4WAL SPEED SENSOR CIRCUIT MALFUNCTION AND DIAGNOSIS. Chevrolet Blazer

SUBJECT: WHEEL SPEED SENSOR CIRCUIT MALFUNCTION (DIAGNOSIS) 4WAL EQUIPPED

VEHICLES AFFECTED: 1990 M/L VANS, 1991 S/T 4-DOOR UTILITY VEHICLES

PRIMARY INDICATORS OF SPEED SENSOR CIRCUIT MALFUNCTION

The following items can be indications of malfunction in the speed sensor circuit.

* Premature EHCU (Electro Hydraulic Control Unit) activation (valve cycling during normal braking) WITH OR WITHOUT trouble codes relating to sensor failures being set, is a strong sign of a wheel speed sensor circuit condition.

* Customer comments of anti-lock light flashing momentarily but not remaining on (soft code may be set).

* Anti-lock light on and one or more of the codes shown below are set.

DIAGNOSIS

In the event of a wheel speed sensor malfunction on 4WAL equipped vehicles the EHCU may provide a means of determining which speed sensor circuit may be the root cause. The following trouble codes identify a rear sensor condition.

Code Description

28 Erratic Sensor (1 to 2 "Dropouts") 29 Simultaneous "Dropout" of all four sensors 31 RR Wheel speed sensor has incorrect resistance 32 RR Wheel speed sensor has incorrect voltage output 33 RR Wheel speed sensor has erratic voltage output 35 LR Wheel speed sensor has incorrect resistance 36 LR Wheel speed sensor has incorrect voltage output 37 LR Wheel speed sensor has erratic voltage output 38 Wheel speed sensor error (2:1 signal ratio above 12 MPH)

NOTE: A "dropout" is defined to be an interruption in the speed sensor signal to the EHCU

Technicians should closely follow diagnostic fault trees given in RWAL UPDATE 4WAL PREVIEW manual 15.005.05-1. Also the following guidelines may help the cause of the problem.

Any rear axle servicing may inadvertently affect speed sensor signals if wires are accidentally shorted or the sensor to tone wheel air gap becomes larger than 1.27mm (.05 inch). This could cause premature anti-lock valve operation during normal braking. To check the sensor air gap and connector the wheel, brake hub and drum and primary shoe must be removed. Refer to bulletin 90-5-113.

For trouble codes which indicate a problem with only one sensor, the entire sensor circuit from the 8 pin connector to the sensor should be checked for loose, corroded or dirty connections, shorted wires and open circuits.

Speed sensor resistance, when taken at the connection on the sensor, should be 1000 to 2000 ohms. Resistance outside of this range would indicate a bad sensor which would require replacement.

Speed sensor resistance, when taken at the 8 pin connector (terminals A and H for right sensor and terminals D and E for left sensor) should also be 1000 to 2000 ohms. Resistance measured outside of that range would indicate a short or open circuit.

Speed sensor output should be 1 volt AC when wheel is turned by hand 8 km/h (5 MPH) minimum. Voltage should be checked at the 8 pin connector (terminals A and H for right wheel sensor, terminals D and E for left wheel sensor).

For trouble codes which indicate a problem with more than one sensor the shared 8 pin connector is probable loose, broken or corroded.

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.