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Audi ur-quattro - Anti-Block System (ABS) pinout and basic diagnostics

The ABS system is a safety feature and - if fitted - should always be maintained in working order.
As always, do not work on safety-related systems if you are unsure of your abilities

Many early ur-quattros did not have ABS. It was at first an optional feature (M 432 - see the service book) and became standard during the 1984 model year. For earlier cars, the best way to tell if ABS is fitted is to check for the ABS pump in the engine bay - this is a rectangular box-like structure with five brake pipes and a thick cable going to it, mounted just behind the main radiator. If ABS is fitted, it is an MoT requirement that the "ABS off" warning light comes on when the ignition is switched on. As of January 2011, the ABS light must go off when the engine is started. The ABS system is not connected to the voice synthesizer - the only warning of a defective system is the panel light.

Many cars with ABS do not show such a light when the ignition is switched on. If this happens, check that the dashboard bulb is present and in working order.

ABS may lengthen braking distances, even on dry roads. Its function is not to shorten braking distances, but to keep the wheels rotating so that most drivers have more control of a car under extreme braking. It is NEVER safe to drive in such a manner that an ABS failure may cause difficulties.

The ABS system consists of five components:

The ABS Combination relay

This relay determines whether the ABS system should be enabled or not. Its inputs are:

The relay "combines" these signals (hence its name) to produce a go/no go signal for the ABS system. If the decision is "go", the relay energises and supplies power to the ABS controller and the relays in the pump and valve unit through the red 10A fuse on top of the relay. The easiest check is to remove the cover from the pump and look for +12v on pin 2 (black/blue wire) with the ignition switched on. If 12v is not present, the combination relay has not engaged ABS.

The relay will also disable ABS if the battery voltage is too low. The reason is that the ABS pump needs power to operate during ABS use - if the battery voltage is too low, it might be insufficient to operate the pump properly. As a safety feature, therefore, ABS is disabled and the warning light on the dashboard lights.

The ABS Combination Relay Pinout

The combination relay is in the auxiliary relay panel behind the glovebox on RHD cars - easily identifiable by the fuse in its top. The relay base can be freed from the frame so the relay can be measured in situ. Use the wire colours to identify the pins:

1/S1Brown/blackSense wire from rear differential warning lightIf +12v absent, rear differential is locked
2/30RedUnfused input power for ABS via fuse in top of relayif +12v absent, break in loom
3/S2Brown yellowSense wire from center differential warning lightIf +12v absent, centre differential is locked
4/31BrownChassis groundContinuity to chassis
5/15BlackFrom ignition switch+12v with ignition on
6/87Black/blueSwitched output to ABS+12v if ignition on and ABS enabled
8/TBlack/yellowABS switch+12v if ABS disabled by driver

The ABS Pump and valve unit

This consists broadly of four components:

Both of these rails are connected via sense wires to the ABS controller, so the controller can see that the relays have switched, although it cannot tell whether any of the driven devices (pump and valves) are actually in working order. There are no pressure sensors or similar in the system.

The default (n/c) position of J106 (the valve power relay) grounds the ABS warning light through a diode - thus the ABS warning light will come on if for any reason J106 does not switch and provide power to the valves. This is a redundant safety feature - the controller can sense the valve power rail directly and switch on the light itself.

ABS Pump Pin Assignments

The connector can be accessed directly with the pump cover removed. Use the wire colours to identify the pins:

2Black/Blue+12v from combination relay to power valve and pump relaysIf +12v absent, the combination relay has disabled ABS
3YellowGrounded by controller to operate valveSee pins 2 and 4
4Red+12v unfused from battery to run valves+12v - if not, break in loom
5Yellow/blackGrounded by controller to operate valve12V
6Green/redEarthed by controller to switch on valve power rail+12v with ignition on - 0V with engine running
7WhiteGrounded by controller to operate valveSee pins 2 & 4
8BrownGround to operate ABS warning light if power not available to valvesContinuity to chassis ground
9Red/whiteSense wire to controller confirming voltage applied to pump by J105Not testable without engaging ABS while driving
10Yellow/red+12v - grounded by pump to operate ABS warning light if relay J106 fails to switch power to valvesNot testable without engaging ABS while driving
11Brown/yellow+12v - grounded by controller to operate pump relay (and thus pump)See pin 2
12Red/whiteSense wire to controller confirming voltage applied to valves by J106+12v with engine running
13Red+12v unfused from battery to run pump+12v - if not, break in loom
-BrownMain pump ground - wire on side of unitCheck continuity

The ABS Controller

The ABS controller is the heart of the system. It receives signals (rapid pulses relative to wheel rotation) from inductive sensors in all four hubs and compares them to detect a wheel that stops rotating - starts skidding - when the others are still going round. When it detects this, it grounds the relevant valve in the pump and valve unit to release the brake on that particular wheel. The ur-quattro system is three-channel - if one rear wheel locks, both are released. This is in part a consequence of the ur-quattro's front/rear rather than diagonal brake circuit split.

Once rotation is restored, the valve is closed and normal braking continues. This process may be repeated rapidly, causing the ABS "stutter".

The controller uses a sense wire from the brake light switch to determine that braking is occurring - on cars with an autocheck or voice synthesizer, this signal is taken from the wire that drives the brake lights through the rear light warning controller.

The system can be disabled for a number of reasons:

ABS Controller pin assignments

The following discussion does not apply to the Sport quattro.

