PROM - Lack of Power/Transmission Hot Mode
GMC NUMBER: 92-7A-102Group: 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.
4. Check Thermac door in air cleaner for correct operation. Door must open and close freely with no sticking or binding.
SERVICE PARTS INFORMATION
In case of limited inventory, parts will be placed on 400 control to waive VIP surcharges. Only verifiable emergency VIP orders will be accepted. GMSPO will make every effort to obtain parts. However, the part will be shipped premium transportation at dealer's expense and all other order types will be placed on backorder until the 400 control is removed.
1991 - 4.3L/RPO LB4
1992 - 4.3L/RPO LB4
1991 - 7.4L/RPO L19
1992 - 7.4L/RPO L19
1991-92 - 5.7L/RPO L05
WARRANTY INFORMATION
Labor Operation: T0500 - MEMCAL
Labor Time: 0.6 HL on C/K, R/V, and P models
0.5 Hr on G models