If you have lived life for any reasonable length of time, you have come to learn that technology in general waits for no one. It seems that as soon as you buy a new electrical device, the next day there is a new and improved version being marketed. It is the same with today’s vehicles. They have become a self-contained mobile network offering up a wide variety of services such as personal comfort, hand free communications, safety via airbags, ABS and traction control, custom tailored shift feel and scheduling and the list goes on. From years ago having two-speed automatic transmissions controlled mechanically and hydraulically to nine- and 10-speed transmissions being completely computer controlled. Once you buy a new car with 10 speeds, the next year it will have 12! It’s crazy and at times it seems as if it is getting out of hand but it has become the norm in the days we live.
Another way this type of heat can be felt is with a sensor malfunction that for one reason or another does not set a code yet it alters system operations. One such device is the Transmission Fluid Temperature (TFT) Sensor. Back in the day when transmissions were not computer controlled, this sensor did not exist for obvious reasons. But when they did make their first appearance, the use of this information was quite limited. Actually, if I remember correctly, they began as a switch and quickly upgraded to a sensor.
Fast forwarding to today’s clutch to clutch six-, eight- or 10-speed skip shifting transmissions, the TFT sensor now has a broad range of influence and affects. Manufacturers use it for a variety of reasons utilizing different methods, strategies and self-system checks. They compare Engine Coolant Temperature Sensor readings with the TFT sensor, they compare it to calculated “in gear” engine run time, they may utilize two TFT sensors; one as a main input while another is redundant. Or one monitors sump temperature while the other monitors converter temperature. As a side note, in some applications Ford has deviated from the TFT sensor being the primary input for converter clutch control and is using a cylinder head temperature sensor instead. This is just another example of a different strategy and control by a manufacturer.
Besides shift feel, shift scheduling and converter clutch control, the TFT sensor plays a role in a variety of failsafe controls which too is manufacturer dependant. Transmission temperature is critical, not only for the life of the fluid itself but it ultimately increases the life expectancy of the transmission. Heat is not a good thing for the engine or the transmission and so great measures are taken to control both. 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. Again each manufacturer has their individual programming strategies offering up different methods in accomplishing similar temperature control.
BMW 3 and 5 series have a thermal bypass valve above the heat exchanger (Figures 1-3). When this fails causing ATF temperature to
increase, a strategy kicks in that extends the shift patterns as well as prohibiting a shift into high gear (5th). This may or may not set a code.
Late model Hyundais such as a Sonata with low mileage starts having harsh engagements into gear described by the driver as a clunk followed by a sudden surge in forward or reverse. Intermittent harsh shifts, some worse than others may accompany the complaint and may or may not set any TFT codes yet the sensor is bad.
There are two temperature sensors used on AS68RC transmissions in Dodge incomplete chassis vehicles behind 6.7L diesels. Depending on which sensor fails the transmission may experience an early full apply of the converter clutch. This complaint may also be accompanied with no PTO operation. Or, the complaint is a loss of 5th, 6th and TCC apply.
When one or both of these sensors malfunction a diagnostic trouble code is suppose to be stored. The problem is that there have been reports of these sensors failing and causing the effects, yet the TCM does not set the codes making the cause of the malfunction elusive.
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 volt meter (Figures 10 and 11) to compare the voltage value with the actual temperature reading. Typical voltage range is as high (cold) as 4.94 volts to as low (hot) as .078 before the computer should set a code. 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. In situations like this, there may be a day when working on a vehicle that you might discover a resistor placed into the TFT sensor circuit. This is a practice done to fool the computer into thinking temperatures are less exists rather than replacing the computer.
As a closing tip, for those that have heavy duty trucks and works them hard, for s additional cooling to help keep transmission fluid temperatures down, take a look at custom pans, such as those from PML.