Changing the automatic transmission filter and fluid is a great preventive maintenance practice, ensuring a clean and proper working transmission assembly. The most important factors are installing the correct filter for the vehicle application and the correct installation procedures of the filter. An incorrect filter installation or the wrong filter will ultimately cause erratic shifts and possibly cause damage to the transmission assembly.
Some, if not all transmission filters, are equipped with what is known as a lip seal. Installing the filter with the lip seal as a unit will not seat the filter properly, cause ingestion of air around the lip seal and dislodge the filter from the pump bore.
The most common cause of transmission erratic shifting after a filter and fluid service is the ingestion of air. When air is introduced inside automatic transmissions, a customer will notice: transmission slippage after 3 to 5 miles of driving; the vehicle will stall or engine will shut off when coming to a stop, or an MIL light illuminates, setting an incorrect pressure ratio DTC.
So where is the problem? We cannot condemn the transmission clutch assemblies because of slippage, nor can we blame the torque converter because of the stalls and engine shut offs and, finally, we cannot say that the pressure regulator solenoid or valve is stuck or worn because of a trouble code.
What to do now? Let us follow the first three important steps:
1. Hear the customer’s complaint;
2. Check the level of the ATF; and
3. Test drive the vehicle.
Now that we have done all three and have encountered a problem, let us pull that good ol’ pressure gauge from the toolbox. A scan tool will not help you in this situation. Once you have connected the pressure gauge and the vehicle is running, any rapid fluctuations of the gauge needle, vibration or humming of the gauge indicates that the pump maybe pulling air.
How can we assume that the pump is pulling in air? Easy: Overfill the transmission with ATF! Overfilling the transmission with fluid raises the fluid level beyond the area to where the air is being introduced. Also, pay attention to the pressure gauge; the needle of the gauge will smooth out once you have overfilled the transmission and no air is present.
Now that we know that air is being pulled into the transmission, we have to find the source. The removal of the transmission pan is now required. We have to make sure that the filter’s rubber lip seal was not damaged when it was driven up into the pump bore. If the filter is attached to the valve body, gently apply downward pressure of the filter’s body to see if the filter is properly lodged into the lip seal. Inspect the filter for any cracks or separation of the seams.
