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

VEHICLES AFFECTED: 1987-89 'C' and 'H' CARS

TACHOMETER DIAGNOSIS

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

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

(Does Not Apply to Olds)

The illustrated procedures can be cut out and taped into the appropriate position in the 1989 Electrical Systems Manual (ESM).

1987-88 SERVICE MANUALS

Section 8A-81, Gauges 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.

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, replace Instrument Cluster; see Section 8C.

(Applies to both Buick and Olds)

Use this procedure when a J-33431 tester is avail ble:

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, 3nd 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.

(Does Not Apply to Olds)

SECTION 8A-82, DIGITAL 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.

2. Turn ignition switch to RUN.

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

a) If first bar on Tachometer display momentarily lights, replace Ignition Module.

b) If first bar does not light, check tachometer wire (121) for open or short to ground. Also, check that Digital Cluster connectors are clean and tight. If circuits and connections are okay, replace Digital Cluster; see Section 8C.

Use this procedure when a J-33431 tester is available:

1. Connect J-33431-10 harness connector is 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 read lead to tachometer wire (121) on Ignition Module connector.

4. Connect J-33431 tester to 120 bolt 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 indicates about 1200-1400 rpm, replace Ignition Module.

b) If Tachometer does not operate, check tachometer wire (121) for open or short to ground. Also, check that Digital Cluster connectors are clean and tight. If wire and connectors are okay, replace Digital Cluster; see Section 8C.

1989 ELECTRICAL SYSTEMS MANUAL (ESM)

Make the following changes in your copy of the ESM:

1. On page 237, delete the note "Greater than 10 volts with ignition switch at run and engine not running".

2. On Page 241, change the descriptive note for connector C1, terminal 18, to read: "Tachometer input from Computer Controlled Coil Ignition (CCCI) Module. Voltage varies with engine rpm. Do Test F."

3. On Page 242, replace Test F with the new Test F which accompanies this bulletin.

4. On Page 255, replace Test D with the new Test D which accompanies this bulletin.

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.