Mazda TCM Troubleshooting

If you're still having issues with your TCM after it has been refurbished. Here are some general tips to help you further troubleshoot the issue:

• First, check that the vehicle's car battery is fully charged and getting a good connection.
• Check the TCM fuse.
• Inspect the pins on both the harness plug as well as the TCM and make sure the pins are not pushed back on the harness plug. If some of the tabs are broken on the TCM plug the harness will have to be manually pushed in further towards the TCM to ensure a good connection.
• Clean the pins with electronic cleaner on both the harness plug as well as the TCM itself and inspect for any dirty or receding pins.
• Check the transmission fluid. If it's very dark and the filter is full of debris, it'll show trouble codes and the A/T along with the check engine light might come on. This is highly likely if the transmission fluid was never serviced and the vehicle has over 75,000 miles. 

Perform a hard battery reset with the TCM plugged back into the vehicle to clear any remaining codes and reset the TCM:

• Remove the key from the ignition.
• With the battery removed, to ensure there is no power, short the battery cables in the vehicle and connect the positive and negative cables together for at least 20-30 minutes.
• Reconnect the battery cables starting with the positive cable on the battery first.

This will reboot all the computer modules including the TCM and make the vehicle relearn its own values.

It's recommended to have the vehicle idle for a few minutes then take it for a drive at least 15-20 minutes after the hard battery reset. 

It might idle rough at first and shift strangely but it should smooth out after the drive. 

If you're not confident in doing the hard battery reset, simply leave the car battery disconnected overnight. The main goal is to drain the voltage in the computer modules to reboot them. 

Troubleshoot 'No Communication' by using a multimeter, and check that there is continuity between the battery and the pins on the harness plugs. Testing the wiring harness pins for any connectivity issues:

Refer to the image above for the pinout of the Mazda TCM Wiring Harness 

• Power: I, O (Red)
• Ground: X (Black)
• Can-Hi: B (Orange)
• Can-Lo: E (Blue)

There should be continuity between the positive post on the battery and the red pins (I & O).

There should be continuity between the negative post on the battery and the black pin (X). 

For any communication issues, with the ignition-on, make sure that you do an ohm test between Pin B (Orange) & Pin E (Blue) on the harness plug for any remaining U0100 and U0101 (no communication) issues that are still present after the repair. There should be 120-ohm resistance. 

If there is no ohm resistance present when you test between pins B & E then the issue is not the TCM. Check the pins and wiring all the way to the OBDII connector pinout.

Check the wiring harness for any pinched or torn wiring, check & clean the connector pins on the harness plug, and inspect closely with a flashlight for any corrosion or receding pins.

Pin B on the TCM plug should have continuity to Pin 6 on the OBDII connector under the dash driver-side. Pin E on the TCM plug should have continuity to Pin 14 on the OBDII connector. 

P0715 - Input Turbine Speed Sensor Malfunction

If you are having this code show up on the OBDII scan tool then it's most likely a defective speed sensor that needs to be replaced. 

The speed sensors on the Mazda 3 and Mazda 5 are actually commonly known to fail and are relatively inexpensive to replace. 

P0753 - Shift Solenoid 'A' Malfunction
P0762 - Shift Solenoid 'B' Malfunction
P2709 - Shift Solenoid 'F' Malfunction

If this code is still present and won't clear after the TCM has been refurbished, please make sure to check the transmission fluid and filter. If the fluid is dirty and the filter is clogged up these codes might not clear. 

Once the transmission fluid and filter have been inspected/serviced the code should clear with an OBDII scan tool. If not, it may be a defective Shift Solenoid A (P0753), B (P0762), or F (P2709) which needs to be replaced.