SERVICE MANUAL UPDATE SEC.8A-ABS REVISED DIAGNOSTICS

VEHICLES AFFECTED: 1986-1989 "Y" CORVETTE

Use the attached revised tests when servicing the Antilock Brake System for the following Service Manuals. Test procedures, resistance values and connector information have been updated or added to enhance the ABS diagnostics.

1986 Corvette Electrical Diagnosis Service Manual Supplement pages 44-10, 44-11 and 44-12.

1987, 1988 and 1989 Corvette Electrical Diagnosis Service Manual Supplement pages 44-9, 44-10 and 44-11.

Test B: Solenoid Valve Resistance Test

Test E: Lateral Acceleration Switch Test

Test F: Wheel Speed Sensor Test

Test G: Pump Motor Relay Test (table 2)

B: SOLENOID VALVE RESISTANCE TEST

Measure: RESISTANCE (200 ohm scale)

At: MODULATORVALVE(Connector (Disconnected)

Measure Correct Result For Diagnosis Between ------- 1 & 4 0.8 to 1.7 See 1 ohms 2 & 4 0.8 to 1.7 See 1 ohms 5 & 4 0.8 to 1.7 See 1 ohms

If all measurements are correct, check that all measurements are within 0.1 ohms of each other. If not, replace the Modulator Valve. If measurements are OK, return to the Symptom Directory.

1. Replace Modulator Valve.

E: LATERAL ACCELERATION SWITCH (TABLE 1)

Measure: RESISTANCE

At: CONNECTOR C400 FEMALE HALF (Disconnected)

Condition:

Ignition Switch: OFF

Measure Correct For Diagnosis Between Resistance ------------ ------- ----------- B & H Less than 100 See 1 ohms

If measurement is correct, check wires to Module Relay and Control Module for an open (see schematic).

1. Go to Table 2.

E: LATERAL ACCELERATION SWITCH (TABLE 2)

Measure: RESISTANCE

At: LATERAL ACCELERATION SWITCH (Disconnected)

Conditions:

Ignition Switch: OFF Switch removed from vehicle Switch held in same relative position as when installed on vehicle.

Measure Correct For Diagnosis Between Resistance ------- ---------- ------------- A&B Less than 100 See 1 ohms

Tilt switch away from horizontal position.

A&B Greater than See 1 2.5 M ohms

If both measurements are correct, check wiring to connector C400 for an open or short to ground (see schematic).

1. If either measurement is incorrect, replace the Lateral Acceleration Switch.

F: WHEEL SPEED SENSOR TEST

Measure: RESISTANCE

At: CONTROL MODULE CONNECTOR PINOUT BOX (Control Module Disconnected)

Conditions:

Ignition Switch: OFF

J135592 Pinout Box connected to Control Module Connector.

Measure Correct For Diagnosis Between Resistance ------- ---------- -------------- 4 & 6 900 to 1100 See 1 ohms

11 &21 900 to 1100 See 2 ohms

9 & 8 900 to 1100 See 3 ohms

24 & 26 900 to 1100 See 4 ohms

Bend and gently pull on wheel speed sensor wires at the wheel and sensor connectors while measuring resistance to check for intermittent opens.

1. Check the Left Front Wheel Speed Sensor connector at the wheel to insure that it is properly mated. Check sensor wires and connectors for opens. If all wiring is good and sensor resistance is incorrect, replace the sensor.

2. Check Right Front Wheel Speed Sensor connector at the wheel to insure that it is properly mated. Check sensor wires and connectors for opens. If all wiring is good and sensor resistance is incorrect, replace the sensor.

3. Check Left Rear Wheel Speed Sensor connector at the wheel to insure that it is properly mated. Check sensor wires and connectors for opens. If all wiring is good and sensor resistance is incorrect, replace the sensor.

4. Check Right Rear Wheel Speed Sensor connector at the wheel to insure that it is properly mated. Check sensor wires and connectors for opens. If all wiring is good and sensors resistance is incorrect, replace the sensor.

G: PUMP MOTOR RELAY TEST (TABLE 2

Measure: VOLTAGE

At: MODULATOR VALVE HARNESS CONNECTOR (Disconnected)

Condition:

Ignition Switch: RUN

Measure Correct For Diagnosis Between Voltage ------- ------- -------------- 10 (GRY/ BLK) & Battery See 1 Ground 12 (RED) & Battery Ground See 2

If voltages are correct but were not present in Table 1, replace the Modulator Valve.

1. Check the GRY/BLK wire for an open back to the Module Relay (see schematic). If wire is good, do Test A.

2. Check Fusible Link J and the RED (2) wire for an open (see schematic).

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.