UPDATE-TACH, GAGES & DIGITAL CLUSTERS (DIAGNOSIS PROCEDURE)

Models Affected: 1987-89 BONNEVILLE MODELS

This bulletin cancels and replaces bulletin 89-8-26 issued 9/89.

TACHOMETER DIAGNOSIS

The following procedures should be used to accurately diagnose an inoperative Tachometer.

The written procedures are for use with the 1987 thru 1989 Service Manuals, Section 8A.

1987-1988 SERVICE MANUALS

Section 8A-81, Gages Cluster

Use this procedure when a J 33431 tester is not available:

1. Disconnect Computer Controlled Coil Ignition Module connector which contains tachometer wire (circuit 121); see appropriate schematic or refer to connector illustration in Section 8A.

2. Turn ignition switch to RUN.

3. While watching Tachometer (or having an assistant watch it) momentarily jumper the tachometer wire (121) terminal "E" at Ignition Module connector to battery voltage, rapidly making and breaking jumper connection several times.

a. If Tachometer pointer moves, replace Ignition Module.

b. If pointer does not move, check circuits 39, 121, and 153 for proper voltage, and for open and short. Also, check that instrument cluster connectors are clean and tight. If circuits and connectors are okay, replace instrument cluster; see section 8C.

Use this procedure when a J33431 tester is available:

1. Connect J 33431-10 harness connector to J 33431 tester.

2. Disconnect Computer Controlled Coil Ignition Module connector which contains tachometer wire (circuit 121) see appropriate schematic.

3. Connect J 33431-10 black lead to ground, and red lead to tachometer wire (121) on Ignition Module connector.

4. Connect J 33431 tester to 120 volt AC outlet, set tester frequency signal switch to 60 Hz, speed signal switch to 54 mph, and turn on power switch.

5. Turn ignition switch to RUN, and observe tachometer.

a. If Tachometer shows 1350 rpm, replace Ignition Module.

b. If Tachometer does not operate, check circuits 39, 121, and 153 for proper voltage, and for open and short. Also, check that instrument cluster connectors are clean and tight. If circuits and connectors are okay, replace instrument cluster; see Section 8C.

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.