by Jim Kerr

Summer holiday travelling can be very hard on your automatic transmission. I recently talked to a couple of automotive technicians working in Kelowna, British Columbia, about transmission problems, and found them to be busier than ever repairing transmissions for holiday travellers. Here is why:

Pulling a trailer or climbing long grades can quickly overheat the transmission. When the transmission fluid becomes overheated, it can no longer lubricate all the moving parts inside the transmission, and the parts begin to fail. The only way of preventing this failure is to keep the transmission fluid cool.

There are two main reasons the fluid heats up inside the transmission. One, is the heat of the engine transfers to the transmission fluid. This heat is passed through the engine coolant to the transmission cooler located in the radiator. Normally, the transmission cooler works to cool the transmission fluid, but if the engine is running hot, then the transmission will operate too hot as well.

Adding an auxiliary transmission cooler will help to keep the transmission operating at a safe temperature, but it only helps. The engine must also operate at the proper temperature. Keep the radiator free of bugs and leaves to allow cool air to flow through the fins. A bug screen may be necessary in some areas to prevent the fins from becoming plugged quickly, but the screen also blocks a lot of airflow through the radiator. Only use the bug screen if necessary. If your vehicle operates on the high end of the temperature scale, then try removing the screen.

Any object in front of the radiator can block a lot of air. I often see trucks towing trailers that have bicycles or the spare tire mounted in front of the truck. Even the license plate can deflect a lot of cool air from the radiator if it is mounted directly in front of an opening. Usually these items can be moved to a different location to aid in airflow through the radiator.

Keeping your engine in a good state of tune will also reduce the amount of heat it produces. If one or two engine cylinders are misfiring, then the gas pedal must be pushed farther to the floor to keep the vehicle going. This puts extra load on the working cylinders and produces more heat. Correct ignition timing and air/fuel mixture also help to keep the engine operating cool. The use of fuel injection on vehicles has greatly reduced the number of vehicles that operate with an incorrect air/fuel mixture, but the ignition timing can still go out of specification on many vehicles due to wear in the engine.

A second cause of the transmission fluid heating up is the heat generated by the torque converter inside the transmission. A torque converter is like a fluid clutch. It slips internally to allow us to bring the vehicle to a stop without shifting the transmission into neutral or stepping on a clutch pedal. The torque converter also internally multiplies the power of the engine approximately two times when it is slipping at its maximum. This occurs when the vehicle is moving slowly and the driver steps on the gas pedal to accelerate fast.

The easiest way to visualize a torque converter operating, is to imagine two fans facing each other. One fan is blowing (this is like the part of the converter connected to the engine) and the other fan is being rotated by the air flowing past it (this is like the part of the converter connected to the transmission). If both fans are turning at close to the same speed, there is little resistance to airflow (transmission fluid flow in the converter) but if the second fan is held still, the air resistance (fluid flow in the converter) is high. This generates a lot of heat!

The driver is the biggest factor in controlling the amount of heat generated in a torque converter. The slower the vehicle is going and the more you step on the gas pedal, the more heat generated. When pulling a trailer, climbing even a short hill can generate a lot of extra heat inside the torque converter. The transmission fluid temperature will quickly rise at least 5 degrees C even if you do not step on the gas pedal any harder. Climbing a long hill or trying to maintain your speed by stepping hard on the throttle can cause the torque converter to generate so much heat the transmission fluid overheats, breaks down, and causes transmission failure. The transmission technicians in British Columbia told me they have noticed 10 times as many transmission failures since the completion of the Coquihalla toll road in that province. This highway has some of the longest and steepest uphill climbs of any road in North America and improper driving methods can quickly destroy an automatic transmission.

The proper way to operate an automatic transmission when pulling a heavy load or climbing long hills is to shift down at least one gear range. Let the lower mechanical gear ratio inside the transmission increase the engine’s torque and power to the rear wheels rather than letting the torque converter do all the work. Keep the engine speed up. If you find yourself stepping more than half way on the gas pedal to maintain vehicle and engine speed, then you should be using a lower gear. Use less throttle, a higher engine speed, and lower gear ranges in your transmission to make it last a long time.

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