Anti-Lock Brake System - Rear Wheel — GMC S15 Pickup (1987-1991)

DESCRIPTION & OPERATION

Note. For procedures not outlined in this article, refer to appropriate BRAKE SYSTEM article in this section.

General Motors light duty trucks use a Rear Wheel Anti-Lock (RWAL) system to prevent wheel lock-up during heavy braking. This is done by regulating fluid pressure to rear wheel cylinders. Pressure regulating is adjusted by a control valve located under the master cylinder. Control valve is controlled by an Electronic Control Unit (ECU) located next to master cylinder.

The ECU receives signals from a speed sensor, located in the transmission on 2WD models or in transfer case on 4WD models, and stoplight switch. Speed sensor signal is sent through a Digital Ratio Adapter Controller (DRAC) which is part of the instrument cluster. If axle ratio or tire size changes on C and K Series, recalibration of the DRAC must be performed. On all other models, DRAC must be replaced. The system is disabled when vehicle is in 4WD.

On models equipped with automatic transmission, an in-line resistor, located between module and stoplight switch, prevents battery voltage from energizing the transmission converter clutch solenoid. The RWAL system is connected to the existing brake warning light on the instrument panel. When ignition is turned on, warning light should come on for approximately 2 seconds to indicate system is operating.

DRAC CALIBRATION ADJUSTMENT

CAUTION

DO NOT touch pins on rear of instrument cluster or damage will occur.

C & K Series

Remove instrument panel. Plug Digital Ratio Adapter Calibration Service Kit (25085515) connector into rear of instrument panel. (Scheme 1) Momentarily touch wires across vehicle's battery terminals. See INSTRUMENT PANEL CALIBRATION TABLE. Remove appropriate pins from clip. Install clip in instrument panel. Install instrument panel.

AXLE RATIO	TIRE SIZE	RPO	HWY/OOR AT/ALS	PINS TO REMOVE
2.73 (GU2)	P195/75R15	YAR & YAS	ALS	1, 2, 4 & 5
"	P205/75R15	YCE & YCG	ALS	1, 4 & 5
"	P225/75R15	YET, YEV & YEU	ALS	2, 3 & 5
"	P235/75R15	YFL, YFM & YFN	ALS	3 & 5
3.08 (GU4)	P195/75R15	YAR & YAS	ALS	2, 3, & 6
"	P205/75R15	YCE & YCG	ALS	3 & 6
"	P225/75R15	YET, YEV & YEU	ALS	6
"	P235/75R15	YFL, YFN & YFM	ALS/OOR	2, 3, 4 & 5
3.42 (GU6)	P205/75R15	YCE, YCG	ALS	1, 3, 4 & 6
"	P195/75R15	YAR & YAS	ALS	1, 2, 3, 4 & 6
"	P225/75R15	YET, YEV & YEU	ALS	1, 4 & 6
"	P235/75R15	YFL, YFN & YFM	ALS	4 & 6
"	LT225/ 75R16	YHR, YHJ & YHN	OOR	2, 3 & 6
"	LT225/ 75R16	YHE, YHV, YHP & YHQ	ALS	2, 3 & 6
"	LT245/ 75R16	YGK, YBN & YBX	OOR	2 & 6
"	LT245/ 75R16	YHH, YBK & YBL	ALS	2 & 6
"	LT265/ 75R16	YGL & YGM	OOR	1, 2, 3, 4 & 5
"	7.50-16LT	YPF	HWY	1, 2, 3, 4 & 5
3.73 (GT4)	P195/75R15	YAR & YAS	ALS	1, 2, 3, 5 & 6
"	P205/75R15	YCE & YCG	ALS	1, 3, 5 & 6
"	P225/75R15	YET, YEV & YEU	ALS	1, 5 & 6
"	P235/75R15	YFL, YFN & YFM	ALS	1, 2, 3, 4 & 6
"	LT225/ 75R16	YHE, YHV, YHQ & YHP	OOR	1, 3, 4 & 6
"	LT225/ 75R16	YHR, YHS & YHN	OOR	1, 3, 4 & 6
"	LT245/ 75R16	YGK, YBN & YBX	OOR	2, 4 & 6
"	LT245/ 75R16	YHH, YBK & YBL	ALS	2, 4 & 6
"	LT265/ 75R16	YGL & YGM	OOR	2, 3, & 6
"	7.50-16LT	YPF	HWY	2, 3, & 6
4.10 (GT5)	LT225/ 75R16	YGL & YGM	OOR	1, 2, 3, 4 & 6
"	LT225/ 75R16	YHP & YHQ	ALS	1, 2, 3, 5 & 6
"	LT225/ 75R16	YHR	OOR	2, 3, 5 & 6
"	LT245/ 75R16	YGK, YBN & YBX	OOR	2, 5 & 6
"	LT245/ 75R16	YGK, YBN & YBX	OOR	2, 5 & 6
"	LT245/ 75R16	YHH, YBK & YBL	ALS	1, 2, 5 & 6
"	7.50-16LT	YPF	HWY	1, 2, 3, 4 & 6
4.56 (HC4)	LT225/ 75R16	YHP & YHQ	ALS	2 & 7
"	LT225/ 75R16	YHR	OOR	1 & 7
"	LT245/ 75R16	YGK	OOR	3, 4, 5 & 6
"	LT245/ 75R16	YHH	ALS	3, 4, 5 & 6
"	7.50-16LT	YPF	HWY	4, 5 & 6

INSTRUMENT PANEL CALIBRATION

TESTING & DIAGNOSIS

Note. Ensure brake warning light is on before trying to retrieve trouble code. If light is not on, grounding ALDL connector will lead to false CODE 9.

System can be diagnosed through trouble codes. To activate trouble codes, connect a jumper wire between terminals "A" and "H" on ALDL connector and observe flashing on brake warning light. Sequence should be one long flash followed by short flashes (i.e. one long flash followed by 4 short flashes indicates CODE 5).
System will repeat code as long as ALDL connector is grounded. Long flash after short flashes indicates repeat of code. If there is more than one failure, only the first failure code will be stored and displayed. See «TROUBLE SHOOTING CHARTS»(/gmc/s15-pickup/1987-1991/remont/anti-locktraction-control/#anti-lock-brake-system-rear-wheel) .
On C and K Series trucks with dual rear wheels, a "chirping" noise may occur when applying brakes. This is a mechanical problem, not RWAL. To correct, remove brake drums and spray two coats of Graphite Compound (12345268) onto brake linings. Allow compound to dry between coats.

Note. If ECU connector retaining clip is broken when unplugging connector, service clips are available from manufacturer.

BRAKE BLEEDING PROCEDURE

Brake system may be bled the same as a conventional system. If manual bleeding, wait 15 seconds between strokes. If pressure bleeding, pressurize brake bleeder to 20-25 psi (kg/cm 2 ). Brake bleeding sequence is as follows: right rear, left rear, right front, left front, isolation dump valve. Use DOT 3 brake fluid only.