quote:

A/T - Intermittent Downshift/Slip/Cycling Diagnosis

File In Section: 07 Transmission/Transaxle

Bulletin No.: 99-07-30-016B

Date: October, 2002

INFORMATION

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 start-up 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 c

_____________________________

Kyle-

04 Rainier
My Restoration Projects
Please Do Not PM for Tech Help

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