Component Tests and General Diagnostics

*TOW /HAUL OVERDRIVE OFF SWITCH (DIESEL)





For the Transmission circuit diagram Refer to Transmission Control Systems/Diagram.
For a complete wiring diagram refer to diagrams.

Theory of Operation
The tow/haul overdrive OFF (control) switch is located in the shift lever arm. The switch is a momentary contact device that signals the ECM/PCM to toggle current status of the overdrive function. At ignition on, overdrive operation is allowed. Pressing the Tow/Haul Overdrive Off switch once causes the Tow/haul Overdrive OFF mode to be entered and the Tow/Haul lamp to be illuminated. Pressing the switch a second time causes normal overdrive operation to be restored and the Tow/haul lamp to be turned off. The Tow/haul Overdrive OFF mode defaults to ON after the ignition switch is cycled OFF and ON. The normal position for the control switch is the ON position. The switch must be in this position to energize the solenoid and allow a 3-4 upshift. The control switch indicator light illuminates only when the Tow/haul Overdrive Off switch is turned to the OFF position, or when illuminated by the transmission control module.

Possible Causes

- TOW/HAUL OVERDRIVE OFF SWITCH SENSE CIRCUIT SHORT TO GROUND
- TOW/HAUL OVERDRIVE OFF SWITCH
- ENGINE CONTROL MODULE

Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding.

Diagnostic Test





1. TOW/HAUL OVERDRIVE OFF SWITCH SENSE CIRCUIT SHORT TO GROUND
Turn the ignition off. Disconnect the ECM C2 harness connector. Disconnect the Tow/Haul Overdrive Off switch harness connector. Measure the resistance between ground and the Tow/Haul Overdrive Off switch sense circuit in the ECM C2 harness connector.

Is the resistance below 5.0 ohms?
Yes >>Repair the Tow/Haul Overdrive Off switch sense circuit for a short to ground.
No >>Go To 2

2. CHECK THE TOW/HAUL OVERDRIVE OFF SWITCH





Reconnect the Tow/Haul Overdrive Off switch harness connector.
Measure the resistance between ground and the O/D Tow/Haul Switch Sense circuit in the ECM C2 harness connector. Press the Tow/Haul Overdrive Off switch button.

Does the resistance change from above 5.0 ohms to below 5.0 ohms while pressing the Tow/Haul Overdrive Off switch button?
Yes >>Using the schematics as a guide, check the Engine Control Module (ECM) pins, terminals, and connectors for corrosion, damage, or terminal push out. Pay particular attention to all power and ground circuits. If no problems are found, replace and program the ECM per the Service Information.
Perform RE TRANSMISSION VERIFICATION TEST VER 1 (DIESEL). Verification Tests
No >>Replace the Tow/Haul Overdrive Off switch per the Service Information.