Is It the Transmission? Can You Feel the Heat?

May 1, 2023 | Expert Tips, News, Sponsored

 By Wayne Colonna

When it comes to automotive repair, and, in particular, automatic transmission repair, being on the diagnosing and repairing end of increasing technology is an incredible challenge.

When some unexplainable malfunction occurs with the way the computer controls the transmission, it can become a real nightmare on several levels. Keeping track of temperature can save you time and money when it comes to diagnosing a transmission (or possibly not a transmission) problem.

A device that is crucial to temperature monitoring is the Transmission Fluid Temperature (TFT) Sensor (Figure 1.) If there is a sensor malfunction, or does not set a code, it can alter systems operations.

Figure 1

At the start, the computer would use this temperature information primarily for converter clutch control. Once a predetermined temperature level was reached, the computer would apply the converter clutch.

Fast forwarding to today’s transmissions, the TFT sensor now has a broad range of influence and affects.

For example:

•  Compare Engine Coolant Temperature Sensor readings

•  Compare it to calculated “in gear” engine run time

•  Utilize two TFT sensors: one monitors sump temperature while the other monitors converter temperature.

•  Shift feel and scheduling

The TFT sensor also plays a role in a variety of failsafe controls. Transmission temperature is critical, not only for the life of the fluid itself, but for the life expectancy of the transmission. If the engine is overheating, the converter clutch may not apply to prevent the engine from being under load. Likewise, if the transmission is overheating, the converter clutch may stay applied longer in an attempt to cool it down.

Many transmissions didn’t have a TFT sensor originally, so it utilized in-gear run time since start, ambient temperature and ECT to determine TFT. They would then place the transmission shift logic into one of the following categories: Extreme Cold, Super Cold, Cold, Warm and Hot. Shift logic then varied to match the condition of the weather and the temperature of the fluid.

Rear-wheel drive transmissions use a temperature sensor within the original TFT. They added another shift logic category to the five just mentioned called Overheat. Extreme cold means anything below -27° C (16° F). The transmission will have reverse, 1st and 3rd gear only in Drive, 2nd only in manual 2 or L and no converter clutch. To the extreme, “Overheat” which means transmission fluid temperatures above 115°C (240°F) or Engine Coolant above 118°C (244°F.) The computer will delay the 2-3 and 3-4 up-shifts. Full converter clutch apply will only be available in 3rd gear from 30-48 mph with partial apply capabilities only above 35 mph.  

There is no doubt that the TFT signal has become an intricate part of the operation of transmissions today on so many levels. As important as this sensor is, it is so often times overlooked when they malfunction and do not set a code. Possible issues could include:

•  Improper clutch adaptations

•  Line pressure control

•  Shift scheduling

•  Delayed engagements,

•  Late shifts,

•  Flared shifts,

•  Hard shifts

•  Loss of high gear and/or TCC operation crop up

One way to inspect a possible TFT sensor issue is to compare scan data temperature readings and/or gauge readings (Figure 2) with an actual reading using an Infrared Thermometer Gun on the transmission pan and cooler out line (Figure 3). If the high temperature reading appears to be valid then there is a transmission malfunction. Overheat issues can be caused by a defective torque converter, a defective pump (turned stator shaft), or a problem in the cooling system. Such as a malfunctioning thermal bypass valve, heat exchanger, or radiator.

Figure 2
Figure 3

If the reading taken by the thermal gun is normal yet the scan tool reports a much higher temperature, an electrical malfunction has occurred either with the sensor itself, the wiring, or the computer. At this point the TFT sensor wire can be checked with a voltmeter to compare the voltage value with the actual temperature reading. If the voltage matches the temperature reported there is a defective sensor or wiring. If it matches the thermal gun conflicting with scan tool or gauge data, there is a computer issue.

As a closing tip, be sure to frequently check on temperature before going down any diagnosing rabbit holes. It might not always be the transmission’s fault!

Wayne Colonna has been with the Automatic Service Group for over 30 years. As President of ATSG and Director of Quality at ETE REMAN, Wayne is helping to usher the transmission industry to the highest of standards. When he’s not at ETE in Milwaukee, he can be found with his wife in Miami taking her out to eat, playing Spanish guitar, or maybe enjoying a bottle of fine wine.

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