DTC Decoded: P1780
Code Meaning: Transmission Control Switch Circuit Out of Self Test Range
What’s Wrong: The transmission control switch is not changing states.
Possible Causes: See diagnosis for possible causes.
How To Diagnose and Fix Possible Causes
The transmission control switch is a component related to electronic automatic transmissions that come in the form of a button near the automatic shift lever. This button is usually labeled as “OD off” or “OD cancel”, but some trucks are labeled as “Tow/Haul”. Basically, these disable the transmission from going into overdrive. For example, a car with a four speed automatic would be limited to the three gears when the driver pushes this switch. There are a few important diagnosis steps to take to pinpoint the problem. These include:
Check the fuse. With the key in the off position, check the resistance on the fuse. If it’s less than 5 ohms, it needs to be replaced.
Check the circuit for a short to the ground. Disconnect the PCM and then measure the resistance between PIN 41 and ground — the resistance should be greater than 10,000 ohms. If less, repair the circuit.
Check the transmission control switch circuit for voltage. Disconnect the PCM, turn the key to the on position, measure the voltage between Pin 41 and ground while cycling the the TCS switch. If the voltage cycles, the PCM needs to be replaced.
Check for an open circuit. With the key in off position, disconnect the central junction box and transmission control switch. Measure the resistance between the CJB fuse 2.13 Pin 10 and power side of the TCS Pin 2 in the harness connector. Now measure the resistance between CJB fuse 2.13 Pin 10 and the power side of the TCS Pin 1 in the harness connector. These should be the same. If not, repair the circuit.
Check circuit for short to power. Measure the resistance between Pin 41 and Pin1. If the resistance is not greater than 10,000 ohms, repair the the circuit.
If you worked through all of these steps and didn’t find the problem, the transmission control switch needs to be replaced. However, if you are unsure of your tests, follow the steps, in order, and retest accordingly.
The transmission control switch is located in various places on Ford models. Typically, it will be a part of the shift knob or shift plate. Most of the time, you’ll have to replace the entire attached assembly and cannot only replace the switch since it’s attached. This is just a switch and should be confused with the transmission control module that controls automatic transmissions.