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Service bulletin 990421003

A bulletin is a manufacturer's communication to its dealers, as NHTSA indexes it. The summary is NHTSA's. The document is the manufacturer's: read it at NHTSA. A bulletin is not a recall, and listing a model year does not mean every vehicle of it is concerned.

The communication, as NHTSA indexes it (as of 2026-10-19)
TypeService bulletin
NHTSA id606091
Published by NHTSA1999-08-06
Manufacturer's number990421003
Manufacturer's date1999-06-01
NHTSA componentPOWER TRAIN:TRANSFER CASE (4-WHEEL DRIVE)

NHTSA's summary

SOME OWNERS MAY COMMENT ABOUT A SLIPPING TYPE CONDITION AT THE FRONT WHEELS WHILE THE VEHICLE IS IN FOUR WHEEL DRIVE (4WD). *TT

The document

No address for NHTSA's copy can be built for this communication. Ask the dealer for it by the manufacturer's number.

Components

Transmission and driveline, Transfer case

Vehicles listed
CHEVROLET BLAZER1999
CHEVROLET ENVOY1999
CHEVROLET JIMMY1999
CHEVROLET K SERIES1999
CHEVROLET SIERRA1999
CHEVROLET SILVERADO1999
CHEVROLET T SERIES1999
GMC BLAZER1999
GMC ENVOY1999
GMC JIMMY1999
GMC K SERIES1999
GMC SIERRA1999
GMC SILVERADO1999
GMC T SERIES1999

Listing a model year does not mean every vehicle of it is concerned: the document says which.

Other bulletins for the same vehicles and component
PIP3366C2018-08-20. Service bulletin. This Preliminary information communicates to the dealer technician thatLight Duty Trucks equipped with automatic transmissions and 4 wheel drive may exhibit a condition of grinding/ratcheting type noise when shifting the automatic transmission into park while the transfer case is in 4 wheel drive low (4LO) range. Dealers technician's are advised this condition should be deemed a normal characteristic of 4WD low operation.
34242005-11-02. Service bulletin. ELECTRONIC TRANSFER CASE UNABLE TO COMPLETE SHIFT. *TT
33892005-09-28. Service bulletin. FOUR WHEEL DRIVE INOPERATIVE AND THE 4WD SERVICE LIGHT WILL ILLUMINATE. DIAGNOSIS MAY REVEAL THAT THE ATC FUSE COULD BE OPEN, POSSIBLE DTC C0308, OR VARIOUS U CODES MAY BE SET. THE ENCODER MOTOR MAY ALSO BE SHORTED OR THE TCCM MAY BE DAMAGED. *TT
35262005-09-22. Service bulletin. INTERMITTENT SERVICE 4WD LIGHT COMES ON WITH DIAGNOSTIC TROUBLE CODE (DTC) C0306 OR C0308 STORED IN TCCM. *TT
33492005-05-10. Service bulletin. 4WD INOPERATIVE AND DIAGNOSTIC TROUBLE CODE (DTC) U1026 ATC FUSE OPEN. *TT
3012B2004-08-23. Service bulletin. THE NVG 136 ALL WHEEL DRIVE (AWD) TRANSFER CASE AND THE 236 AND 246 NVG SELECTABLE TRANSFER CASES OPERATING IN THE AUTO MODE MAY EXHIBIT A CONDITION OF A VIBRATION/SHUDDER DURING LOW SPEED TURNS SUCH AS A PARKING LOT MANEUVER, UP TO APPROXIMATELY 5 MPH. *TT 3012A NOT YET RECEIVED. *TT
014092003-11-22. Service bulletin. THE NVG 136 ALL WHEEL DRIVE (AWD) TRANSFER CASE AND THE 236 AND 246 NVG SELECTABLE TRANSFER CASE SHUDDER ON TURNS. *TT
0204210042002-10-10. Service bulletin. SOME CUSTOMERS MAY COMMENT ON THE SERVICE 4WD INDICATOR ILLUMINATING WITH DIAGNOSTIC TROUBLE CODE B2725 SET. *TT