Tips to Prevent Transmission Overheating

Medium and heavy duty truck transmissions undergo a lot of stress and abuse throughout their lifetime. With the elevated loads that the transmissions undergo, an abundant amount of heat is generated due to higher internal friction. If heat generation is left unchecked, it can lead to complete failure of the transmission.

With this in mind, there are several root causes of heat-related failures, some of which depend on the type of transmission being diagnosed (manual, automated, automatic).

Low Fluid Level


Graphic 1 – Example of proper lubrication level information in TruckSeries.

Low fluid level can result in increased friction due to inadequate lubrication, which generates more heat. It also reduces the efficiency of the cooling system because the lower amount of fluid means a lower capacity to absorb heat. If a low fluid condition exists, the source of the leak should be identified and corrected. See Graphic 1 (at right – click images to expand to full size) for an example of proper lubrication level information.

Excessive load

Excessive load can cause overheating when the transmission endures load levels beyond what it was designed to handle. Extra load through the transmission generates more heat than the cooling system has capacity for, therefore overheating can occur. Terrain can also affect heat production when the vehicle encounters steep grades frequently. For applications that do not originally include a fluid cooler, one can be fitted to help bring operation temperatures back to an ideal range. See Graphic 2 – below.

Specific Cases for Automatics

Automatic transmissions may see additional overheating symptoms occur. In an automatic transmission, the pump that provides clutch pressure for transferring torque also handles lubrication and cooling flow duties. Pump failure can cause clutch slip because of reduced operating pressure. The extra slip contributes to even more heat, and with cooling system capacity reduced, the unit can quickly rise to critical operating temperatures. See Graphic 3 – below.

The torque converter in an automatic transmission also produces a significant amount of heat. This mainly occurs during standstill where the most slip occurs. Conversely, during driving, a faulty lockup clutch can allow the torque converter to continue to slip, causing excessive heat production.

Graphic 2 – Example of lubrication and temperature information in TruckSeries

Graphic 2 – Example of lubrication and temperature information in TruckSeries



Graphic 3 – Example of transmission service information in TruckSeries.

Controlling heat is a critical factor in getting the most service life out of the transmission. Proper service and maintenance are also important to ensure your transmission can properly control heat production at full capacity. Mitchell 1 TruckSeries provides information to properly diagnose and service various types of transmissions in Class 4-8 vehicles, assisting technicians to maintain the transmission system for optimal performance throughout its life.

Read more:

Posted in:
About the Author

Julius Hairston

Julius Hairston joined the Mitchell 1 commercial vehicle group in 2012 as associate editor and is currently a technical editor for the TruckLabor product. He is certified as an ASE Master Technician for medium/heavy truck and worked as a technician for three years prior to joining Mitchell 1. He holds an associate degree in automotive technology from Cuyamaca College and a bachelor's degree in business administration from Everest University.