One of the first you need to know when diagnosing and fixing any transmission is to understand how it works. Once you understand what does what and when it’s supposed to do it, you’re more likely to know what to check first and what you don’t need to check at all.
It’s never any fun to get the transmission on the bench, only to find nothing wrong internally. But if you know what the component is and what it’s supposed to do, then you can move forward to diagnosing and fixing the problem.
Transmissions in today’s cars are more complex and require a more detailed diagnosis before you remove them.
FIRST THINGS FIRST
Always start by determining if there are any codes present in any of the vehicle’s computers. Check for codes, write them down, clear them, and see whether they come back.
Even if a code doesn’t return right away, that doesn’t mean the problem disappeared. It may take several key cycles before a code returns, or it may require some very specific driving conditions that you haven’t met.
Today computers look at more than just TPS and VSS to control the transmission’s shifts. Did you know that the wrong type of fuel could affect how your transmission shifts?
For example, if you were to put E-85 fuel in a non-flex-fuel vehicle, you’ll end up with O2 sensor and knock sensor codes because the computer can’t adjust the fuel trim and timing correctly. You can change sensors all day long and not fix anything if this happens.
If you suspect this might be the problem, ethanol test kits are available for testing the type of fuel in the tank. This quick test could save you a lot of time and trouble.
When the computer identifies a problem that could affect the transmission, it’ll usually raise mainline pressure to protect it. Your scan tool can provide you with information to help you monitor sensor and computer commands that could affect how the computer alters transmission control.
Knowing which inputs and output are involved with the transmission will help with diagnosis. Codes in the body, electronic brake, and traction control computers may affect transmission performance. Any engine codes that have a direct effect on engine load will also cause the transmission to function improperly.
So your first step in diagnosing any transmission problem should be to check for codes and computer system issues.
Always check for any factory TSBs or reflashes before removing the transmission from the vehicle. Some shops have removed a transmission to find nothing wrong internally. Later, after they get it together and back in the car, they discover the problem simply required a software update.
INTERNAL ELECTRONIC COMPONENT TESTING
The electronic components connect internally to the TEHCM. Because of this internal connection, there isn’t much external testing possible. You still need to make sure you have power and ground to the TECHM, and that the computer is receiving a brake light and P/N signal. Finally, you need to make sure there’s serial data going to and from the TECHM.
Scan data is the only way to examine solenoid commands, input speed sensor (ISS), output speed sensor (OSS), and internal mode switch (IMS) function.
Here’s an example of the harness connector pin identification for a 2010 Chevrolet Cruze with a 1.4L turbo engine (figure 1).
3 Park/Neutral Signal
4 Not Used
5 Not Used
6 High Speed GMLAN Serial Data (+)
7 High Speed GMLAN Serial Data (+)
8 High Speed GMLAN Serial Data (–)
9 Not Used
10 Not Used
11 Not Used
13 Serial Data
14 High Speed GMLAN Serial Data (–)
Pin ID and location may vary with year and model, so always verify with factory information.
The speed sensors are 2-wire sensors that create a digital signal. As the transmission component rotates past the sensor, the sensors produce a digital signal. The TCM monitors the frequency of the signal to determine the input or output speed.
The TECHM provides voltage for the sensor operation.
The input speed sensor (ISS) is located on the rear cover, with the harness routed inside to the TECHM. It generates a signal as the 3-5-R clutch assembly rotates past it. The computer uses the ISS signal to calculate gear ratio and slip rates.
The output speed sensor (OSS) is located underneath the valve body (figure 2). It generates a signal as the park gear rotates past it. The computer uses the OSS signal to indicate vehicle speed for shift pattern control and ratio calculations.
SOLENOID FUNCTION TEST
One important test while diagnosing a 6T40 with solenoid functional codes present is to check the solenoids functionally; not just electrically. These systems have an automated process available to help clean debris from the solenoid assembly.
A capable scan tool will instruct the TCM (TEHCM) to cycle the solenoids while the system’s pressurized to clean the solenoids. You don’t need to disassemble the transmission to perform the cleaning process. Simply follow the instructions on the scan tool to activate the cleaning program. Always perform this cleaning process before attempting to diagnose the transmission.
If the cleaning process is unsuccessful, you’ll need to diagnose the problem with a Kent Moore DT47825 tool kit (figure 3). This is a must-have tool!
- Remove the control solenoid assembly from the transmission.
- Install tool DT 48616 onto the control solenoid assembly (5 Nm / 44 lb-in).
- Apply regulated shop air (90–100 PSI) to the tool.
- Connect your scan tool to the control solenoid assembly using cable DT48616-10.
- Command the solenoid on and off. Air pressure should be present on the gauge and it should exhaust as you cycle the solenoid. If the solenoid or valve isn’t operating properly, the gauge pressure won’t change as you cycle the solenoid. In that case, replace the complete control solenoid assembly.
