GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2001 🢒 GMC C Sierra - 2WD 🢒 Transmission/Transaxle 🢒 Automatic Transmission - Allison 🢒 Tow/Haul Switch/Indicator Always On or Inoperative

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 0 volts. This signals the transmission control module (TCM) to extend the length of time between upshifts. Cycling the tow/haul switch again disables tow/haul mode and returns the transmission to a normal shift pattern.

Diagnostic Aids
    • The tow/haul switch is hard wired to the BCM where it is then broadcasted on Class 2 Serial Data link to the TCM.
    • Observe tow/haul status on the scan tool to see if the status changes.
    • Inspect the wiring for poor electrical connections at the BCM. Inspect for the following conditions:
       - A bent terminal
       - A backed out terminal
       - A damaged terminal
       - Poor terminal tension
       - A chafed wire
       - A broken wire inside the insulation
    • When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. This step tests for a faulty tow/haul switch.

    2.

  2. This step tests for voltage input from the BCM to the tow/haul switch.

    3.

  3. This step tests for ground integrity.

Step

Action

Yes

No

Schematic Reference: Automatic Transmission Controls Schematics

1
  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. Cycle the tow/haul switch while observing Tow/Haul Mode on the scan tool.

Does the scan tool display Active when the switch is first pressed, then Inactive when the switch is pressed again?

Go to Testing for Intermittent Conditions and Poor Connections

Go to Step 2

2

  1. Remove the connector from the tow/haul switch.
  2. Using the J 35616 GM terminal test kit, install a fused jumper wire from terminals of the tow/haul switch connector.

Does the scan tool Tow/Haul Mode indicate a status change?

Go to Step 7

Go to Step 3

3

Using the DMM and the J 35616 , measure the voltage at the low reference of 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 voltage at the tow/haul switch signal circuit of the BCM.

Is ignition voltage measured?

Go to Step 5

Go to Step 8

5

Test the tow/haul switch signal circuit of the tow/haul switch for an open or shorted condition. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 9

Go to Testing for Intermittent Conditions and Poor Connections

6

Test the ground circuit of the tow/haul switch for an open or shorted condition. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 9

Go to Testing for Intermittent Conditions and Poor Connections

7

Replace the tow/haul switch. Refer to Shift Lever Replacement - On Vehicle.

Did you complete the replacement?

Go to Step 9

--

8

Test the BCM for proper operation. Refer to Diagnostic Starting Point - Body Control System.

Did you find and correct the condition?

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, then Inactive when the switch is pressed again?

System OK

Go to Step 1