ECM - False Setting of Trouble Code 42
Number: 84-218Section: 6E (VIt)
Date: Nov., 1984
Subject: FALSE SETTING OF TROUBLE CODE 42 CALIFORNIA EMISSIONS ONLY
Model and Year: 1984 "S/T" TRUCK 2.8L RPO LR2 WITH YF5 CALIFORNIA EMISSIONS AND 1984 "G" VAN 4.1L, 5.0L, 5.7L RPO LE3, LF3, LS9 WITH YF5 CALIFORNIA EMISSIONS, WITHOUT GAUGE PACKAGE
TO: ALL CHEVROLET DEALERS
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 or 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.
1984 COMPUTER COMMAND CONTROL "S/T" AND "G" VANS:
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 illustration), 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.