Modern electronic transmissions use sensors, solenoids and relays to control 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 will be a separate transmission control module (TCM) to oversee the operation of the transmission – and that is the aim of this article: how the TCMs and PCMs interact.
The same as the PCM, the TCM requires accurate information to do its job properly. If the transmission computer gets bad information from the transmissions own internal sensors or bad inputs from the PCM or other engine sensors, it will have a bad effect on how the transmission functions. 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.
Electronic transmissions seem to get 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 the engine performance and wear in the transmission friction elements in the clutches. By making subtle changes to the shift points and engagement, the computer attempts to maintain consistent shift quality.
The transmission computer checks 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 requires certain inputs from the engine and PCM. The transmission computer has to know how fast the car is traveling. This information is given by the vehicle speed sensor. It also has 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 crank position sensor and TCM to give 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 also be determined from throttle position, intake vacuum and airflow. Depending on what type of fuel injection system is in use (speed density or airflow), engine load inputs may be 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 pass go through the PCM and forwarded to the TCM over the data bus.
What are the Effects of Bad Engine Sensor Inputs on the Transmission
Because the transmission has 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 issues for the transmission computer.
The throttle position sensor signal takes the position 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 impact on the 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 offered, 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 functioning 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 dependent on the self-diagnostic strategy, how sensitive it is to faults and how easily the PCM can detect issues.
Transmission Limp-In Modes
Under certain conditions, that 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. When a serious problem is diagnosed (such as loss of an internal speed sensor signal) or a problem happens in the wiring circuit to any of the shift solenoids, the TCM will kill the power to the transmission control relay and deenergize all of the shift solenoids. This often causes the transmission to freeze in 2nd or 3rd gear. The transmission will remain in limp-in mode until (1) the problem is diagnosed and repaired, or (2) power to the TCM is momentarily turned off to “reset” the computer. This may restore normal operation temporarily, but as soon as the TCM detects the fault again, it will go back into limp-in mode.
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 functioning 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 happening.
It is not unusual to blame some engine driveability issues 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 make the engine to buck and stall when coming to a stop.
Any time you encounter an issue with an electronic automatic or find a transmission code, make sure the engine is running correctly and there are no engine codes that could impact the operation of the transmission. In other words, take care of any engine issues first before attempting to diagnose a transmission problem.
If a car 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 issue 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 part that is attached to the bus.
The data bus is monitored any time the ignition key is on. If no messages are gotten from the PCM for 10 or more seconds, it tells the TCM something is wrong and it sets a data bus code.
If the transmission can get an engine speed signal from the crankshaft position sensor or PCM, the transmission can be forced into the limp-in mode. The issue 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 an issue 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.
|Steps for Troubleshooting An Electronic Automatic Transmision
The recommended steps for troubleshooting an electronic automatic transmission issues involves seven steps: