UNDER THE HOOD
Watch your transmission’s temperature when hauling trailers
Q:My wife and I took the plunge and bought a fifth-wheel trailer and a truck to tow it. Everything was supposedly chosen to work seamlessly together and, so far, it tows great. My question is about the transmission temperature gauge. I’ve never had one before and wonder what to look for when towing. —Kirk M.
A: Sounds like a lot of fun! The added weight and drag of your trailer will increase transmission temperature, especially in hilly terrain. Normal transmission fluid temperature is about 160-175 degrees. It might rise as high as 200-220 degrees under demanding conditions but hopefully it won't go any higher. Transmission fluid degrades rapidly at higher temperatures. Cooked fluid and high operating temperature are transmission killers!
If you are ascending steep grades and find the transmission fluid temperature exceeding 220 degrees, I'd consider pulling over and enjoying at least a half hour of relaxation with the engine idling and transmission in park. Also know that downhill stretches will cool the transmission more quickly than being parked or idling.
Automatic transmissions typically employ a liquid transmission cooler within the engine-cooling system radiator. This allows engine heat to bring the transmission temperature up to the correct range quickly and then does a fairly good job of maintaining fluid temperature under moderate towing conditions. Depending on your truck's powertrain options, there's a good chance it may be equipped with an auxiliary transmission cooler (a finned rectangular device mounted in front of the radiator/air conditioning condenser). If not, you may want to consider adding one.
If you find the transmission frequently working at temperatures higher than desired, a larger auxiliary cooler, with fan, and perhaps a deep sump transmission oil pan (adds about three quarts to fluid capacity) may help. Adding a drain plug to the oil pan—if it doesn't already have one—and doing at least a partial fluid change every 15,000 miles can't hurt (about a third of the fluid drains out through the pan).
There's a good chance your truck is equipped with a tow-haul feature. Using it in hilly terrain will ease strain on the engine and transmission, as the software orders quicker downshifts, holds gears longer, allows differing operating characteristics of the torque converter clutch, and enhances engine braking on downgrades or when slowing. Tow-haul does decrease fuel economy a bit, so you may find it's best left off unless conditions warrant its use. ABOUT THE WRITER
Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at bradbergholdt@ gmail.com; he cannot make personal replies.