The only difference between the pre 85-H-900252 (Bosch 0 0265 103 003) and later controllers (Bosch 0 265 103 021, Audi 857 907 379B) is that a mercury motion detector was fitted in the later cars. This is connected between pins 1 (power feed from the combination relay) and 16. The controller for later cars carries a sticker "Nur in Verbindung mit λ-Beschleunigungschalter". This controller CANNOT be used in earlier cars. The accelerometer is designed to cater for the extremely rare condition of all four wheels locking simultaneously, in which case the ABS controller is deprived of input from the hub sensors that the car is still moving.

In theory, the earlier controller may be used in later cars with the loss of the motion detector function.

To perform the tests, the connector should be detached from the controller. A single small crosshead screw retains the cover - this can be slid off the connector and it can then be replaced on the controller.

Use the wire colours to identify the pins:

1Black/bluePower - switched and fused by J156, ABS Relay
Feed to F113 - Accelerometer (1987-1991)
2YellowABS valve, front left - grounded to activate2
4RedG47 - Wheel sensor, front left3
8WhiteG46 - Wheel sensor, rear left3
9WhiteG46 - Wheel sensor, rear left3
10BrownChassis ground near controller4
14Red/white+12v when power on return pump2
15BlueC1 - Voltage stabiliser on alternator5
16Black/greenReturn from accelerometer F1136
18WhiteABS valve, common rear - grounded to activate2
20BrownChassis ground4
21GreenG45 - Wheel sensor, front right3
22RedG47 - Wheel sensor, front left3
23GreenG45 - Wheel sensor, front right3
24YellowG44 - Wheel sensor, rear right3
25Red/blackBrake lights
26YellowG44 - Wheel sensor, rear right3
27Green/redGrounded to switch J106 (on pump) and power valves7
28Brown/yellowGrounded to switch J105 (on pump) and power pump7
29Yellow/redK46 - ABS warning lamp (earthed for "On")2
32Green/yellowSense wire from valve common supply rail2
34BrownChassis ground near controller4
35Yellow/blackABS valve, front right - grounded to activate2

Spare pins for the connector are available from Audi as part number 000 979 225A - this is actually a wire with a new pin connected to each end as few garages possess the correct crimper. The repair is thus a butt splice to one of these wires cut in half. Old pins can be released with a dressmaker's pin pushed down outside each flat of the pin from the face of the connector to release the tangs.


  1. Battery voltage with ignition on and combination relay engaged. This is a prerequisite for the other tests.
  2. With the engine running, battery voltage. If not - assuming Test 1 is passed - replace the relays in the top of the pump unit.
  3. The wheel sensors are connected to the controller by pairs of wires with solid colours, i.e., there are no trace colours applied. Each sensor is tested by jacking up the relevant wheel and spinning it with a millivoltmeter connected across that pair of pins. Use care when testing the back wheels because the handbrake must be released - securely chock both wheels on the opposite side of the car. The values obtained for the sensors should be broadly similar - no signal indicates an open circuit. One cause for this is the use of cable ties to secure the sensor wires to the strut - the wire can flex at the joint and break. If one signal is merely too weak, remove that sensor from the hub, clean everything thoroughly, and reseat the sensor with a new ablative cap. Sometimes loosening the clamp a fraction and applying a light tap with a soft-faced hammer is sufficient. Sensor failure is rare. Cross-testing is possible, but the ur-quattro sensors are different front-to-back and the Sport's sensors different left-to-right. The wheel sensors have O-rings - faults are commonly caused when inexperienced mechanics think the sensor is fully home when the O-ring touches the hub - the Allen screw that holds the sensor in should be some distance down the slot in the tang on the hub. If in doubt, remove the Allen screw - the sensor should be difficult to move. The chisel point at the active end of the sensor should be parallel to the axis of the hub.
  4. Low resistance to chassis.
  5. +12v to +14V with engine running - 0V with ignition on but engine not running. Usual problem is corrosion in a single-pole red connector behind the right headlight. A good current supply is ESSENTIAL to ABS operation, and the ABS controller will disable ABS if there is the slightest doubt about the availability of electrical power for its functions. A good alternator is a prerequisite.
  6. +12v to +14V with ignition with or without engine running. Shaking the vehicle should produce intermittent breaks in the voltage, though these may be hard to see with a digital multimeter. Detaching the connector from the accelerometer under the rear seat should remove the voltage - this connector is also the normal source of problems.
  7. Close to 0V with the engine running - 12V with the ignition on and the engine not running.

Pump and valve housing relays

Four-wheel lockup was first reported in Ford Granadas in the mid-1980s. The Granada was the first vehicle of its class fitted with ABS as standard. After an incident near Newmarket, some experiments were conducted on the car park at Wellingborough Swimming Pool. It proved possible to lock up the ABS system if the surface was extremely slippery and the brakes were applied very quickly. This lockup is also a vulnerability of the early ur-quattro ABS system - it can be simulated in the later systems by pulling the connector off the accelerometer and jumpering its pins. EXTREME care should be taken in such experimentation - use a regulated skid pan or similar.

Hub corrosion presents problems in cars that have been used in high-salt environments, especially if they have been garaged. The metal moth loves warmth as much as salt. The East Coast of England and Scotland have significant quantities of air-borne salt - the North York Moors see a lot of road salt in winter because the roads are too uneven to plough. The front right hub is usually the first to show symptoms - this can be verified by a simple experiment. Most traffic roundabouts on UK dual carriageways have an accumulation of dust and grit on the outside of the outside lane at the entry point of the roundabout. With no other traffic in view, approach such a junction in the outside lane and at low speed - less than 10 mph. At the entry, observe that the front right wheel will pass over such a patch of detritus and tap the brakes as the vehicle slows to a halt. A pronounced stutter suggests the ABS sensor is marginal.

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