INSTRUMENT PANEL DIAGNOSIS SERVICE MANUAL UPDATE. Buick Century 1988

VEHICLES AFFECTED: 1988 CHEVROLET AND PONTIAC A CAR

SERVICE MANUAL UPDATE

The following pages should replace the current pages in the specified Manuals.

Page 8A-80-11 1988 Chevrolet A Car

8A-82-12-STE 1988 Pontiac A Car

INSTRUMENT PANEL: INDICATORS CLUSTER

N: TACHOMETER TEST ------------------ Measure: VOLTAGE At: TACHOMETER CONNECTOR (Disconnected) Conditions: - Ignition Switch: RUN - Park Lights: ON

Measure Correct For Diagnosis Between Voltage of Incorrect Results ----------------------------------------------- A (PNK/BLK) Battery See 1 & Ground ----------------------------------------------- A (PNK/BLK) Battery See 2 & C (BLK) ----------------------------------------------- E (WHT) & C 10 volts or See 3 (BLK) greater ----------------------------------------------- Greater than 5 F (GRY) & C volts variable See 4 (BLK) with Dimmer Control ----------------------------------------------- D (BRN) & C Battery See 5 (BLK) ------------------------------------------------

- If all voltages are correct, replace the Tachometer.

1. Check the PNK/BLK (39) wire for an open.

2. Check the BLK (150) wire for an open.

3. Check the WHT (121) wire for an open. If OK, see Section 6D.

4. Check the GRY (8) wire for an open (see Section 8A-117).

5. Check the BRN (9) wire for an open (see Section 8A-12).

O: SHIFT INDICATOR WIRE TEST ---------------------------- Disconnect connector C3 from the Electronic Control Module (ECM). Put the Ignition Switch in RUN and measure the voltage at connector C3 terminal F6 (see schematic).

- If battery voltage is present, see Section 6E.

- If no voltage is present, check the TAN/BLK and TAN/WHT (456) wire for an open or short to ground (see schematic).

P: SPEEDOMETER AND ODOMETER ARE NOT ACCURATE (J33431-10 Harness Connector available) ---------------------------------------------------------- 1. Disconnect connector C210. Plug in the J33431 Instrument Panel Cluster Tester and connect the J33431-10 harness connector to terminal N and G round. Turn the Ignition Switch to RUN. Set the Tester Controls to ON, 54 MPH and 60Hz. Observe the Speedometer. It should read 54 mph +/- 2 mph.

- If the Speedometer reads correctly, check for correct tire size and proper inflation. If tire size and inflation are OK, replace the ECM.

- If the Speedometer does not read correctly replace the Instrument Cluster (see Section 8C).

CIRCUIT OPERATION

If the complete circuit of an indicator is not shown on Instrument Panel schematics, see the circuit referred to for the complete operation of the indicator. Only the indicators and gages that do not appear in other schematics are described in this section.

Oil Indicator

When starting the engine, the OIL Indicator lights as a bulb test. The bulb circuit is case grounded through the Oil Pressure Switch. After the engine starts, and the oil pressure rises above 27 kPa (4 PSI), the Oil Pressure Switch opens. With no path to ground, the OIL Indicator goes out. If the oil pressure fails, the Oil Pressure Switch closes. The OIL Indicator bulb lights for one side is again grounded through the Oil Pressure Switch.

Temperature Indicator

The TEMP Indicator bulb is tested in BULB TEST and START by grounding through the Ignition Switch. The indicator lights in RUN when the engine coolant temperature is above 258 degrees F (126 degrees C) and the Coolant Temperature Switch closes.

Tailgate Ajar Indicator

Voltage is applied at all times through the GAGES Fuse to the TAILGATE AJAR Indicator. When the Tailgate in unlatched, the Tailgate Ajar Switch is closed. This provides a ground for the TAILGATE AJAR Indicator which allows the bulb to light.

Fuel Gage

The pointer of the Fuel Gage is moved by the magnetic fields of two coils. The coils are at right angles to each other. Battery voltage is applied to the "E " coil and the circuit divides at the opposite end of this coil. One path continues to ground through the "F" coil. Another goes to ground through the variable resistor of the Fuel Gage Sender.

