Chevrolet Silverado / GMC Sierra. Manual - part 735
Closed throttle and engine RPM increases above 1500 RPM while in Park Test Setup or at
entrance of the test modes.
Park/Neutral position switch is properly adjusted and functioning.
Line pressure control is able to provide 1000 kPa and is within specifications.
Vehicle is not moving, or vibrating excessively.
Clutches are properly assembled.
TOW/HAUL SWITCH/INDICATOR ALWAYS ON OR INOPERATIVE
Fig. 5: View Of Tow/Haul Switch/Indicator
Courtesy of GENERAL MOTORS CORP.
CIRCUIT/SYSTEM 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 control solenoid
(w/body and TCM) valve assembly to extend the length of time between upshifts and elevate
transmission line pressure. Cycling the tow/haul switch again disables tow/haul mode and returns
the transmission to a normal shift pattern. The tow/haul switch is a momentary switch and is
normally open. The internal spring in the switch will always cause the switch state to return to
open when not held in to the closed position.
DIAGNOSTIC AIDS
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 Symptoms -
Automatic Transmission.
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Automatic Transmission - 6L50/6L80/6L90 - Cab & Chassis Sierra, Cab & Chassis Silverado,
Sierra & Silverado
REFERENCE INFORMATION
Schematic Reference
Automatic Transmission Controls Schematics
Connector End View Reference
Component Connector End Views
Electrical Information Reference
Circuit Testing
Connector Repairs
Testing for Intermittent Conditions and Poor Connections
Wiring Repairs
Scan Tool Reference
Control Module References for scan tool information
CIRCUIT/SYSTEM VERIFICATION
With a scan tool observe the Tow/Haul switch parameter in Body Controls data list. With the
ignition On or engine running, operate the Tow/Haul switch several times, and observe the
parameter as you operate the switch. The parameter should toggle between Active and Inactive
when the switch is operated.
CIRCUIT/SYSTEM TESTING
1. Ignition Off, transmission in Park, disconnect the Tow/Haul switch connector.
2. Test for less than 1 Ohm of resistance between the Tow/Haul switch signal circuit terminal
B and ground.
If greater than the specified range, test the ground circuit for an Open/High resistance.
3. Ignition On, verify the scan tool Tow/Haul parameter displays Inactive.
If not the specified value, test the signal circuit for a short to ground. If the circuit tests
normal, replace the BCM.
4. Install a fused jumper between the signal circuit and ground. Verify the scan tool parameter
displays Active.
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Automatic Transmission - 6L50/6L80/6L90 - Cab & Chassis Sierra, Cab & Chassis Silverado,
Sierra & Silverado
If not the specified value, test the signal circuit for a short to voltage or an open/high
resistance. If the circuit tests normal, replace the BCM.
5. If all circuits test normal, replace the Tow/Haul switch.
COMPONENT TESTING
1. Ignition Off, transmission in Park, disconnect the transmission Tow/Haul switch assembly.
2. Test for infinite resistance between the Tow/Haul switch with the switch in the Open
position.
If not the specified value, replace the Tow/Haul switch assembly.
3. Depress and hold the Tow/Haul switch.
4. Test for less than 1 Ohm of resistance between the Tow/Haul switch signal circuit terminals
B and A while the switch is depressed.
If greater than the specified value, replace the Tow/Haul switch assembly.
REPAIR PROCEDURES
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
Body Control Module (BCM) assembly replacement. Refer to Control Module
References for replacement, setup, and programming.
Perform the Shift Lever Replacement .
TRANSMISSION FLUID CHECK
This procedure checks the transmission fluid level, as well as the condition of the fluid itself.
Before checking the fluid level, perform the following:
1. Start the engine and park the vehicle on a level surface. Keep the engine running.
2. Apply the parking brake and place the shift lever in PARK (P).
3. Depress the brake pedal and move the shift lever through each gear range, pausing for about
3 seconds in each range. Then, move the shift lever back to PARK (P).
4. Allow the engine to idle 500-800 rpm for at least 1 minute. Slowly release the brake pedal.
5. Keep the engine running and observe the transmission fluid temperature (TFT) using the
NOTE:
Always use the proper automatic transmission fluid listed. Using
incorrect automatic transmission fluid may damage the vehicle.
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Automatic Transmission - 6L50/6L80/6L90 - Cab & Chassis Sierra, Cab & Chassis Silverado,
Sierra & Silverado
Driver Information Center (DIC) or a scan tool.
6. Using the TFT reading, determine and perform the appropriate check procedure. If the TFT
reading is not within the required temperature ranges, allow the vehicle to cool, or operate
the vehicle until the appropriate TFT is reached.
COLD CHECK PROCEDURE
1. Locate the transmission dipstick at the rear of the engine compartment, on the passenger's
side of the vehicle.
2. Flip the handle up, and then pull out the dipstick and wipe it with a clean rag or paper
towel.
3. Install the dipstick by pushing it back in all the way, wait three seconds and then pull it back
out again.
4. Check both sides of the dipstick, and read the lower level. Repeat the check procedure to
verify the reading.
5. Inspect the color of the fluid on the dipstick. Refer to Fluid Condition Inspection in this
procedure.
6. If the fluid level is below the COLD check band, add only enough fluid as necessary to
bring the level into the COLD band. It does not take much fluid, generally less than one pint
(0.5L). Do not overfill.
7. If the fluid level is in the acceptable range, push the dipstick back in all the way, then flip
the handle down to lock the dipstick in place.
8. Perform a hot check at the first opportunity after the transmission reaches a normal
operating temperature between 160°F to 200°F (71°C to 93°C).
IMPORTANT: Use this procedure only as a reference to determine if the
transmission has enough fluid to be operated safely until a
hot check procedure can be made. The hot check procedure
is the most accurate method to check the fluid level. Perform
the hot check procedure at the first opportunity. Use this
cold check procedure to check fluid level when the TFT is
between 80°F and 90°F (27°C and 32°C).
IMPORTANT: Always check the fluid level at least twice. Consistent
readings are important to maintaining proper fluid level. If
inconsistent readings are noted, inspect the transmission
vent assembly to ensure it is clean and unclogged.
2008 Chevrolet Silverado 1500
2008 TRANSMISSION Automatic Transmission - 6L50/6L80/6L90 - Cab & Chassis Sierra, Cab & Chassis Silverado,
Sierra & Silverado