Transmission Diagnostic Procedures
Transmission failures can occur in many ways, from a small light on the dash with no noticeable problems, to whining or grinding noises that leave you stranded. Because many transmission problems are easily and inexpensively corrected, a missed diagnosis could cost you thousands of dollars. The shop you choose should have the equipment and expertise necessary to isolate the small problems from the big ones.
Diagnosis of Simple Transmission Problems
Before work is performed, the transmission problem should have an initial diagnosis. Because we commonly see the same failures within the same type of transmission, some diagnosis can be done in as little as 20 minutes and usually while you wait.
Diagnosis of Complex Computer Controlled Transmission Problems
The transmissions in almost all vehicles built in the last fifteen years are computer controlled. Data is collected from various sensors on the engine and transmission, and calculations are made to control shift timing and firmness. Quite often a failed electronic component will create the illusion of an internal transmission problem.
Late model transmissions are controlled by a computer that operates shift and pressure control solenoids inside the transmission. Therefore the computer needs to be electrically scanned by a transmission professional to determine if any electronic control problems exist. When the computer “sees” any parameter from various sensors that do not fit established criteria, the computer will set a code for each error, which will be evident by a “check engine” and/or a flashing overdrive light. However, there are some errors that will be present that the computer will not sense because they are within the normal range. In that case, the system has to be diagnosed to determine if applicable sensors or other electrical components are performing incorrectly and creating an abnormal transmission operational response. The following sensors are usually part of the transmission: MPLS (Manual Linkage Position Sensor), input and output speed sensors, shift solenoids, governor sensor, etc.
Please be aware that some sensors are not part of the transmission itself, but in other areas of the vehicle. These include the TPS (Throttle Position Sensor), the MAS (Mass Airflow Sensor), MAP (Manifold Absolute Pressure sensor), PCM (Power Control Module or Computer), DSS (Differential Speed Sensor), CTS (Coolant Temperature Sensor), IAT (Intake Air Temperature), etc.
Also, be aware that a scan done at a “parts store” will not show all of the information necessary to form a conclusion about an electrical control problem. A lot of problems in the control system do not set a code in the computer, but can still cause transmission problems.
When the shifts of a computer controlled transmission are excessively firm, the pressure regulator may set to maximum pressure by the computer if it senses a problem from one or more sensors.
The results of this diagnostic testing should indicate whether the problems are in the control system, the transmission, or both.
When the ignition key is cycled, the computer is reset and will control the transmission normally until an error is registered. Then the transmission will be placed in “limp” mode (2nd gear) again.
I’ve found that it much better to evaluate the situation “hands-on” than to speculate on the phone. Over the years, I’ve been amazed at the number of times the conclusion made before seeing a vehicle changes after it is inspected and the situation is diagnosed.
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