Special Policy 99046 - High/Low/High ABS Brake Anomaly

THIS SPECIAL POLICY IS IN EFFECT UNTIL DECEMBER 1, 2002

DUE TO THE AVAILABILITY OF PARTS, THIS SPECIAL POLICY WILL BE ADMINISTERED IN PHASES. THE FIRST PHASE WILL BE THE REPROGRAMMING OF THE VCM IN 2WD AND 4WD 1994-1996 S/T PICKUPS AND 1995-1996 S/T UTILITIES EQUIPPED WITH A VCM. VEHICLES INVOLVED IN PHASE I ARE SHADED IN THE TABLE BELOW.

YOU WILL BE NOTIFIED OF THE NEXT PHASES VIA DCS MESSAGE.

The federal government's highway safety agency, the National Highway Traffic Safety Administration (NHTSA) has identified, and General Motors Corporation has confirmed, the existence of a condition in the antilock braking system of some Chevrolet, GMC, and Oldsmobile S/T utilities, 1994-1996 Chevrolet and GMC S/T pickups equipped with a V6 engine, 1993-1995 Chevrolet and GMC M/L vans, and 1993-1996 Chevrolet and GMC G vans, all equipped with the Lucas Varity three-sensor ABS system. On rare occasions, this condition can result in longer stopping distances during certain antilock brake applications, as explained below.

If the customer is driving on a road surface that supports good traction and they begin to stop by applying the brake pedal firmly, and both front wheels of their vehicle then pass onto a slippery surface (such as an ice-covered or wet patched asphalt part of the road), the antilock brake system will adjust the brakes at each of the wheels to take advantage of the available traction. This will allow the customer to steer and maintain stability, which is normal ABS operation, as their owner's manual explains in more detail.

However, if the customer is still braking while the vehicle leaves the slippery surface and both front wheels get back on a higher-traction surface, the ABS may perform as if the vehicle were still on the slippery surface and the vehicle may not stop as quickly. However, this will not happen every time these conditions are encountered. It depends on several additional factors, such as vehicle speed and the length of the slippery surface.

The ABS system was designed with increased sensitivity to wheel slip in order to improve vehicle steerability while braking on very slippery surfaces. This improvement for steerability, however, made it possible for reduced front braking effectiveness to occur as described above.

This special policy adjustment covers the condition described above until December 1, 2002, regardless of vehicle mileage or ownership. Dealers are to reprogram the VCM. Use the following Service Procedure. This will be performed at no charge to the customer during this time. Other conditions that may cause similar or different brake complaints that are not a result of the condition listed above are not covered by this special policy. The customer should be informed that any further service that is not covered by this special policy would be their responsibility, if they elect to have the service performed.

Involved are 1993-1996 S/T utilities, 1994-1996 S/T pickups equipped with a V6 engine, 1993-1995 M/L vans, and 1993-1996 G vans built within the following VIN breakpoints:

This phase requires no parts. Calibrations are available in October, 1999 on TIS 2000 CD#21 or Techline CD #20/21 or later versions.

Registered letters will be sent to known owners of record located within areas covered by the U.S. National Traffic and Motor Vehicle Safety Act, as amended. For owners outside these areas, the attached suggested dealer letter is recommended and should be sent by dealers.

VCM Programming

Important: Check the Service Parts Identification (SPID) label on the inside of the glovebox to determine if the vehicle has a VCM. If there is an RPO of "K29", the vehicle has a PCM and is NOT programmable.

The new calibration will be available in October, 1999 on TIS 2000 CD #21 or Techline CD #20/21 and later versions. The calibration is programmed into the vehicle's VCM via a Techline Tool. Use a Techline Terminal or scan tool to perform the learn procedure and program the VCM.

Important: Use the calibration file "Special Policy 99046" on TIS 2000 CD #21 or Techline CD #20/21 or later versions.

1. To ensure VCM programming/RPO configuration, confirm that the following conditions exist in order to prepare for VCM programming:

•	The battery is fully charged.

•	The ignition switch is in the RUN position.

•	The Data Link Connector (DLC) is accessible.

2. Refer to the latest Techline Terminal and equipment user's instructions.
3. Clear the diagnostic trouble codes (DTCs) after the programming is complete.

Submit a claim with the information indicated below.

Suggested Dealer Letter

As the owner of a General Motors truck equipped with the Lucas Varity three-sensor antilock brake system (ABS), your satisfaction with our product is of utmost concern to us.

The federal government's highway safety agency, the National Highway Traffic Safety Administration (NHTSA) has identified, and General Motors Corporation has confirmed, the existence of a condition in the antilock braking system of some Chevrolet and GMC 1994-1996 S/T pickups equipped with a V6 engine, and 1995-1996 S/T utility vehicles. On rare occasions, this condition can result in longer stopping distances during certain antilock brake applications, as explained below.

If you're driving on a road surface that supports good traction and you begin to stop by applying your brake pedal firmly, and both front wheels of your vehicle then pass onto a slippery surface (such as an ice-covered or wet patched asphalt part of the road), your antilock brake system will adjust the brakes at each of the wheels to take advantage of the available traction. This will allow you to steer and maintain stability, which is normal ABS operation, as your owner's manual explains in more detail.

However, if you are still braking while the vehicle leaves the slippery surface and both front wheels get back on a higher-traction surface, the ABS may perform as if the vehicle were still on the slippery surface and the vehicle may not stop as quickly. However, this will not happen every time these conditions are encountered. It depends on several additional factors, such as vehicle speed and the length of the slippery surface.

Your ABS system was designed with increased sensitivity to wheel slip in order to improve vehicle steerability while braking on very slippery surfaces. This improvement for steerability, however, made it possible for reduced front braking effectiveness to occur as described above. Therefore, GM has developed a software change that will make your vehicle less sensitive to wheel slip under the circumstances described above.

Upon your request, your GM dealer will make a change to your antilock braking software to prevent this phenomenon from occurring. This software change will have only a slight effect on vehicle steerability during braking on very slippery surfaces and is designed to have no effect on normal ABS or other braking operations. This change should not affect how your brakes feel or create any perceptible difference in the steerability of stability of your vehicle while braking. This modification will be performed for you at no charge at any time until December 1, 2002.

Your GM dealer will modify your vehicle's ABS software. We estimate that it will take your dealer 45 minutes to perform this modification. Additional time may be required to schedule and process your vehicle. If your dealer has a large number of vehicles awaiting service, this additional time may be significant. Please ask your dealer if you wish to know how much additional time will be needed.

Repairs and adjustments qualifying under this special coverage must be performed by a GM dealer. You may want to call the service department to arrange a convenient appointment.

GMODC

General Motors Corporation