Todays electronic transmissions use sensors, solenoids and relays to regulate the operation of the clutches, gears and torque converter. On applications where these functions have not been integrated into the powertrain control module (PCM), there is a separate transmission control module (TCM) to oversee the operation of the transmission - and that is the focus of this article: how TCMs and PCMs interact.
Just like the PCM, the TCM needs accurate information to do its job properly. If the transmission computer receives bad information from the transmissions own internal sensors or bad inputs from the PCM or other engine sensors, it will have an adverse effect on how the transmission operates. It may not shift smoothly. It may shift at the wrong rpm. It may even go into a "limp-in" mode and remain frozen in second or third gear.
Essential Information
Electronic transmissions are getting smarter all the time. Most of the current generation units have fully adaptive control systems that "learn" the best shift points based on real-time sensor inputs and feedback. The transmission computer adapts the shift strategy to compensate for changes in engine performance and wear in the transmission friction elements in the clutches. By making subtle changes to the shift points and engagement, the computer tries to maintain consistent shift quality.
The transmission computer monitors what is happening inside the transmission with various speed and gear range sensors that tell it if the gears are shifting correctly and at what speeds. But to pick the shift points, it needs certain inputs from the engine and PCM. The transmission computer needs to know how fast the vehicle is traveling. This information is provided by the vehicle speed sensor. It also needs to know engine speed (rpm) and load.
On some applications, the rpm signal is hard-wired directly to the transmission computer as well as the PCM. There is a dedicated circuit between the crankshaft position sensor and TCM to supply the rpm signal. On other applications, the rpm signal goes only to the PCM and the PCM forwards it to the TCM via a data bus circuit.
Engine load can be determined from throttle position, intake vacuum and airflow. Depending on what type of fuel injection system is used (speed density or airflow), engine load inputs may come from the throttle position sensor (TPS), manifold absolute pressure sensor (MAP) and/or a vane airflow sensor (VAF) or mass airflow sensor (MAF). As with the rpm signal, the information may be shared directly with the transmission computer or it may go through the PCM and forwarded to the TCM over the data bus.
Effects of ATF Oil Temperature on the Transmission
Regardless of how the information is generated and how it gets there, the transmission computer juggles the various inputs to come up with the best shift points. But because automatic transmissions use hydraulics to operate the clutches and gear changes, oil temperature is another factor that needs to be considered to maintain consistent shift quality. An automatic transmission fluid (ATF) oil temperature sensor inside the transmission provides information the TCM needs to adjust shift pressures when the ATF is cold and when it is hot. If the thermistor that provides the oil temperature input is not working, the TCM can go to the PCM for help and estimate oil temperature based on engine coolant temperature and how long the vehicle has been running.
For example, on Chrysler 45RFE electronic automatics (which are used in Jeep Grand Cherokees and are similar to Chryslers 41TE and 42LE transmissions), the TCM can look at the engine coolant temperature sensor reading when the vehicle is first started and use that as a reference point for estimating the ATF oil temperature. Once this has been done, the TCM updates the estimated transmission oil temperature as the vehicle is being driven based on torque converter slip speed, vehicle speed, gear position and engine coolant temperature. The estimated oil temperature will be pretty close to the real oil temperature if the vehicle is driven normally and there are no other problems. But if the transmission is overfilled with fluid, the transmission oil cooler is restricted, the engine is running hot, or if the vehicle is driven aggressively in low gear, the TCM will underestimate the oil temperature causing the transmission to shift at the wrong points.
Chryslers 45RFE uses several "shift schedules" that are based on ATF oil temperature. There is an "extreme cold" schedule when the oil temperature is below -16 degrees F that allows Park, Neutral, Reverse, first and third gears only. If the oil temperature is between -12 degrees and 10 degrees F, it uses the "super cold" shift schedule that delays 2-3 and 3-4 upshifts, and provides an earlier 4-3 and 3-2 coastdown shifts. High speed 4-2, 3-2 and 2-1 kickdown shifts are prevented in this mode. When the ATF is between 10 degrees and 36 degrees F, the "cold" schedule takes over and the transmission shifts at higher throttle openings and high speed 4-2, 3-2 and 2-1 kickdown shifts are still prevented. Also, there is no torque converter clutch lockup in the cold, super cold or extreme cold ranges.
Once the ATF is above 40 degrees, the TCM goes to the "warm" schedule, which allows normal upshifts, kickdowns and coastdowns - but still not torque converter lockup. When the ATF reaches 80 degrees F, the TCM changes to the "hot/normal" mode and begins to engage the torque converter when vehicle speed is above about 22 mph.
If the ATF gets too hot (above 240 degrees F), or the engine starts to overheat (coolant above 244 degrees F), the TCM will employ an "overheat" schedule that delays 2-3 and 3-4 upshifts and changes the torque converter lockup points. If the ATF gets really hot (above 260 degrees F), the TCM goes into a "super overheat" mode which further delays 2-3 and 3-4 upshifts and prevents the torque converter clutch from unlocking above 22 mph unless the throttle angle is less than 4 degrees or a wide open throttle 2-1 kickdown is made.
