glowshifts temp sender shit the bed a couple weeks ago but mine was reading zero temp.

Have you noticed a significant loss in MPG? Maybe the transmission is fine but something is bound up and it's having to work extra hard.

You could try disconnecting the line after your external cooler and see how the flow is when the truck is running. If it's just dribbling out, its plugged up again somewhere. It takes approximately 10 seconds for these trucks to pump a quart of fluid while in park, you can use that for reference.

If it's a circulation problem (and I agree it sounds like that), then it's unrelated to the previous blockage. That was purely my idiocy: I tried my setup with external cooler only, and blocked the rad ports with little screws to avoid dirt getting in. I then decided to go back to having both rad and cooler in the lop, but didn't take out the screws

I am using an AutoMeter guage to read the temps, and I am considering the possibility that the t-junction where the probe sits may be causing some kind of flow impediment.

It does also feel like I've lost some MPG lately. Where and what do you think could bind it?

What I was trying to get at earlier is if you are really overheating. Gauges and gauge installs can produce bad readings if the right parts are not used, not installed correctly or the gauge fails of course. If you know of someone who has a ScanGauge II or comparable, that would be nice to try as well...

I would first try to confirm that are you are truly running hot ATF.

Unfortunately, my truck is a 1996 diesel surf, they didn't have ODB2 until 2000 or 2002

I will be trying another temp sensor and will be investigating blockages.

If you have a temp gun check your pan temp.


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I drove my new trd pro on the beach in 4 high gear for about 4 miles. Did not take air out of tires. Got “transmission”fluid running hot “or overheating”
I switched to 4 low and it went off. Could this mean I need to let air out of tires?

It means you need to use 4 low instead of 4 high. The gear reduction of 4 low helps your transmission not work as hard and the fluid temperature will stay lower vs 4 high.

Yeah that torque converter and lugging in low RPM's really ramps up the transmission temperatures. Using a lower gear (shifting to L or 2) and/or using 4 low like mentioned above makes a big difference.

Same goes for driving up steep grades on highways. Turning off overdrive allows the engine to not work so hard and reduces both engine and transmission temps.