1989-1997 Chevrolet S-10 ABS Light On: Common Causes and Fixes

By Core Brake Parts Tech Team — Circleville, OH warehouse

An ABS light on a 1989-1997 Chevrolet S-10 means the Kelsey-Hayes anti-lock computer has found a fault and switched the anti-lock function off. Your normal brakes still work, but the wheels can lock in a panic stop until the fault is found. On these trucks the usual culprits are a speed sensor or its wiring, low or dirty brake fluid, a weak electrical supply, or the ABS valve and control unit itself, and the system will flash its own trouble code so you can find out which.

Which ABS System Your S-10 Has

S-10s from this era did not all get the same anti-lock system, and the fix depends on which one is on your truck. Both are Kelsey-Hayes designs.

• RWAL (Rear Wheel Anti-Lock). Controls only the rear brakes, as one pair. The front brakes are conventional. Many 2WD S-10 pickups and 2WD Blazers carry this system, sold as 2-wheel ABS. RWAL can store only one trouble code at a time.
• 4WAL / EBC4 (four-wheel anti-lock). Kelsey-Hayes introduced 4WAL in 1990 and the later EBC system in 1993. It has three control channels: left front, right front and the rear axle, with a pump and a brake pressure modulator valve (BPMV) bolted to the control unit. It can store several codes at once.

Not sure which you have? Look at the front wheels. A four-wheel system has a speed sensor at each front knuckle and a modulator with an electric pump; RWAL does not use front wheel sensors.

What the Warning Lights Are Telling You

These trucks use two lamps, and they mean different things:

• Yellow ANTILOCK light: an electrical or other failure has disabled the ABS. Base braking is still there, just without anti-lock.
• Red BRAKE light: comes on with the parking brake applied or when there is a hydraulic problem such as low fluid. A red BRAKE light that stays on with the parking brake fully released is a stop-and-check item, not something to drive around with.

Both lamps should light briefly at key-on as a bulb check and then go out. If a lamp never comes on at all during that check, suspect the bulb or its circuit before anything else, because a dead bulb also hides flash codes.

How to Read the Codes Without a Scan Tool

Kelsey-Hayes built these systems so you can pull flash codes by hand. On GM trucks the procedure is the same as on GM cars of the period: with the ignition on and engine off, jumper terminal H to ground (terminal A) at the under-dash diagnostic connector, often called the ALDL. Then count the lamp flashes. A scan tool that covers the Kelsey-Hayes system reduces the chance of miscounting, and on these systems a scan tool is also the normal way to clear codes.

Two habits save a lot of wasted parts:

1. On RWAL, fix one code, then re-check. Because it only remembers one fault, a second problem can hide behind the first. Clear the code after the repair, drive, and read it again.
2. Look up the code before buying anything. The code tells you whether the fault is a sensor circuit, a valve circuit, or the control unit. Use the code chart in the factory service manual for your exact year, since the lists differ between RWAL and 4WAL.

Common Causes of an S-10 ABS Light

1. Speed sensor or sensor wiring

This is the most common fault family on any ABS. On four-wheel systems the front sensors sit on the knuckle dust shields, where road debris, rust and corrosion get at them. MOTOR magazine gives a resistance range of 1,000 to 3,000 ohms for each 4WAL wheel speed sensor, but a sensor can measure in range and still drop its signal while the wheel turns, so a lab scope is the better test. It also notes that tires rubbing a sensor harness can cause signal dropout, a classic cause of a light that comes and goes on bumps or turns.

2. Low charging voltage or a poor ground

The 4WAL pump and solenoids can pull up to 45 amps. A tired battery, loose battery cable or weak alternator can trip a fault. Check the battery, charging output and the module grounds before condemning the control unit.

3. Low, old or contaminated brake fluid

Low fluid can bring on the red BRAKE lamp, and dirty fluid causes valve trouble. MOTOR describes a GM truck RWAL case where contamination kept the isolation valve from closing, letting the rear wheels lock. If the fluid in your reservoir is dark, a full flush with the fluid type printed on the reservoir cap is cheap insurance. MOTOR also warns that a leaking master cylinder can let fluid intrude into control-unit circuits on 4WAL trucks.