- If the solenoid checks out okay, install the gauge on another solenoid port and repeat the process.
There may be times when a solenoid performance code is caused by a problem with the pressure switches in GEN I models. There are repair kits available from the aftermarket for the GEN I pressure switches.
These switches were eliminated in 2012 because of high failure rate. One issue is that the pressure switch retaining rivets become loose. When repairing existing pressure switches, always check for loose rivets (figure 4).
Any solenoid performance code found on any transmission could indicate a pressure leak in the clutch circuit controlled by the solenoid.
FALLING OUT OF GEAR AT A STOP
The customer brings his car in because he says the transmission is falling out of gear at a stop. A check reveals code P2723 — pressure control solenoid 5 performance: stuck off.
Check the piston carefully for any cracks or defects. It may have shrunk from heat. If so, it won’t create any resistance as you slide it in and out of the housing.
Always replace these bonded pistons during a rebuild.
OVERFILLING SETS CODE P0218 OVERHEATS THE TRANSMISSION
Overfilling will cause the transmission to overheat and set code P0218 — transmission fluid over temperature. The 6T40/45 uses a thermal element to control the oil level in the unit, similar to other GM units.
Known as a fluid level control valve (figure 6), the unit is basically a thermally controlled standpipe. The fluid level control valve is attached to the transmission case, next to the control valve body assembly, and is designed to control the fluid level in the control valve body cover.
The fluid level control valve contains a bimetal strip that reacts to fluid temperature changes and opens or closes a fluid passage. The system controls the maximum fluid level by allowing fluid to overflow from the top of the fluid level control valve pipe and drain into the case sump.
At temperatures below 60ºC (140ºF), the thermostatic element allows fluid to drain from the control valve body cover area into the case sump.
As the temperature of the transmission fluid increases, the thermostatic element traps fluid in the control valve body cover area and the fluid level rises.
This maintains the fluid level to provide adequate fluid for the transmission’s hydraulic system. A damaged or loose thermostatic element could cause fluid foaming or incorrect fluid level.
The fluid temperature must be at operating temperature to get a proper measurement when checking the fluid level. Checking the fluid level with the fluid temperature below operating temperature will result in the fluid level being too low.
If you’re changing fluid, make sure the transmission is cold. If you remove the drain plug in the bottom of the case while the transmission is still warm, only about 50% of the fluid will drain.
You can check the fluid temperature from the driver information center on some models, or with a scan tool. It’s critical that the fluid be at the correct temperature or you could over-or underfill the transmission.
These units are easily overfilled because the synthetic fluid’s expansion rate is very sensitive to temperature, giving false level readings. As little as half a quart overfull can cause fluid to leak from the vent.
Always check the fluid level with the engine running, fluid temperature 85ºC–95ºC (185ºF–205ºF), in park, through the plug near the axle seal in the case (figure 7).
The transmission adaptive values learn is a procedure for 6-speed automatic transmissions in which the TCM “learns” individual clutch characteristics. Once the TCM learns the clutch data, the procedure translates it into the adaptive data cells, which the TCM uses for clutch control during shifts.
The scan tool lets you initiate the transmission adaptive values learn procedure, which you should perform after any of these transmission repairs:
- Transmission internal service or rebuild
- Valve body repair or replacement
- Control solenoid valve assembly replacement
- TCM software or calibration update
- Any service in response to shift quality
Failure to perform the procedure after one of these repairs may result in poor transmission performance or set a trouble code.
Before you begin the transmission adaptive values learn procedure, make sure of these conditions:
- Block the drive wheels
- Apply the parking brake
- Apply the service brake
- Zero percent throttle and no external engine RPM control
- Transmission fluid temperature (TFT) between 70ºC–100ºC (155ºF–215ºF)
- Cycle the transmission gear selector from park to reverse three times to purge air from the reverse clutches
If, at any time during the procedure, the required conditions aren’t met, the process may abort and you’ll need to start over from the beginning.
Now you’re ready to begin the transmission adaptive values learn procedure:
- Use your scan tool to navigate to Transmission Adaptive Values Learn:
- Module Diagnosis
- Transmission Control Module
- Configurations/Reset Functions
- Transmission Adaptive Values Learn
- Select the Transmission Adaptive Values Learn procedure. During the procedure, the scan tool data display will provide operator instructions. Follow the instructions as required.
- Once the procedure is complete, shut the engine off and power down the TCM. You’ll lose communication to the scan tool.
- Restart the engine. This will complete the transmission adaptive values learn procedure. After the procedure is completed, the transmission may remain in neutral.
- Turn the ignition off and remove the scan tool.
That’s all there is to it. Your rebuild should be ready to be delivered and should perform like new. And that’s not just smart; that’s street smart!