4L60E temp and sensor location

I put one in the pan on a '90 4L60 and one in the out line from the stacked plate aux cooler with a 3 position switch and one gage.
Interesting watching fluid temps out on the road all day runs.....in city driving....towing in 110 plus temps .......zero temps and towing vs non towing a small 8660 lb 5th wheel trailer.

On my '97 4L60E, I installed the probe in the pressure test port located just above where the shifter cable attaches to the trans (driver side). The Autometer gauge I used threaded right in without an adapter, and the gauge was mounted to the A-Pillar with a vehicle specific gauge pod.

The fluid is Dextron VI full synthetic. The fluid runs from the transmission to the radiator cooler, factory external cooler, aftermarket, then back to the transmission. More than likely will never see any temps below 0, hence this particular truck is not my daily driver.

You really want to know bulk temperature in the transmission sump/pan. If you place the sensor in transmission out as Bessie-Hunter recommended you will be measuring the temperature coming out of the torque converter which is the hottest oil in the transmission.

From a plumbing stand-point the in-take transmission cooler in the radiator should be the first cooler you flow through, than your oil to air coolers, then back to the transmission. If you flow the air to oil first you run the risk of actually heating your transmission fluid back up in the in-tank cooler. If you are running two external coolers I would be a bit concerned about the pressure drop in extreme cold, if you get an air to oil cooler cold enough (i.e. -30 to -40 fahrenheit you can get enough pressure drop that oil will not flow through the cooler. I am not overly familiar with the 4L60E but taking a quick look at the hydraulic I do not see a pressure bypass valve for the cooler circuit so if you get too much restriction in the coolers you can cut-off cooler flow.

What fluid are you running in your transmission? Newer fluids are more thermally stable (i.e. Dexron VI is more thermally stable than prior generations of Dexron). Older vehicles without a thermal bypass valve will run cooler unloaded than modern automatics (150 - 170 fahrenheit). Under heavy loads peak temperature should never exceed 248 fahrenheit. Towing on flat ground a stabilized temperature of 200 fahrenheit is acceptable, particularly if you are running Dexron VI.

Use any one of the transmission fluid pressure test ports.

I installed a external cooler and temp gauge on a 98 Safari. Had the temp sensor located on the line out of the trans to the external cooler.
The external cooler was first to get the hot oil (after the temp sensor) then into the factory cooler in the radiator. Pulled a 27 foot Surveyor (around 5600 lbs) for 8 years and never saw the gauge get over 210 when the torque converter was unlocked. Ran 180 on the long haul (always locked) and in 3rd gear (drive).

Drove the van for another 5 years not towing and the trans was quite and smooth as if it had never pulled anything. Did regular fluid and filter changes while towing. Would still have the van, but did not need it any more and the brake lines rusted out,