Jim Grant's Tech Tips

'98 Chevy S-10, Overheating Transmission

Q: I have a '98 Chevy S-10 pickup and the engine light comes on when the transmission heats up. I have replaced the transmission fluid, filter, gaskets twice and it lasts only about a day before the light comes on again. The truck only has 47k miles on it and runs perfectly otherwise. It also won't shift into overdrive when it heats up, stays in 3rd gear. Any suggestions?"

A: The first and most important question that has to be answered, is the transmission really overheating? If so, what is causing the overheat problem? Heat is the biggest enemy to an automatic transmission. The overheated transmission oil transforms from a lubricant and fiction modifier to an acid with built in sand paper. Burnt transmission oil just tears the inside of a transmission apart. Because your transmission is computerized there is a possibility that it is not overheating. It could be the Transmission Fluid Temperature (TFT) sensor or wiring for TFT sensor that has a problem. The computer for the transmission has what you could call a fail-safe strategy to minimize damage to the transmission in the event the transmission is overheating. The strategy is what you're experiencing when each shift becomes more pronounced and the transmission is prevented, by the computer, from using all the gears. This strategy is in an effort by the computer to minimize shifting to reduce transmission temperature. You must have this problem diagnosed and determine as mentioned before if it is a faulty sensor and/or wiring or if the transmission is, in fact, overheating. If the transmission is overheating then the transmission cooler should be inspected. Check lines from the transmission to the cooler, be sure they are not bent/kinked which would restrict fluid flow to the cooler. The cooler should be checked for blockage and for flow rate. In any event do not delay, overheating your transmission is going to put your check book in the red.