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Diagnostic Information for Intermittent Transmission Downshift, Slip, Busy/Cycling TCC or Noisy Cooling Fan

Subject:Diagnostic Information For Intermittent Transmission Downshift, Slip, Busy/Cycling TCC or Noisy Cooling Fan

Models:1999-2000 Cadillac Escalade
2002-2003 Cadillac Escalade, Escalade EXT
1988-2003 Chevrolet Astro, Blazer, S-10, Silverado, Suburban
1989-2003 Chevrolet/Geo Tracker
1995-2003 Chevrolet Tahoe
1996-2003 Chevrolet Express
1988-1994 GMC S-15
1988-1999 GMC Suburban
1988-2003 GMC Safari, Sierra
1995-2003 GMC Sonoma, Yukon, Yukon XL
1996-2003 GMC Savana
1999-2001 GMC Envoy
1991-2001 Oldsmobile Bravada
2003 HUMMER H2
with Air Conditioning


This bulletin is being revised to change the Model information and text. Please discard Corporate Bulletin Number 99-07-30-016A (Section 07 -- Transmission/Transaxle).

Some customers may comment that at times the transmission seems to slip, or that there is a loud roar from the engine with slow acceleration. This condition is most noticeable after the vehicle has sat idle for 12 or more hours, or on hot days when the A/C is on and the vehicle moves slowly with traffic. Typical comments from customers may include the following conditions:

    • Intermittent slipping
    • Intermittent downshift followed by an upshift, both with no apparent reason
    • Busyness or cycling of the TCC (torque converter clutch) at steady throttle conditions and on a level roadway
    • Noisy cooling fan

The type of concern described above requires further definition. The customer should be asked the following questions:

    • Is the situation more pronounced with higher vehicle loads such as when pulling a trailer?
    • Do warmer ambient temperatures make the situation more pronounced ?
    • When the condition occurred, did you have the A/C on, and were you driving in stop and go city traffic?
    • Does the condition exhibit itself on the first startup after sitting more than eight hours?

If the customer indicates that these conditions apply, and your observation confirms that the vehicle is operating properly, provide the customer with the vehicle operating description included at the end of this bulletin. Further action may not be necessary. A service procedure follows if further definition is required.

Diagnostic Aids

Cooling fan operation or the resulting sound varies. The cooling fan clutch may be described as a continuously variable clutch. If the vehicle engine is running, the fan blade is always turning unless the fan clutch is non-functional. The speed of the fan in relation to engine speed is temperature dependent. Maximum fan speed (air flow and related fan noise) through the engine compartment is experienced under two conditions.

  1. When the vehicle sits in an unused condition for several hours, the viscous fluid within the thermostatic fan clutch assembly migrates and fully engages the clutch of the fan. After a short drive, the viscous fluid will migrate to the storage area in the fan clutch and the fan clutch will slip, reducing the noise (roar of the fan). This is normal thermostatic fan clutch operation.
  2. When the engine is running, and the air being drawn by the fan or pushed through the radiator from the vehicle movement reaches a high enough temperature, the fan clutch will fully engage the fan clutch, drawing additional air through the radiator to lower the engine coolant temperature and A/C refrigerant temperature. When the cooling fan clutch fully engages, fan noise increases (for example; this is the same as switching an electric household fan from low to high speed). Some customers have interpreted this sound increase to be an increase in the engine RPM due to transmission downshift, transmission slipping, or TCC cycling. As the engine coolant temperature decreases, the fan clutch will begin to slip, lowering the actual speed of the fan blade and the resultant sound.

Service Procedure

When diagnosing an intermittent transmission downshift, slip, or busy/cycling TCC, follow these steps:

  1. Verify the transmission fluid level and the fluid condition. Refer to the Automatic Transmission sub-section of the appropriate Service Manual.
  2. Test drive the vehicle under the conditions described by the customer (ambient temperature, engine coolant temperature, trailering, etc.). It may be necessary to partially restrict airflow to the radiator in order to raise the engine coolant temperature to match the customer's conditions.
  3. Monitor the engine RPM and the engine coolant temperature using a scan tool.
  4. Listen for an actual increase in the engine RPM. Use either the vehicle tachometer (if equipped), the Tech 2 RPM or transmission slip speed as an indicator, rather than just the sound.

If the engine RPM display on the tachometer or the Tech 2 increases, verify the scan tool RPM and coolant temperature readings. If the noise increase is due to the engagement of the fan, the engine RPM will not increase and the engine coolant temperature will begin to decrease after the fan engages. As the fan runs, the engine coolant temperature will drop and the fan will disengage, reducing noise levels. The engine RPM will not decrease. This cycle will repeat as the engine coolant temperature rises again.

If the above procedure shows the condition to be cooling fan-related, no further action is necessary. The vehicle should be returned to the customer and the condition explained.

If the above procedure shows the condition to be other than cooling fan-related, refer to the Automatic Transmission sub-section of the appropriate Service Manual for transmission diagnosis information.

The following information regarding the operation of the engine cooling fan should be photocopied and given to the customer.

Intermittent Transmission Downshift

All light duty trucks are equipped with a thermostatic engine cooling fan. This fan is designed to provide greater fuel efficiency and quieter operation than a standard fan. These benefits are possible through the addition of a thermostatic clutch to the fan drive.

When the engine is cool (if the engine has been run in the last few hours), the clutch allows the fan to "slip" or turn at a speed slower than the engine. By turning at a slower speed, the fan uses less horsepower, which saves fuel, and is quieter. When the engine temperature reaches a preset temperature or if the engine has not been run for several hours, the fan "engages" and turns at the same speed as the engine.

"Engagement" of the fan provides increased airflow through the radiator to cool the engine. As the airflow increases, fan operation becomes clearly audible. This increase in noise can easily be mistaken for an increase in engine RPM and may be incorrectly blamed on the automatic transmission. When operating an unloaded vehicle in cooler ambient temperatures, the thermostatic clutch usually won't fully engage. However, if the vehicle is pulling a trailer, is heavily loaded or is operated at high ambient temperatures, the thermostatic fan clutch may cycle on and off as the engine coolant temperature rises and falls.

The sound of fan operation under the conditions described above is a sign that the cooling system on your vehicle is working correctly. Replacement or modification of the cooling system or the transmission parts will not change or reduce the noise level. Attempts to reduce this noise may cause you, the customer, to believe that your vehicle is not reliable and will inconvenience you by causing your vehicle to be out of service.