RWAL Brake Warning Light - False Code 7: Overview

90chevy24
GMC NUMBER: 91-5E-21

GROUP: 5E-ANTI-LOCK BRAKES

DATE: SEPTEMBER, 1990

CORPORATE
NUMBER: 065013
SUBJECT: RWAL CODE 7/BRAKE WARNING LIGHT ILLUMINATES

MODELS: 1989-91 S/T AND 1988-90 C/K WITHOUT AIR CONDITIONING
Some 1989-91 S/T and 1988-90 C/K trucks without air conditioning may experience a condition in which the brake warning light is illuminated and a false code 7 is set to the RWAL ECU memory. This condition is most often observed on trucks equipped with manual transmissions.

It has been determined that a false code 7 can be set on these vehicles if the following conditions are met:

1. The blower motor is set on high and

2. The ignition is turned off and the brakes are applied while the truck is still moving. Because of the unusual circumstances owners may drive for some time between occurrences of code 7.

This condition is caused by slow blower motor "wind down". When the ignition is cycled off the blower motor will wind down and may supply the RWAL ECU with sufficient power to set a false code 7. To correct this concern in production a change was made to ECU modules in August, 1990.

Technicians can identify the new ECUs from the old by examining the long bar code sticker on the unit. Old ECUs have 15 digit alphanumeric codes at the bottom of the sticker beginning with 912349K. The new ECUs also have a 15 digit alphanumeric code but the code begins with 919220K.

To repair this condition a relay should be installed between the blower motor and the resistor block to prevent any feed back of current to the RWAL system from the high blower line.