The following technical tips provide repair information about specific conditions on a variety of vehicles. If you have a tough or unusual service repair, the TSS Technical Assistance Hot Line can help.
Call 1-800-825-5886, prompt #2, to speak with a technical expert with the latest OEM information.
Allison Transmission Shift Conditions
The 2001-2007 Silverado Classic, Sierra Classic; 2007-2010 Silverado, Sierra; and 2003-2010 Kodiak and Topkick, equipped with the Allison LCT1000 transmission (RPO M74, MW7) may have a delayed engagement or no engagement into Drive, a delayed engagement or no engagement into Reverse, or a range inhibit with several possible DTCs. These conditions may be more pronounced in cold climates during a cold start.
When DTC P0701, P0751, P0756, P0761, P0843, P0848, P0873, P0877 or P0894 set, it could indicate no or excessively low pump pressure as a result of a no prime or loss of prime condition within the transmission pump. Follow the diagnostic steps outlined in the Service Information.
All of these DTCs, excluding P0894, could be set due to a fluid service/filter change, transmission service involving removing the pan, low fluid level caused by a leak, or long periods of storage.
Also check the transmission fluid level using the Hot Check Procedure. Low fluid level may cause any of the listed DTCs to set, particularly during a cold ambient and cold vehicle start. In cold climates, ensure the fluid level is at the top of the HOT band.
After checking the fluid level using the Hot Check Procedure, if any of the DTCs reset during a cold start, inspect the internal suction filter for possible cracks.
Replacing the TCM will not likely correct any of the listed DTCs or low main line pressure.
Inoperative, Intermittent or Noisy A/C
Some owners of GM vehicles may comment that their A/C is inoperative, operates intermittently or is noisy. No DTCs are set. After further investigation and diagnosis, it may be determined necessary to replace the Thermal Expansion Valve (TXV). (Fig. 17)
(Fig. 17)
However, before replacing TXVs, always follow the A/C Performance Diagnostic in the Service Information to verify the valve performance and review the diagnostic tips.
The TXV meters the amount of liquid refrigerant that can flow into the evaporator. Located at the evaporator inlet, the TXV is the dividing point between the high and the low pressure sides of the A/C system. As the refrigerant passes through the TXV, the refrigerant pressure is lowered. Due to the pressure differential, the liquid refrigerant will begin to boil at the TXV outlet before entering the evaporator core. This characteristic is observed as a drop in temperature between the TXV inlet tube and the evaporator inlet tube.
TXV Failure Modes
Possible causes of inoperative or intermittent A/C:
· Bulb Sensor
Temperatures on inlet and outlet tubes - Are the temperatures on both sides (inlet and outlet tubes) of the evaporator thermal expansion valve tube at the expansion device similar? If yes, replace the damaged or faulty thermal expansion valve.
Pressures - Are both the low side and high side pressures within the specified values? Refer to the appropriate HVAC zone diagnostic.
· Incorrect refrigerant charge - Diagnose and repair potential leak and/or fill A/C system with correct charge.
· Leaking interfaces (Incorrect fastening at TXV or seal damage) - Apply correct torque or replace seals if necessary.
· System contamination - Is the liquid line extremely cold at the expansion device location and warm beyond the expansion device location? The expansion device is restricted. Inspect for contamination (brown, powdery residue or metal flake) and replace TXV.
Possible causes of A/C noise:
· Leaking interfaces (Incorrect fastening at TXV) - Fill system with correct charge.
· Incorrect refrigerant charge - Fill system with correct charge.
· Non-optimized TXV/evaporator/plumbing - Verify if noise is being produced by the TXV or other A/C system components.
Brake Pedal Kickback
1997-2007 Ford E-Series; 1999-2008 Ford F-Super Duty; 2000-2005 Ford Excursion
Some vehicles equipped with the Hydro-Boost brake booster system may exhibit a brake pedal kickback and/or grabbing condition when applying the brakes that may be caused by contamination within the power steering system.
Filter the power steering fluid by installing a temporary filter in the power steering system in line with the power steering hose. The filter needs to go in the low pressure line that goes from gear to cooler to reservoir, where it would be most accessible.
After the filtering procedure is completed, if the brake pedal kickback persists, it may be necessary to replace the Hydro-Boost brake booster.
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