944 Anti-lock Braking System (ABS) — Service, Sensors & Torques

Service data for the optional anti-lock braking system (ABS), order code M593, offered on the 944, 944 S and 944 Turbo from the 1987 model year. The system is a three-channel Bosch ABS and is essentially the same design as the one fitted to the 928 S. It does not change the mechanical brakes — the discs, pads, calipers and their torques are covered in the brake data page. This page collects the ABS-specific service facts: what the layout is, how the wheel-speed sensors and their air gap are handled, the notes for the hydraulic unit and relays, the bleeding rule, and the one tightening torque the factory quotes for ABS parts. Temperatures switch between °C and °F; the sensor torque switches between N·m and ft·lb. Dimensions in millimetres, tooth counts and gauge markings are written as-is.

Warning: The hydraulic unit and the electronic control unit must not be repaired or disassembled. Never disconnect or reconnect the control unit's multi-pin plug with the ignition switched on, and disconnect that plug before doing any electric welding on the car.

System layout

The three-channel ABS has a separate speed sensor at each wheel. The car keeps the 944's front/rear (black-white) brake-circuit split: one circuit (the push-rod circuit) acts on the front axle, the other (the floating circuit) on the rear axle. The main ABS components are:

• Tandem brake master cylinder and hydraulic unit (the ABS modulator).
• Wheel-speed sensors — cross-pole type at the front, flat-pole type at the rear.
• Electronic control unit, mounted on the wheel arch in the passenger-side footwell.
• ABS pilot lamp in the instrument cluster, plus the stop-light feed.

The hydraulic unit sits beneath the right-hand front fender, in the rear of the right front wheel arch. On the 944 S and 944 Turbo a braking-force regulator is fitted to the hydraulic unit; the base 944 does not have it.

Note: The control unit is matched to the approved tyre sizes. Fitting unapproved tyres can give the axles different rolling radii; if the difference is large enough the control unit reads it as different road speeds, switches the ABS off and lights the ABS pilot lamp. Keep the wheels and tyres to the approved sizes.

Wheel-speed sensors and air gap

Each wheel-speed sensor works against an impulse ring with 45 teeth. The two front impulse rings are pressed onto the front-wheel hubs; the two rear rings are milled into the wheel shafts. The sensors are permanent-magnet units, so a few handling rules matter:

• Do not remove a new sensor from its protective packing until just before fitting — it loses permanent magnetism if left exposed.
• Check the magnetic tip for metal chips before fitting, and clean them off. Iron particles collect on the magnet and corrupt the signal.
• Grease the sensor and its bore with Molykote Longterm 2 and fit a new O-ring on the sensor before installing. This applies to both the front (steering knuckle) and rear (trailing arm / wheel carrier) sensors.
• Insert the sensor without force and secure it with its hex socket-head bolt.

The air gap between the sensor and the impulse ring is a fixed design feature and cannot be adjusted. There is no clearance to set: the sensor bottoms in its bore against the knuckle or wheel carrier, and correct spacing follows from that. After fitting, the sensor function is confirmed with the ABS tester rather than by measuring a gap.

Hydraulic unit, relays and control unit

The hydraulic unit carries two of the system's three relays under its cover — the solenoid-valve relay (No. 1) and the pump-motor relay (No. 2). Both can be replaced individually. The third relay, the ABS relay, lives on the central electric board (position G20 on the 944; G22 on the 968) and is an electronic relay with overvoltage protection; it feeds the control unit, the pump-motor relay and the solenoid-valve relay. Remove or refit the ABS relay only with the ignition switched off.

Two model-year changes are worth knowing when servicing an older or a mixed car:

• From model year 1988 the hydraulic unit was modified: the blocking diode for the ABS pilot lamp moved out of the hydraulic unit and into the valve relay. The later unit is identified by a recessed cover marked "ABS", and its solenoid-valve relay became a 6-pole part (the earlier one is 5-pole; the pump-motor relay stays 4-pole).
• The electronic control unit can be confused with the visually similar 928 unit (model year 1986 and earlier, which uses 90-tooth impulse rings, not the 944's 45-tooth rings). Externally the two differ only by the Bosch or Porsche number, so verify the number when replacing — the wrong unit will misread wheel speed.

When removing the hydraulic unit, plug or cap the disconnected brake lines immediately (insert stoppers, or drain the reservoir and cover the lines) to keep dirt out, and mark the six line ports so they go back to the correct fittings.

Bleeding after ABS work

There is no special ABS bleeding sequence or pressure for this system. After refitting the hydraulic unit or opening the brake circuits, bleed the brakes exactly as on a car without ABS, then check for leaks — see Brake Fluid Replacement and Bleeding Brakes for the wheel sequence and method. Confirm the ports and routing of the six brake lines first, then bleed.

Function test and thermal limits

Whether an ABS function test is required depends on what was touched:

• Work that did not touch ABS parts (brake pads, hoses, discs, calipers, the tandem master cylinder, brake cables, or parking-brake lines not connected to the hydraulic unit) needs only a simple function check: with an intact system, the ABS pilot lamp in the cluster must go out when the engine is started.
• Work on ABS parts — the hydraulic unit, the control unit, the wheel-speed sensors or their lines, or repairs after accident damage — requires a full function test with the ABS tester. The control unit has self-diagnosis, so it is only checked as part of an ABS test carried out for another reason.

Caution: During paint drying the control unit tolerates a short-term maximum of 95 °C203 °F and a long-term maximum (about two hours) of 85 °C185 °F. Disconnect the battery from the electrical system before fast-charging, do not fast-charge to start the engine, and retighten both battery cable clamps after refitting the battery.

ABS component tightening torques

The factory quotes one ABS-specific tightening torque; the mechanical brake fasteners are listed on the brake data page.

Fastener	Thread	Torque
Wheel-speed sensor to steering knuckle (front)	hex socket-head bolt	10 N·m7 ft·lb
Wheel-speed sensor to wheel carrier / trailing arm (rear)	hex socket-head bolt	10 N·m7 ft·lb

Both front and rear sensor bolts take the same 10 N·m7 ft·lb. There is no clearance to set afterwards — the air gap is fixed by design.