Circuit Description
Tow/haul mode enables the operator to achieve enhanced shift performance when towing or hauling a load. When tow/haul mode is selected, the tow/haul switch input signal to the body control module (BCM) is momentarily toggled to zero volts. This signals the powertrain control module (PCM) to extend the length of time between upshifts and increase transmission line pressure. Cycling the tow/haul switch again disables tow/haul mode and returns the transmission to a normal shift pattern.
Diagnostic Aids
If the electrical circuit tests are OK and the tow/haul shift pattern is not occurring, there may be a mechanical/hydraulic condition that prevents tow/haul operation. Refer to Symptoms - Automatic Transmission .
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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This step tests for a faulty tow/haul switch.
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This step tests for voltage input from the BCM to the tow/haul switch.
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This step tests for ground integrity.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
1 |
Does the scan tool display Active when the switch is first pressed, and then Inactive when the switch is pressed again? | -- | Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems | Go to Step 2 |
Does the scan tool Tow/Haul Mode indicate a status change? | -- | Go to Step 7 | Go to Step 3 | |
Using the DMM and the J 35616 , measure the tow/haul switch signal voltage at the tow/haul switch connector. Is ignition voltage measured? | -- | Go to Step 6 | Go to Step 4 | |
4 | Using the DMM and the J 35616 , measure the tow/haul switch signal voltage at the body control module (BCM). Is ignition voltage measured? | -- | Go to Step 5 | Go to Step 8 |
5 | Test the signal circuit of the tow/haul switch for an open or shorted condition. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 9 | Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems |
Test the ground circuit of the tow/haul switch for an open or shorted condition. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 9 | Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems | |
7 | Replace the tow/haul switch. Refer to Tow/Haul Switch Replacement . Did you complete the replacement? | -- | Go to Step 9 | -- |
8 | Replace the BCM. Refer to Control Module References in Computer/Integrating Systems for replacement, setup and programming. Did you complete the replacement? | -- | Go to Step 9 | -- |
9 | After the repair is complete, observe Tow/Haul Mode on the scan tool display, while cycling the tow/haul switch. Does the scan tool display Active when the switch is first pressed, and then Inactive when the switch is pressed again? | -- | System OK | Go to Step 1 |
