Tow/Haul Switch/Indicator Always On or Inoperative 4.3L
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 vehicle control module (VCM) 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
| • | Inspect the wiring at the BCM, the mid bussed electrical center (MBEC) and all other circuit connecting points for the following conditions: |
| - | A backed out terminal |
| - | A damaged terminal |
| - | Reduced terminal tension |
| - | A chafed wire |
| - | A broken wire inside the insulation |
| - | Moisture intrusion |
| - | Corrosion |
| • | When diagnosing for an intermittent short or open condition, massage the wiring harness while watching the test equipment for a change. |
| • | If the electrical circuit checks 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 Symptom Diagnosis . |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
This step tests for a faulty tow/haul switch.
-
This step tests for voltage input from the BCM to the tow/haul switch.
-
This step tests for voltage through the MBEC.
-
This step tests for ground integrity at the splice pack.
Step | Action | Value(s) | 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 Diagnostic Aids | |
Does the scan tool Tow/Haul Mode indicate a status change? | -- | |||
Using the J 39200 digital multimeter (DMM) and the J 35616-A connector test adapter kit, check the voltage at terminal B5 of the I/P wiring harness connector (C201B). Is ignition voltage indicated? | -- | |||
4 | Using the J 39200 DMM and the J 35616-A connector test adapter kit, check the voltage at terminal C1-B12 of the body control module (BCM). Is ignition voltage indicated? | -- | ||
5 | Inspect circuit 553 (LT BLU) for an open or shorted condition. Refer to General Electrical Diagnosis in Wiring Systems. Did you find an open or shorted condition? | -- | Go to Diagnostic Aids | |
6 | Inspect the BCM for proper operation. Refer to Diagnostic System Check - Body Control System in Body Control System. Is the inspection complete? | -- | -- | |
7 |
Is ignition voltage indicated? | -- | ||
Using the J 39200 DMM and the J 35616-A connector test adapter kit, check for voltage at terminal C1-F2 of the MBEC. Is ignition voltage indicated? | -- | |||
Using the J 39200 DMM and the J 35616-A connector test adapter kit, check for voltage at terminal E of splice pack 203 (SP203). Is ignition voltage indicated? | -- | |||
10 | Inspect circuit 1850 (BLK) between terminal B2 of the I/P wiring harness connector (C201B) and terminal C2-C9 of the MBEC for an open or shorted condition. Refer to General Electrical Diagnosis in Wiring Systems. Did you find an open or shorted condition? | -- | Go to Diagnostic Aids | |
11 | Replace the MBEC. Refer to Fuse Block Replacement in Instrument Panel, Gauges and Console. Is the replacement complete? | -- | -- | |
12 | Inspect circuit 1850 (BLK) between terminal C1-F2 of the MBEC and terminal E of splice pack 203 (SP203) for an open or shorted condition. Refer to General Electrical Diagnosis in Wiring Systems. Did you find an open or shorted condition? | -- | Go to Diagnostic Aids | |
13 | Repair the circuit. Refer to Wiring Repairs in Wiring Systems. Is the repair complete? | -- | -- | |
14 | Repair splice pack 203 (SP203). Refer to Wiring Repairs in Wiring Systems. Is the repair complete? | -- | -- | |
15 | Replace the tow/haul switch. Refer to Shift Lever Replacement - On Vehicle in Steering Wheel and Column - Tilt. Is the replacement complete? | -- | -- | |
16 | 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 |
Tow/Haul Switch/Indicator Always On or Inoperative 4.8L and 5.3L
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
| • | Inspect the wiring at the BCM, the mid bussed electrical center (MBEC) and all other circuit connecting points for the following conditions: |
| - | A backed out terminal |
| - | A damaged terminal |
| - | Reduced terminal tension |
| - | A chafed wire |
| - | A broken wire inside the insulation |
| - | Moisture intrusion |
| - | Corrosion |
| • | When diagnosing for an intermittent short or open condition, massage the wiring harness while watching the test equipment for a change. |
| • | If the electrical circuit checks 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 Symptom Diagnosis . |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
This step tests for a faulty tow/haul switch.
-
This step tests for voltage input from the BCM to the tow/haul switch.
-
This step tests for voltage through the MBEC.
-
This step tests for ground integrity at the splice pack.
Step | Action | Value(s) | 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 Diagnostic Aids | |
Does the scan tool Tow/Haul Mode indicate a status change? | -- | |||
Using the J 39200 digital multimeter (DMM) and the J 35616-A connector test adapter kit, check the voltage at terminal B5 of the I/P wiring harness connector (C201B). Is ignition voltage indicated? | -- | |||
4 | Using the J 39200 DMM and the J 35616-A connector test adapter kit, check the voltage at terminal C1-B12 of the body control module (BCM). Is ignition voltage indicated? | -- | ||
5 | Inspect circuit 553 (LT BLU) for an open or shorted condition. Refer to General Electrical Diagnosis in Wiring Systems. Did you find an open or shorted condition? | -- | Go to Diagnostic Aids | |
6 | Inspect the BCM for proper operation. Refer to Diagnostic System Check - Body Control System in Body Control System. Is the inspection complete? | -- | -- | |
7 |
Is ignition voltage indicated? | -- | ||
Using the J 39200 DMM and the J 35616-A connector test adapter kit, check for voltage at terminal C1-F2 of the MBEC. Is ignition voltage indicated? | -- | |||
Using the J 39200 DMM and the J 35616-A connector test adapter kit, check for voltage at terminal E of splice pack 203 (SP203). Is ignition voltage indicated? | -- | |||
10 | Inspect circuit 1850 (BLK) between terminal B2 of the I/P wiring harness connector (C201B) and terminal C2-C9 of the MBEC for an open or shorted condition. Refer to General Electrical Diagnosis in Wiring Systems. Did you find an open or shorted condition? | -- | Go to Diagnostic Aids | |
11 | Replace the MBEC. Refer to Fuse Block Replacement in Instrument Panel, Gauges and Console. Is the replacement complete? | -- | -- | |
12 | Inspect circuit 1850 (BLK) between terminal C1-F2 of the MBEC and terminal E of splice pack 203 (SP203) for an open or shorted condition. Refer to General Electrical Diagnosis in Wiring Systems. Did you find an open or shorted condition? | -- | Go to Diagnostic Aids | |
13 | Repair the circuit. Refer to Wiring Repairs in Wiring Systems. Is the repair complete? | -- | -- | |
14 | Repair splice pack 203 (SP203). Refer to Wiring Repairs in Wiring Systems. Is the repair complete? | -- | -- | |
15 | Replace the tow/haul switch. Refer to Shift Lever Replacement - On Vehicle in Steering Wheel and Column - Tilt. Is the replacement complete? | -- | -- | |
16 | 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 |
