DELAYED SHIFT IN LOW GEAR FOUR WHEEL DRIVE MODE-NEW PROM

Subject: DELAYED SHIFTS IN LOW GEAR - FOUR WHEEL DRIVE MODE (NEW MEM-CAL/ PROM)

Model and Year: 1991 K2/K3, V2/V3 TRUCKS WITH 4L80-E-4L80-EHD AUTOMATIC TRANSMISSION

Some HYDRA-MATIC 4L80-E/4L80-EHD Transmissions may experience delayed shifts and malfunction code 85 generated when four wheel drive low gear is selected. This condition is caused by the PCM/TCM selecting the output speed sensor located on the transfer case instead of the output speed sensor from the transmission. Malfunction code 24 will not set if the output speed sensor from DRA (Transfer) is selected.

A new MEM-CAL/PROM is released for K and V trucks.

Date of Production Change: (Figure 1) -------------------------------------- Starting January 15, 1991 (Julian Date 015), all ACP, BJP, CKP, and DLP transmission model applications were built with a new MEM-CAL/PROM.

Service Procedure:

Refer to Section 7A of your Service Manual for Output Speed Sensor check for four wheel drive.

To correct this condition, a new service calibration MEM-CAL/PROM should be installed for all four wheel drive vehicles.

Service Parts Information:

Body Engine/RPO Current PCM/TCM New MEM-CAL/PROM -------- --------- ------------- --------------- K 4.3L LB4 16154381 16161422 K 5.7L L05 16154421 16161192 K 5.7L L05 16154431 16161196 K 5.7L L05 16154441 16161201 K 5.7L L05 16154451 16161205 V 5.7L L05 16154461 16161217 V 5.7L L05 16154471 16161433 K/V 6.2L LL4 16161529 16161710 K 7.4L L19 16154241 16161151 K 7.4L L19 16154251 16161156 K 7.4L L19 16154261 16161166 K 7.4L L19 16154291 16161173 K 7.4L L19 16158731 16161178 V 7.4L L19 16154311 16161183 V 7.4L L19 16154331 16161187 V 7.4L L19 16158741 16161225 V 7.4L L19 16154361 16161230 K 7.4L L19 16154271 16161427

Parts are currently available from GMSPO.

Service Manual Reference:

Refer to the On-Vehicle Service Manual Section of the appropriate service manual for proper replacement procedures.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.