4. The ABS valve or control unit

Codes that point to the isolation valve, dump valve or ECU usually mean the valve assembly or electronic control unit needs replacing. Confirm the power, ground and connector condition first. After any modulator or valve replacement the system must be bled correctly; air trapped in the modulator circuits can cause a spongy pedal that shows up days later.

5. The 2WD/4WD signal on 4x4 trucks

The four-wheel ABS needs to know whether the truck is in 2WD or 4WD. NHTSA recall 99V193000 covers certain 4WD pickups and SUVs built September 1989 through August 1996 where that switch can malfunction and increase stopping distance in 2WD. A related action, 99I006000, covered trucks with 3-sensor EBC4 units that could have extended stopping distances on certain mixed surfaces. If you own a 4WD S-10 or Blazer, run the VIN on NHTSA's recall lookup to see whether that work was ever done.

Fixing It and What to Replace at the Same Time

Once the code points you at the fault, the repair order is usually:

1. Check fluid level and condition, battery voltage, charging output and grounds.
2. Inspect the sensor wiring for rubbing, broken clips and green, corroded connectors. Repair wiring before buying a sensor.
3. Test the sensor itself, then replace it if it fails.
4. Replace the valve or control unit only when the code and your circuit checks agree.
5. Clear codes, road test, and re-read.

A sticking caliper, worn pads or a leaking wheel cylinder will not always set an ABS code, but they cause the pulling and lockup complaints that owners often blame on the ABS. If the brakes are due, do the base system at the same time: brake pads for the S-10, replacement rotors, and fresh rubber brake hoses if the originals are cracked. For hydraulic faults, see our master cylinders and brake calipers. For 2WD Blazers with 2-wheel ABS, we carry the EBC Greenstuff 6000 front pad set DP61146.

Difficulty: reading codes and cleaning connectors is easy driveway work. A sensor swap is moderate. Replacing and bleeding a modulator or RWAL valve is harder because of the bleed procedure, and many owners hand that part to a shop with the right scan tool.

Frequently Asked Questions

Is it safe to drive my S-10 with the ABS light on?

With only the yellow ANTILOCK light on, the normal brakes work but the wheels can lock in a hard stop, so leave extra distance and get it diagnosed. If the red BRAKE light is on with the parking brake released, check the fluid and hydraulics before driving.

Can I read S-10 ABS codes without a scan tool?

Yes. Jumper terminal H to terminal A at the under-dash diagnostic connector with the key on and count the lamp flashes. A scan tool is still the usual way to clear codes.

Why does the light come back after I cleared the code?

Either the fault was not fixed, or on a rear-wheel-only RWAL truck a second fault was hiding behind the first, since RWAL stores one code at a time. Re-read the code after every repair.

Will new brake pads turn off the ABS light?

Not by themselves. Pads do not feed the ABS computer. But a brake job is a good time to inspect the sensor wiring, flush the fluid and fix any leak that is setting the red BRAKE lamp.

Was there a recall on S-10 anti-lock brakes?

Yes. In 1999 GM announced actions covering certain S and T trucks with Kelsey-Hayes ABS, including a 2WD/4WD switch recall on some 4WD models and a computer program fix for EBC4 units. Check your VIN on nhtsa.gov.

Sources

1. AA1Car - Reading ABS Codes - RWAL stores one code; 4WAL introduced 1990, EBC 1993, multiple codes; GM manual flash-code method
2. MOTOR - The Nuts and Bolts of Kelsey-Hayes 4WAL - three channels, 45-amp draw, sensor location and 1,000-3,000 ohm range, lamp meanings, harness rub and bleeding issues
3. MOTOR - Trouble Shooter, March 2000 - GM truck RWAL isolation valve held open by contaminated fluid
4. NHTSA recall data, 1994 Chevrolet S10 - campaigns 99V193000 and 99I006000
5. CBS News - GM Recalls 3.5 Million Vehicles - 1999 Kelsey-Hayes ABS recall scope for S and T trucks