POOR LAUNCH NO POWER ESC UPDATE- REPAIR

MODELS: 1981-1982 C/K/G 15-25 SERIES TRUCKS WITH LE9 (5.0L V8) AND TRANSMISSIONS

Inability to launch vehicles equipped with engine RPO LE9 (305 CID, 4BBL with ESC) and a manual transmission when starting on a grade may be experienced.

The ESC system on the LE9 responds to engine detonation by retarding ignition timing (up to 20 DEG in 1981 and 1982). When under extreme load, the initial driveaway is accompanied by high levels of detonation. This in tum calls for maximum retard from the ESC system and causes significant loss of initial torque.

When launching the vehicle under heavy load, as soon as the clutch is engaged, engine speed drops to under 1000 RPM. When a knock is perceived at this low speed, the timing is retarded resulting in a loss of engine torque. This further decreases the engine/vehicle speed, forcing the driver to disengage the clutch.

A spark trap assembly (consisting of two traps in parallel), P/N 14068679, is being released for the distributor vacuum advance can. The trap assembly contains a bleed which will allow the vacuum advance to return to normal levels after approximately 5 seconds.

During the 5 second trap period, ESC retard will be offset by the trapped vacuum advance. This will enable a normal vehicle launch.

The use of this fix will increase the amount of knock allowed for a short period, and this knock may occur at all shift points. The fix should, therefore, be limited to only those cases where inability to launch due to ESC caused retard is a severe and consistent condition.

To install the spark trap, the following procedure should be followed:

1. Remove the vacuum hose from the vacuum advance can on the distributor.

2. Cut approximately five (5) inches from the end of the hose and discard.

3. Connect the hose end of the spark trap assembly to the vacuum can.

4. Connect the other end of the spark trap assembly into the vacuum hose.

5. Assure that the spark trap is installed in the correct direction. The green ends of the two (2) valves should be towards the vacuum can.

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.