1987-1991 BMW 325i ABS Light On: Common Causes and Fixes

By Core Brake Parts Tech Team — Circleville, OH warehouse

When the ABS light stays on in a 1987-1991 BMW 325i, the anti-lock system has switched itself off after its control unit saw a fault. On these E30 cars the usual culprits are a failed relay (the overvoltage relay or one of the two relays on the ABS module), a wheel speed sensor with damaged wiring or an open coil, or a corroded connector. Your normal brakes still work, but you have lost anti-lock protection until the fault is fixed.

How the ABS on an E30 325i works

The E30 uses a Bosch anti-lock system from the first generation of electronic ABS. Bosch describes the principle simply: a wheel that keeps rolling while you brake stops in a shorter distance and keeps the car steerable, while a locked wheel skids. To keep the wheels rolling, the system uses three groups of parts:

• Wheel speed sensors (BMW calls them pulse generators) at each corner, which report how fast each wheel is turning.
• The ABS control unit, which compares those signals and spots a wheel that is about to lock.
• The hydraulic unit (modulator), which the control unit commands to reduce brake pressure at that wheel for a split second.

Bosch introduced its first production electronic ABS in 1978, and it was first fitted to upper-class cars such as the BMW 7 Series before spreading down the range. By the late 1980s the same basic sensor, control unit and modulator layout was in the E30. The system also depends on relays that feed power to the control unit and the pump, and that is where many E30 faults start.

What the ABS light is telling you

The ABS warning lamp should come on briefly when you switch on the ignition while the system checks itself, then go out. If it stays on, or comes on while driving, the control unit has found something it does not like and has disabled anti-lock braking. How and when the light appears is a useful clue:

On a car this age, also assume that previous owners may have unplugged or bypassed things. Check that every sensor plug is connected before buying parts.

Cause 1: The ABS relays

BMW specialist RTS notes that three relays control the entire E30 ABS system: an overvoltage relay and two relays mounted directly on the ABS module. On early cars the overvoltage relay sits under the driver's knee bolster near the ABS module, and RTS lists pre-1988 cars as using the overvoltage relay with BMW part numbers 34521154894 or 34520005192. The two module relays are reached by removing the cover on the ABS module.

The weak point is a thin wire fuse inside these relays. When it breaks, the ABS loses power and the light comes on. Some owners resolder the fuse wire as a roadside fix, but RTS cautions that doing so removes the protection the fuse is there to provide. The better fix is replacement. Because the relays are the same age and fail the same way, RTS recommends replacing all of the ABS relays at once rather than chasing them one by one.

A 1987 car sits on the early side of the pre-1988 split, while 1988-1991 cars may use a different relay arrangement. Confirm the relay layout on your car against the parts catalog for your VIN before ordering.

Cause 2: Wheel speed sensors and their wiring

Each corner has its own sensor. BimmerWorld lists the front left sensor, BMW part number 34521178981 (superseding 34521155037), as fitting rear-wheel-drive E30 models including the 325i, 325is and 325e. The sensors sit near the hub where they are exposed to heat, water, road salt and brake dust, and after 30-plus years the wire insulation is often brittle.

You can test all four sensors from one spot. RTS describes unplugging the connector at the ABS control unit and measuring resistance across these pin pairs:

RTS says each sensor should read about 1,000 ohms. Some variation between sensors is normal; a failed sensor usually reads very differently from the other three, or shows an open circuit. If one reading is off, test again at that sensor's own connector to find out whether the sensor or the harness between it and the control unit is at fault. Confirm the pinout against a wiring diagram for your exact car before probing.

A sensor can pass the resistance test and still give a weak or erratic signal if its tip is caked with rust or the pickup ring is damaged, so inspect the sensor tip and ring whenever the wheel is off.

Cause 3: The control unit and hydraulic unit

If the relays are good, power reaches the control unit, and all four sensors read correctly, the fault may be in the control unit itself or the hydraulic unit. These are the least common and most expensive causes, so rule out relays, fuses, connectors and sensors first. Look for corrosion on the control unit connector pins and make sure its ground is clean. A professional scan with a tool that can talk to early BMW systems is worth doing before you replace either unit.

Can you drive with the ABS light on?

The base hydraulic brakes on the 325i work independently of the anti-lock function, so the car will still stop. What you lose is the ability to steer while braking hard, because the wheels can lock in an emergency stop. Drive with extra following distance, especially in the wet, and get the fault fixed.

Keep in mind the ABS light only reports on the anti-lock system. It will not warn you about worn pads or a sticking caliper. If you are chasing the ABS fault with the wheels off, it is a good time to check pads and rotors. Browse brake pads and brake rotors, or if you drive your E30 on track, see the Hawk DTC-70 front race pad set for the 1987-1991 325i. Race compounds are made for heat and are not the right pick for a daily driver. Worn soft lines are also worth replacing while you are there; see our brake hoses.

How hard is the fix?

• Relays: easy. Remove a trim panel or the module cover, swap the relays, and clear the fault by cycling the ignition.
• Sensor testing: easy with a multimeter, done from the control unit connector.
• Sensor replacement: moderate. The sensor is held by a bolt, but old sensors are often seized in their bores. Penetrating oil and patience help; avoid prying on the body of a new sensor.
• Wiring repair: moderate. Solder and heat-shrink any repair, and route the wire away from moving suspension parts.
• Control unit or hydraulic unit: advanced. Any work that opens the hydraulic system needs a proper brake bleed afterward.

Frequently Asked Questions

Why does my E30 ABS light stay on right after I start the car?

A light that never goes out after the self-check usually points to the ABS power supply, most often a failed overvoltage relay or one of the two relays on the ABS module, whose internal fuse wire breaks with age.

What resistance should an E30 ABS sensor read?

BMW specialist RTS gives about 1,000 ohms per sensor, measured at the ABS control unit connector. The key is comparing all four: a bad sensor usually reads well away from the others or shows open circuit.

Is it safe to drive a 325i with the ABS light on?

The normal brakes still work, but anti-lock is disabled, so the wheels can lock under hard braking. Drive cautiously and repair it soon.

Should I replace all the ABS relays or just one?

RTS recommends replacing all of them. They are the same age, fail the same way, and are cheap compared with the time spent chasing a second failure later.

Will new brake pads turn off the ABS light?

No. Pads and rotors are not monitored by the ABS system. The light only goes out once the relay, sensor, wiring or control unit fault is corrected.

Sources

1. RTS - E30 ABS pedal pulsating at idle problem - three relays, overvoltage relay location and part numbers, thin fuse wire failure
2. RTS - E30 testing all ABS speed sensors the easy way - control unit pinout and about 1,000 ohm sensor resistance
3. BimmerWorld - Bosch BMW front left ABS sensor 34521178981 - sensor part number, supersession and E30 fitment
4. Bosch - Beginnings of ABS - how ABS works and the 1978 introduction of Bosch electronic ABS
5. Bosch Media Service - 40 years of Bosch ABS - sensor, control unit and hydraulic modulator roles; early use in the BMW 7 Series