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.
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When it comes to automotive repair, and in particular automatic transmission repair, being on the diagnosing and repairing end of this increased 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. You feel the heat from the customer who wants their vehicle and all the while you are not getting paid for the many diagnostic hours you have put into to resolving the problem. And when it is discovered that a hidden corroded connector was the cause of the malfunction, than what do you charge?
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.
Generally speaking, the computer would use this information primarily for converter clutch control. Once a predetermined temperature level was reached, the computer would apply the converter clutch. As each manufacturer began to use these sensors, they each had similar reasons but they also introduced their own unique programming. For example, soon after TFT controlled converter clutch apply hit the streets, they began to have a limited affect with shift scheduling. Some would prohibit high gear when cold while some would also delay the up-shifts. One reason for a delayed up-shift when cold was to heat up the catalytic converter a little quicker. As technology advanced to the place where pressure inside the transmission became electronically controlled, temperature information influenced the control for this as well.
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.