Can you feel the heat?

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.

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.

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.

One of the transmissions that comes to mind which utilizes transmission fluid temperature for a variety of different reasons is the 45/545/68RFE RWD transmission in Dodge, Chrysler and Jeep vehicles. In fact the strategy used in these transmissions date as far back as 1989 when they came out with the 41TE (A604) transmission. This transmission didn’t have a TFT sensor originally so it utilized calculated 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. Much of this same strategy is in the previously mentioned rear wheel drive transmissions.

They do use a temperature sensor, which is inside the transmission range sensor. They added another shift logic category to the five just mentioned called Overheat. What is quite helpful is that Chrysler offers the description of what to expect with each of the five temperature related shift logic. Extreme cold means anything below -27° C (16° F). The transmission will have reverse, 1^st and 3^rd gear only in Drive, 2^nd only in manual 2 or L and no converter clutch. To the extreme, “Overheat” which means TF 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 3^rd gear from 30-48 mph with partial apply capabilities only above 35 mph.

Because this is a clutch to clutch shifting transmission, adaptations are a part of the computer strategy which can be observed in data parameters called CVI, Clutch Volume Index. After repairs, there is a scan tool option called Quick Learn that allows to the computer to quickly determine the approximate volume of fluid it takes to apply a clutch. The vehicle then needs to be driven for it to fine tune quality learned shifts. For a particular objectionable shift there are specific learning procedures which must be performed according to specific temperatures. For example, the procedure to learn a smooth P/N to Drive shift must be performed with temperatures between 27-43°C (80-110°F). Yet to perform a learn procedure for a smooth up shifts temperatures need to be above 27°C (110°F).

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. The variety of problems they can produce can drive you crazy. Improper clutch adaptations, line pressure control and shift scheduling are all up for grabs. Delayed engagements, late shifts, flared shifts, hard shifts, and a loss of high gear and/or TCC operation crop up and can be intermittent to boot.

It does help when there is a code or a message display that clues you in. Some Range Rovers have an over-temp strategy that initiates when it sees ATF temperature exceed a preprogrammed threshold. Once initiated, the driver will notice late shifting and perhaps a sudden neutralizing during highway driving along with a Transmission Overheat message appearing in the instrument cluster. The overheat problem in this case is typically caused by a failed radiator where the lower eight tubes supplying coolant to the transmission cooler gets restricted preventing heat dissipation. The over-heat temp strategy designed to cool the transmission is done by significantly increasing the speed of the shifting points in order to keep engine rpm and cooling to an optimum.

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 (5^th). 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.

TFT Sensor A is located in the valve body sump area as seen in Figure 4, while TFT Sensor B is located in the To Cooler fitting on the transmission (Figure 5). TFT Sensor B monitors converter out temperature which is typically hotter than the sump which TFT sensor A monitors. If an overheat signal is sent to the TCM, the computer strategy is to fully apply the converter clutch at slightly above the 1,200 rpm range. PTO is also disabled. Likewise, if the sensors provide a cold reading to the TCM, the computer strategy is to prohibit 5th, 6th and TCC apply function.

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.

One way to inspect a possible TFT sensor issue is to compare scan data temperature readings and/or gauge readings (Figure 6) with an actual reading using an Infrared Thermometer Gun on the transmission pan and cooler out line (Figure 7). 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 (Figure 8), heat exchanger (Figure 9) or radiator.

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.