FIRM-HARSH SHIFT/NO 4TH GEAR/ NO TCC (DON'T REPLACE TRANS). GMC Truck GMC Suburban

SUBJECT : FIRM-HARSH SHIFT/NO 4TH GEAR/NO TCC/HESITATION/SURGE/ENGINE STALL/CODE 22 (DON'T REPLACE TRANS)

VEHICLE APPLICATIONS: 1991 L/D TRUCKS W/4L80E

TRANSMISSION APPLICATIONS: 4L80-E/4L80-EHD (MT1) AUTOMATIC TRANSMISSIONS

TRANSMISSION MODELS:

SUBJECT: FIRM OR HARSH SHIFTS, NO 4TH GEAR, NO TCC, HESITATION, SURGE, STALL CAUSED BY LOW SIGNAL VOLTAGE AT IDLE FROM THE THROTTLE POSITION SENSOR (TPS), CODE 22

MODELS: 1991 C/K, R/V, G AND P MODEL TRUCKS EQUIPPED WITH GAS ENGINES AND 4L80-E/4L80-EHD (MT1) AUTOMATIC TRANSMISSIONS

Some 1991 C/K, R/V, G and P models equipped with gas engines and 4L80-E/4L80-EHD automatic transmissions may experience firm or harsh shifts, no 4th gear or no TCC caused by a low signal voltage at idle from the TPS (throttle position sensor), often after a short hot-soak period. Engine conditions of hesitation, surging or stall may also be noted. An inoperative TPS will also normally set a code 22 (TPS Signal Voltage Low).

These transmission conditions, referred to as defaults, are the normal result of low signal voltage from the TPS. When low signal voltage is received from the TPS, a code 22 is set and the transmission is commanded to default conditions to minimize the risk of transmission damage. These default conditions are as follows:

Maximum Line Pressure Firm/Harsh Shifts Fixed Shift Points No 4th Gear No TCC

These default conditions will clear on the next ignition cycle and will not reappear unless low TPS signal voltage is detected again. In some of the involved vehicles, this low TPS signal voltage is caused by a buildup of lubricant inside the TPS. This buildup results in poor electrical contact inside the TPS at idle throttle positions and may occur intermittently.

Replacement of the transmission in cases of code 22 will not correct the problem and may result in a comeback for the same condition. A revised TPS featuring a different lubricant will soon be available through CANSPO. When a revised TPS is available, a bulletin will be published.

SERVICE PROCEDURE:

Refer to Electronic Transmission Control, section 7A4-24 of the applicable service manual for code 22 diagnosis and correction and further details on how the TPS interacts with the transmission.

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.