FALSE SETTING - TROUBLE CODE FEEDBACK FROM GEN. LT. CIRCUIT

1984 S/T TRUCK 2.8L RPO LR2 WITH YF5 CALIFORNIA EMISSIONS 1984 G VAN 4.1 L, 5.0 L, 5.7L RPO LE3, LF3, LS9 WITH YF5 CALIFORNIA EMISSIONS, WITHOUT GAGE PACKAGE

A false Trouble Code 42, Electronic Spark Timing (EST) may be stored in the Electronic Control Module (ECM). This condition does not cause a typical EST type condition such as "no start", "starts/will not run" or a change in RPM.

A false Trouble Code 42 can be detected in one of two ways, either the Check Engine Light is on continuously or there is no Check Engine Light on, however, a Code 42 is stored in the ECM.

This condition can be caused by an electrical feedback from the generator light circuit back to the ECM. This condition can be corrected by installing a diode jumper assembly (see Figure No. 1), P/N 12047479, between the generator and the existing harness.

Before this diode jumper assembly is installed, it should be determined that it is a false code and not a legitimate EST problem.

Refer to the service manual for diagnosis of trouble codes.

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.