CONSTANT/INTERMIT. CHK ENG LT. DIAGNOSE PER LH6 CHART

SUBJECT: CONSTANT OR INTERMITTENT CHECK ENGINE LIGHT (DIAGNOSE PER LH6 CHART)

MODELS: 1991-93 C/K, R/V, P TRUCKS AND G-VAN

CONDITION:

Some 1991-93 trucks equipped with the L65 6.5 Turbo Diesel or the LL4 6.2 Diesel and 4L80-E/4L80-EHD exhibit a constant or intermittent check engine light with the engine running and no codes stored.

CAUSE:

The 4L80-E/4L80-EHD use an engine speed sensor and if failure occurs a code will not be stored. Code 12 will be present with the engine running and the ALDL grounded. The electronic transmission control section of the 1991, 1992 and 1993 service manuals does not acknowledge the use of an engine speed sensor or code 12 for the L65 (6.5 turbo) or the LL4 (6.2) electronic transmission applications.

CORRECTION:

Check for proper engine speed sensor operation as follows:

1991 and 1992 Vehicles

For diagnosis, use the code 12 chart from the LH6 Diesel Emission Section of the 1991 or 1992 Service Manual (Pages 6E2-22 and 6E2-23). When following the trouble tree, substitute TCM PIN D7 for ECM PIN Cl and TCM PIN D6 for ECM PIN A8.

1993 Vehicles

For diagnosis, use the code 12 chart from the LH6 Diesel Emission Section of the 1993 Service Manual (Pages 6E2-22 and 6E2-23 for the G Van, Pages 6E2-32 and 6E2-33 for the C and K vehicles). When following the trouble tree, substitute TCM PIN D7 for PCM PIN F5 and TCM PIN D6 for PCM PIN F4.

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.