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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:

• A combination relay in the auxiliary relay panel to control whether or not ABS is enabled
• An ABS pump and valve unit, mounted in the engine bay
• An ABS controller, located in the right C-pillar and accessed from the boot
• Rotation sensors mounted in each hub
• From VIN 85-H-900252 in the 1987 model year onwards, a mercury motion detector (accelerometer) under the rear seat

The ABS Combination relay

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

• The ignition switch
• The ABS on/off switch on the dashboard
• The rear differential lock (taken from the indicator light)
• The centre differential lock (taken from the indicator light - pre-1988 cars only)

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:

The ABS Pump and valve unit

This consists broadly of four components:

• A return pump
• One magnetic valve for each channel:
  • Front right caliper - "r" on top of the unit, fed through "V" halfway down from the brake compensation valve
  • Front left caliper - "l" on top of the unit, fed through "V" halfway down from the brake compensation valve
  • A common valve for both rear wheels - "h" on top of the unit, fed through "H" halfway down from the brake compensation valve
• Two relays, one (J105) for the pump and one (J106) for all three valves - directly under the top cover which is secured by a single slotted screw. These relays are the only parts of the pump unit that can be serviced. The relay coils are powered by the combination relay and activated when the ABS controller grounds the other side of the coil. They both switch power to their respective rails directly from the battery - unfused. These are standard relays and if under suspicion should simply be replaced.

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:

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:

• Before vehicle motion occurs:
  • No power to the controller:
    • The combination relay has decided to disable the system
    • The combination relay is defective
    • The fuse in the combination relay has blown
• Power to the controller but:
  • No power on the valve rail from J106
  • No signal from the alternator (engine stopped or alternator belt failed)
    Note: a corollary of this is that the ABS controller is sensitive to alternator field voltage - the ABS system may be disabled if the alternator is not performing to specification. This condition usually also produces a flickering alternator light.
  • On 1987 and later cars, no signal from the accelerometer under the rear seat.
In these cases, the light will stay on if the vehicle is driven.

• After the vehicle is set in motion:
  • A discrepancy between the signals coming from the hub sensors in the absence of input from the brake light switch. This is the normal cause of the light coming on after a few tens of yards.
    • A sensor has been removed, e.g., when changing a wheel bearing, and not replaced correctly using a new ablative cap.
    • One or more hubs has serious corrosion, common in UK East Coast, Scotland and North Yorkshire cars.
    • One wheel has a different diameter to the others. This condition can be generated by changing one wheel for the small form of emergency spare. Serious underinflation can also cause it.
  In the above cases, if the ABS system is switched off and on the light will go off for a while and then come back on.
  • One (or both, on earlier cars) of the differential locks engaging.
    In this case, switching the ABS off and on will not switch the ABS light off and the system remains engaged. A differential lock warning light should be on, but if the bulb is blown and the differential lock engages, the voltage to the dead bulb is enough to disable ABS.
  • J105 fails to switch power to the return pump. This will cause the light to come on when the ABS system actually operates under braking.

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:

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.

Tests:

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

• J105 (the pump relay) is an unusually large simple relay, Bosch number 0 265 003 002 (Audi 437 927 829 (£27)). From the size of the terminals, it is rated at 70 amps - Audi 443 951 253J or 431 951 254 would substitute. These are often found in junkyards, used as the ignition load reduction relay in later cars.
• J106 (the valve relay) is a normal Bosch 0 332 204 153 (Audi 701 927 841) and can be substituted with any 40 amp relay with crossover contacts - both 87 and 87a terminals. Terminal 87a is used to ground (light) the ABS warning light if the relay is not closed. Note that the switched current flows through the relay in a reverse direction. Audi also approves the use of 191 927 841 (£15) with the note that one pin must be broken off.

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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