FALSE CODE 65 SET 4WAL BRAKE DIAGNOSTICS

Subject: FALSE CODE 65 SET WITH WHEEL SPEED SENSOR CODES 23, 27, 28, 29, 33, 37

Model and Year: 1990 M/L, 1991 S/T 4 DOOR UTILITY TRUCKS WITH 4-WAL

In the event a problem is experienced with the input of speed signal from the wheel speed sensors any of the following codes may be set:

Code 23 RF wheel speed sensor has erratic voltage output Code 27 LF wheel speed sensor has erratic voltage output Code 28 Erratic sensor (1 to 2 "dropouts") Code 29 simultaneous "drop-out" of all 4 sensors Code 33 RR wheel speed sensor has erratic voltage output Code 37 LR wheel speed sensor has erratic voltage output

The root cause of these codes could be a malfunctioning sensor or shorted sensor wires or a loose or damaged 8 pin connector.

A peculiarity within the 4WAL system may set a code 65 at the same time codes 23, 27, 28, 29, 33, and 37 are set. Code 65 usually indicates that the pump motor relay has an open circuit which would require the replacement of the EHCU (Electro-Hydraulic Control Unit). When set with the above sensor codes, however, this is a FALSE code and the EHCU SHOULD NOT BE REPLACED. Once the appropriate sensor repair has been made the codes should be cleared and a function test of the system, using Tech 1 should be made.

A software revision has been made in production to eliminate the false code 65 setting with codes 23, 27, 28, 29, 33, and 37 on vehicles built after the following VIN breakpoints:

M Van LB225290

L Van LB225573

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.