H: TEMPERATURE GAGE SHOWS HOT ----------------------------- Remove the connector from the Coolant Temperature Sender and turn the Ignition Switch to RUN.

- If the display is now blank, the wiring is good. Do Test M.

- If the display still shows hot, check for a short to ground in the DK GRN/WHT (35) wire back to the Instrument Cluster. If that wire is good, replace the Instrument Cluster (see Section 8C).

I: TEMPERATURE GAGE IS INACCURATE OR IS BLANK --------------------------------------------- Remove the connect from the Coolant Temperature Sender. Connect one red clip lead of the J-33431 tester to the harness connector terminal and the other red clip lead to ground. Adjust the resistance dials of the tester to 912 +/- 109 ohms and then to 85 ohms. The Temperature Display should show low temperature, approximately 120 degrees F (49 degrees C) and then hot 260 degrees F (127 degrees C).

- If the display is correct, the cluster is good. Install a new Coolant Temperature Sender.

- If the display is not correct, check the DK GRN/WHT wire for an open or high resistance. If the wire is OK, replace the Instrument Cluster (see Section 8C).

J: SPEEDOMETER AND ODOMETERS ARE INACCURATE (J33431-10 Harness Connector Available) ----------------------------------------------------------------------------- Disconnect Electronic Control Module connector C2. Plug in the J-33431 Instrument Panel Cluster Tester and connect the J33431-10 Harness Connector to C2 terminal B11. Turn the Ignition Switch to RUN. Set the tester con- trols to ON, 54 MPH and 60 Hz. The Speedometer should indicator 54 +/- 2 MPH.

- If the Speedometer reads correctly, check for correct tire size and inflation. If OK, replace Electronic Control Module.

- If the Speedometer does not read correctly, replace the Instrument Cluster (see Section 8C).

K: SHIFT INDICATOR TEST ----------------------- Connect: FUSED JUMPER At: CONNECTOR C210 (MALE HALF) (Disconnected) Condition: - Ignition Switch: RUN

Jumper Correct Result For Diagnosis Between -------------------------------------------- H (TAN/BLK) SHIFT & Ground Indicator See 1 lights ---------------------------------------------- - Remove Jumper

Jumper SHIFT removed Indicator goes See 2 out ---------------------------------------------- - If the results are correct, check the TAN/ BLK (456) wire to the ECM for an open or a short to ground. If the wire is OK, see Section 6E for ECM diagnosis.

1. Check TAN/BLK (456) wire from the Instrument Cluster for an open. If the wire is good, replace the Instrument Cluster (see Section 8C).

2. Check TAN/BLK (456) wire from the Instrument Cluster for a short to ground. If the wire is OK, replace the Instrument Cluster (see Section 8C).

CIRCUIT OPERATION

Audible Alarm and Gages

With the Ignition Switch in RUN, voltage is applied through the FAN/ELEC Fuse to the Battery Power Input terminal of the Gages Module in the Instrument Panel Digital Cluster. With the Light Switch in PARK or HEAD, voltage is applied at all times to the Lights-On Dim Input.

English/Metric Switch

The English/Metric Switch, located in the Display Switch Assembly, by-passes either one of the two light emitting diodes. In the Metric position, the E diode is shorted out and the M diode is conducting and glowing. Battery voltage is also applied to the English/Metric Input of the Digital Cluster to connect the solid state processor to Metric readouts. With the Switch the English position, the M diode is by-passed. The E diode conducts and glows. The English/Metric Input to the Digital Cluster is now grounded.

Fuel Display

The Fuel Gage Sender provides a signal to the cluster that is related to fuel level. With a full tank, the resistance is 90 ohms, and with an empty tank, the resistance is less than 1 ohm. The microprocessor converts this signal and activates a 16-segment bar-graph which displays the fuel level. Each segment corresponds to one gallon of fuel. When the bar-graph Fuel Display changes from two segments to one, the Audible Alarm sounds briefly and the low Fuel Warning Indicator lights.

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.