Automatic transmissions without an oil cooler and no transmission fluid high temperature warning indicator can be damaged towing heavy trailers. What can be done to get around needing the towing package, and what situations make the towing package absolutely necessary?
Automatic transmissions use a torque converter to allow the engine to turn at a different speed than the transmission input shaft. This allows smooth shifting and it allows the engine to run at idle with the vehicle stopped. The problem is that this device is very inefficient at low RPMs and high torque which causes it to generate a lot of heat in the transmission fluid. Most all modern transmissions are equipped with a torque converter lock up clutch that prevents any slip in the torque converter. This allows the transmission to be as efficient as a manual transmission, but it usually only activates at cruising speed when the transmission is in the highest or next to highest gear. When the torque converter lock up engages it often feels like a 5th gear (on a 4 speed).
When towing a trailer, the engine will be operating at much higher power or torque than usual with the vehicle traveling at an unnaturally low speed given the amount of engine power. This situation gets much worse when going up a hill, especially when the transmission fluid is already hotter than normal from pulling the trailer while on a flat road. Other than using a manual transmission, the traditional method to solve this problem is to install a towing package, which usually consists of a separate transmission oil cooler, instead of using the regular oil cooler which is part of the engine cooling radiator. Another method is a transmission fluid temperature warning to alert the driver to stop and let the transmission cool off.
This information is based on a typical 4 speed automatic transmission which has a large gear ratio difference between 1st and 2nd. More modern 5 speed transmissions may be less susceptible to the 2nd gear overheating condition. The early to mid 90s Toyota Camry owner's manual says to not drive continuously while pulling a trailer if the transmission is staying in 1st or 2nd gear, but it doesn't say why. The Volvo 850 has a temperature sensor on the transmission so that it can down shift to reduce heat. I'm going to tell how damage occurs and how it can be prevented without needing a towing package. This has the benefit of increasing fuel economy as well.
How transmission overheating can occur:
Transmission fluid overheating and damage primarily occurs when the transmission is operating in 1st or 2nd gear. Overheating can occur in 1st gear when going slowly up a steep hill, for instance when you are stuck in traffic that is creeping along at only 5 MPH. The added weight of the trailer means that the engine is putting out much more power than normal for this particular speed, and much more heat is being generated in the torque converter. The other situation is when you are going somewhat slowly up a hill in 2nd gear, but not slowly enough for a down shift in to 1st, due to the large difference between 1st and 2nd gear. The vehicle would normally accelerate up to a safe speed but it can't due to the trailer's weight. Speed up in 2nd gear or slow down and get in 1st gear to prevent this.
When in 3rd or 4th gear on the highway, the transmission may run hot reducing your fuel economy, but it shouldn't cause damage. The increased torque will often cause the torque converter lock to disengage. If this happens, engage the overdrive cancel feature. On most vehicles this will allow torque converter lock up to occur in 3rd gear, which is now the highest gear. This can have a large impact on fuel economy, often over 25% improvement with the transmission now running cool.
How to prevent overheating:
This simple rule is to keep RPMs up. When going up a hill, keep the engine above 3000 RPM and downshift the transmission as needed. It may seem like driving slowly up a hill is better for the vehicle, but quite the opposite is usually true. Doing this will keep the torque converter away from dangerous low speeds where excessive heat is generated while harming fuel economy.
If traveling up a long or steep hill in a vehicle that is not equipped with tachometer, then slow down to around 40 MPH and shift in to 2nd gear *. Maintain speed between 35 and 60 MPH. If it is not possible to maintain 35MPH, slow down to around 20 MPH and shift in to 1st gear *. Maintain speed between 18 and 30 MPH.
If you are stuck in traffic going up a hill, make sure you let off on the gas at least half of the time. DO NOT creep along. Move forward while counting the seconds during the time that you are going less than 18 MPH. When you stop, leave your foot off of the gas for at least that many seconds.
* Note: always check with your owner's manual to verify what speeds are permissible when down shifting in to 1st or 2nd gear, as well as what the maximum driving speed is for a particular gear.