The point here is that the temperature of the ATF as well as the engine coolant can have a major effect on how an electronic transmission operates. So too can the inputs from the throttle position sensor, crankshaft position sensor, MAP sensor and other engine sensors.
Chrysler says its 45RFE transmission can be put into an overheat or superheat shift schedule by anything that causes the engine to overheat, by aggressive driving in low gear, by towing a trailer in the "OD" (overdrive) position (using "3" is recommended if frequent gear shifts occur), by driving in heavy stop-and-go city traffic, or by an engine that is idling too fast (stuck AIS motor).
If the engine coolant temperature stays too low too long (due to an open thermostat or faulty coolant sensor), the TCM may go into a cold mode and prevent the torque converter clutch from locking up.
Effects of Bad Engine Sensor Inputs on the Transmission
Because the transmission needs to know engine speed and load as well as vehicle speed to pick the right shift points, a bad sensor input or loss of a signal can create real problems for the transmission computer.
The throttle position sensor signal takes the place of the throttle kickdown linkage on older mechanical automatics. So if the TPS is reading high or low, or has a dead spot, it can affect transmission kickdown shifts when accelerating, as well as normal upshifts and downshifts, too. If the TCM cannot get a good TPS signal, it may substitute a "calculated" throttle angle provided by the PCM over the data bus. Or, if this signal is not available, it may substitute a fixed value for the TPS signal. This will obviously affect the way it shifts (transmission typically hunts for shift points) but not necessarily cause it to go into a limp-in mode.
A faulty throttle position sensor will not always set a fault code. The PCM has to be smart enough to figure out when the TPS is working properly and when it is not. Its diagnostic strategies may compare the TPS signal against engine rpm, MAP signal and/or airflow to determine if the TPS signal makes sense. If the TPS signal does not correspond to other sensor inputs that can be used to measure engine load, the PCM may set a fault code. Then again, it might not. It all depends on the self-diagnostic strategy, how sensitive it is to faults and how easily the PCM can detect problems.
Transmission Limp-In Modes
Under certain conditions, which may include the loss of one or more vital inputs to the TCM, the transmission will go into some kind of limp-in or "default" mode. On Chryslers 45RFE transmission, there are three different default modes:
The point here is if the transmission is in a limp-in or default mode, something is wrong. It may be an internal transmission problem, an engine sensor problem, a data bus problem or even a problem within the TCM or PCM modules.
Transmission Diagnostic Trouble Codes
Like PCMs, TCMs have the ability to self-diagnose faults and set diagnostic trouble codes that can be read with a scan tool. So if the Malfunction Indicator Lamp (MIL) is on and the transmission is not operating properly, the fault may be in the transmission - or it may be in the engine. The only way to find out is to pull the code(s) to see what is going on.
It is not unusual to blame some engine driveability problems on the transmission and vice versa. A torque converter that locks up prematurely or fails to release quickly enough can cause a driveline shudder that may feel like an engine misfire or vibration. If the torque converter fails to release at all, it can cause the engine to buck and stall when coming to a stop.
Any time you encounter a problem with an electronic automatic or find a transmission code, make sure the engine is running properly and there are no engine codes that could affect the operation of the transmission. In other words, take care of any engine problems first before attempting to diagnose a transmission problem.
If a vehicle has a data bus communication problem between the TCM and PCM, you probably will not be able to access any transmission codes until the wiring problem is fixed. Possible causes include an open or short to the ground or battery in the PCI bus circuit, or an internal failure of any module or component that is attached to the bus.
The data bus is monitored any time the ignition key is on. If no messages are received from the PCM for 10 or more seconds, it tells the TCM something is wrong and it sets a data bus code. On Chryslers 45RFE, a data bus problem (codes P1716 or P1719) can cause the transmission to shift poorly, delay 3-4 shifts and prevent torque converter lockup.
If the transmission can find an engine speed signal from the crankshaft position sensor or PCM, the transmission can be forced into the limp-in mode. The problem should set a crank sensor circuit fault code and turn on the MIL. Possible causes include an open or short in the crank sensor circuit, a TCM connector problem, an open or short in the crank sensor ground circuit, or an internal fault in the TCM or PCM.
On OBD II applications, various transmission codes are included in the "generic" list of OBD II codes. If the transmission computer detects a problem that may affect emissions, it will send a request over the data bus to the engine computer to turn on the MIL lamp. A code will be set in the TCM and remain there until it is cleared or no fault is detected during 40 consecutive drive cycles. The MIL may go out but leave the code in memory if no fault is detected during three consecutive drive cycles.
Cover and feature art courtesy of ZF Friedrichshafen AG.
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