A2: Automatic Transmission (Sample Test)

Both Technician A and Technician B are correct.
A TFT sensor is a thermistor, which is a type of resistor that changes its resistance in relation to temperature. The TFT sensor is used to measure the temperature of the transmission fluid and send that information to the transmission control module (TCM). The TCM uses this information to control the shifting of the transmission and to prevent the transmission from overheating.
The TFT sensor can be diagnosed using a scan tool and an ohmmeter. The scan tool can be used to read the TFT sensor’s voltage signal. The ohmmeter can be used to measure the resistance of the TFT sensor at different temperatures. If the TFT sensor’s resistance is not within the specified range, then the sensor is faulty and needs to be replaced.

A 10 Megohm tester should be used for checking the circuits leading to the control module of an electronic automatic transmission. This is because the higher resistance of the 10 Megohm tester is less likely to damage the sensitive electronic components in the transmission control module. Using a tester with a lower resistance, such as a 1 Megohm tester or a 5 Megohm tester, could potentially cause damage to the module. Using a tester with a higher resistance, such as a 20 Megohm tester, may not provide accurate readings.

In an automatic transmission, clutches are used to engage and disengage different gear ratios. If a clutch is slipping, it means that it is not fully engaging and transferring the engine’s power to the transmission. As a result, the engine RPMs will increase without a corresponding increase in vehicle speed.
A faulty torque converter, a restriction in the exhaust system, or the one-way stator slipping are not directly related to the tachometer indicating high RPMs during a stall test. A faulty torque converter might cause issues with power transfer, but it would not specifically result in high RPMs during a stall test. A restriction in the exhaust system or a slipping one-way stator would not directly affect the engine RPMs in this context.

Technician A is correct that test ports for retrieving hydraulic pressure data for the line, clutch, and servo circuits are typically located on the transmission case. These test ports can be used to diagnose problems with the hydraulic system and ensure that the transmission is functioning properly.
Technician B is also correct that running air pressure tests before and after disassembling the transmission can be a useful diagnostic tool. Air pressure tests involve applying compressed air to various parts of the transmission to check for leaks and other problems. This can help identify issues before they become more serious and can provide a baseline for comparison after the transmission has been disassembled and reassembled.
In summary, both technicians are correct, but they are referring to different types of tests. Technician A is referring to hydraulic pressure tests, while Technician B is referring to air pressure tests. Both types of tests can be useful for diagnosing transmission problems.

Technician A is correct that the engine coolant temperature sensor’s input is used by the transmission control module to manage the torque converter clutch. The torque converter clutch is an important component of the automatic transmission system that locks the torque converter’s turbine to the impeller to improve fuel efficiency and reduce heat buildup.

The transmission control module (TCM) uses data from various sensors to determine when to engage or disengage the torque converter clutch. One of these sensors is the engine coolant temperature sensor, which provides information about the engine’s temperature. The TCM uses this information to adjust the torque converter clutch’s engagement and disengagement points to optimize performance.

Technician B, on the other hand, is incorrect in stating that the electronic throttle position sensor (TPS) has taken over the job of the hydraulic governor. The hydraulic governor is a mechanical component that regulates the transmission’s hydraulic pressure based on the vehicle’s speed. The TPS measures the position of the accelerator pedal and sends a signal to the engine control module to adjust the engine’s output torque, but it does not directly control the transmission’s hydraulic pressure like the governor does.

In modern electronic transmissions, the TCM uses various sensors, including the TPS, to determine the optimal shift points and adjust the transmission’s hydraulic pressure. However, the hydraulic governor is still an essential component of the transmission and is used in conjunction with electronic controls to manage hydraulic pressure and shift points.

A variable resistor is a resistor of which the electric resistance value can be adjusted. A variable resistor is in essence an electro-mechanical transducer and normally works by sliding a contact (wiper) over a resistive element.

If a 2-4 band is worn out, it can lead to slipping in both the second and fourth gears. Automatic transmission application charts efficiently identify the specific area of the transmission that is causing shifting issues. By systematically ruling out possibilities, this useful chart can assist in identifying the defective component.

When removing an engine from a vehicle with an automatic transmission, the torque converter remains with the transmission. The torque converter is a fluid coupling that sits between the engine and transmission and allows the engine to continue running while the vehicle is stopped or idling. It also helps to increase torque output and smooth out power delivery.
There is typically no need to flush or drain the torque converter during an engine removal, as it should only contain transmission fluid and not engine oil or other contaminants. However, if the torque converter is being replaced or serviced, it may need to be drained and refilled with fresh transmission fluid.

If a vacuum hose has come detached and is hanging from the transmission pan, causing the automatic transmission to not downshift properly, it should be connected to the modulator.
The modulator is a component of an automatic transmission that uses vacuum pressure to determine the load on the engine and adjust the shift points accordingly. If the vacuum hose that connects the modulator to the engine is disconnected or damaged, it can cause the transmission to shift improperly or not at all.
The governor is another component that helps control the shift points of an automatic transmission, but it does not rely on vacuum pressure. The throttle position sensor and vehicle speed sensor are also important components of a vehicle’s drivetrain, but they do not control the shifting of the transmission.

Both Technician A and Technician B are correct.
A TFT sensor is a thermistor, which is a type of resistor that changes its resistance in relation to temperature. The TFT sensor is used to measure the temperature of the transmission fluid and send that information to the transmission control module (TCM). The TCM uses this information to control the shifting of the transmission and to prevent the transmission from overheating.
The TFT sensor can be diagnosed using a scan tool and an ohmmeter. The scan tool can be used to read the TFT sensor’s voltage signal. The ohmmeter can be used to measure the resistance of the TFT sensor at different temperatures. If the TFT sensor’s resistance is not within the specified range, then the sensor is faulty and needs to be replaced.