C 1500 Truck 2WD V6-262 4.3L VIN Z (1992)
PROM - Programmable Read Only Memory: Customer Interest
PROM - Lack of Power/Transmission Hot Mode
GMC NUMBER: 92-7A-102
Group: 7A Automatic Transmission
DATE: July 1992
CORPORATE NUMBER: 236509R
SUBJECT:
LACK OF POWER/TRANSMISSION HOT MODE (INSTALL MEMCAL/CHECK THERMAC DOOR)
MODELS:
1991-92 TRUCKS WITH 4.3, 5.7, 7.4L ENGINES
CONDITION
Some owners of 1991-92 trucks with 4.3, 5.7, 7.4L engines and 4-speed automatic transmission (4L80E) may comment on the conditions. See
illustrations.
4.3L (RPO LB4 with MTI transmission) 5.7L (RPO L05 with MT1 transmission)
1.
Transmission converter clutch (TCC) is locking too early in 2nd, 3rd, and 4th gears and may be perceived as lack of power at highway speeds.
7.4L (RPO L19 with MT1 transmission)
1.
Transmission converter clutch (TCC) is locking too early in 2nd, 3rd, and 4th gears and may be perceived as lack of power at highway speeds.
2.
Tach needle flutters on decelerations.
3.
Catalytic converter odor.
CAUSE 4.3/5.7L with MT1 transmission
1.
Transmission hot mode, which causes TCC lock-up in 2nd, 3rd, and 4th gears, entry temperature is too low.
2.
Thermac door in air cleaner may not be fully opening. This may cause the engine to operate on warm air, lowering power.
7.4L with MT1 transmission
1.
Transmission hot mode, which causes TCC lock-up in 2nd, 3rd, and 4th gear, entry temperature is too low.
2.
Engine fuel calibration is not optimal.
3.
At idle, the engine goes into an open loop condition which does not provide precise fuel control, possibly leading to catalytic converter odor.
4.
Thermac door in air cleaner may not be fully opening. This may cause the engine to operate on warm air, lowering power.
SERVICE PROCEDURE
4.3/5.7L with MT1 transmission
1.
Transmission hot mode entry temperature raised from 252 degrees F to 270 degrees F. Install MEMCAL per attached charts.
2.
Check Thermac door in air cleaner for correct operation. Door must open and close freely with no sticking or binding.
7.4L with MT1 transmission
Install MEMCAL per charts for corrections 1-3.
1.
Transmission hot mode entry temperature raised from 252 degrees F to 270 degrees F
2.
Engine decel fuel calibration revised.
3.
Engine idle revised from open loop to closed loop.
