There is no denying that heat kills auto- matic transmissions faster than a coronary will take out a fat man who has won free unlimited doughnuts for life. We’ve talked a lot about transmission coolers, but not so much about what is too hot or how to monitor the transmission. The fact of the matter is that you might need to add an auxiliary cooler, but the only way to be sure is to first install a gauge and monitor just how hot your junk is getting. This, of course, opens a whole other can of worms. Where do you put the sensor? How hot is too hot for your transmission? Well, read on and we’ll sort out the mysteries.
You really only need one gauge, but Trasborg runs two full-sweep Auto Meter gauges in his ’98 Cherokee. The red-backlit gauge reads the temperature right as it comes out of the transmission and the green-backlit gauge reads it after the cooler. If you are only running one gauge, then put it in the transmission pan since that is the fluid that the transmission is drawing from. Many aftermarket pans offer locations for temp sensors as well as offering more fluid capacity. Otherwise, we recommend installing it in the line before the fluid returns to the transmission.
The AW4 transmission can be found in ’87-’01 XJs and the ’93 ZJ with an inline-six and has an operating range of between 125-176 degrees. Most other Jeep autos need to run between 150-200 degrees. That includes the TF727 that was used in ’80-’92 Jeeps behind the V-8; the TH400 found in full-size Jeeps from the late ’60s through 1979 and in ’76-’79 CJs; the TF999 that was frequently found behind the 258ci inline-six from 1980-1991; its descendant, the 32RH, that came behind the 4.0L up through 2002; and finally the ’03-’11 42RLE four-speed.
Normally we’ll see an approximate 50-degree drop across our B&M SuperCooler, but 70-80 degrees isn’t uncommon when really working the transmission. If you see over 200 degrees on your gauge, you don’t need to panic or immediately pull over and stop. Just bear in mind that the longer it runs over 200 degrees (or 176 degrees for an AW4), the shorter the life of the fluid will be. The severe-duty maintenance schedule for our ’98 Cherokee suggests changing the automatic transmission fluid every 12,000 miles, but we usually end up doing it around every 10,000 miles (every other oil change) because we frequently see temperatures over 176 degrees.
As shown here, there is a direct correlation between how hot the transmission gets and how long it is expected to last. Even synthetic fluids are subject to this kind of accelerated wear, and while the mileage values might bump up, all fluids will break down if you get them hot enough-so keep it cool.