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09/03/2005

  ABS Fault Finding

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ABS System & Fault Finding

 

The Anti-Lock Braking System fitted to the Scorpio is either the Teves Mk IV or Mk 20, depending on year. The systems are identical, but the later Mk20 is updated with smaller control systems and is of lighter weight. The ABS sensors remained the same throughout production.


Experienced drivers will know that when the brakes are fully applied the friction of the brakes will prevent the wheels from turning and the locked wheels produce a skid. When a front wheel is not turning it becomes a sliding surface and inertia takes over, the vehicle slides in a straight line regardless of where the steering wheels are pointing. Professional drivers use cadence braking techniques to produce fierce braking and retain steering control, this means jabbing the brake pedal quickly and forcibly while making steering movements and this enables the vehicle to be steered around hazards while still braking.

ABS uses the same effect as cadence braking. It is a peculiarity of the modern tyre that it produces its maximum retardation just before the point at which it commences to skid. ABS systems monitor the rate at which each wheel revolves and when it senses the point of lockup it releases the pressure to that wheel, even though the driver has white knuckles and is standing on the brake pedal. The ABS then re-applies the braking force to the wheel, and does this many times a second, much more efficiently than any cadence braking technique. In the Scorpio the control frequency of the Teves system can approach 20 Hz (operations per second). Because of the speed of this pressure-release cycle it is not possible to use brake drums with ABS: the design of the brake shoes produces a self-servo action and this would hamper the ABS operating cycle.

Learner drivers are taught the emergency stop and are supposed to feather the brakes when the vehicle commences to skid to bring the vehicle to a stop without leaving smoking rubber behind them. With ABS though, the technique is different - the driver stands on the brake using as much force as he can apply - he will feel the ABS modulator varying the pressure to the calipers by a clattering sensation through the brake pedal - but he will retain steering control throughout. Even on wet and greasy roads, retardation rates with ABS can be amazing and have saved many lives.

On higher spec Scorpios, the Traction Control system is added. It is piggy-backed onto the ABS controller and shares the rear wheel ABS sensors, using input from these to compare the speeds of the driven wheels. If the separate TCS module detects that one wheel is beginning to spin it uses the ABS module to brake this wheel and a servo attached by cable to the throttle body forces the throttle to close, so that the driver feels the accelerator forced upward against his pressure. This intervention ceases as soon as the controller detects that order is restored. The module defaults to 'ON' - so that the TCS is not operating when the button in the centre console is pressed and while the light shows in the instrument panel.

The ABS system on the Scorpio consists of a Module, Modulator and four wheel sensors.

The ABS module receives the signals produced from the sensors in each wheel hub. The sensors are magnets surrounded by a coil, and are mounted close to an ABS sensing ring which revolves with the road wheel. These have segments, so that as the wheel turns the protrusions produce a magnetic flux in the sensor which is detected by the module. It is by comparing the frequency of these signals that the module can determine which of the wheels is about to lock up and will order the modulator to release the brake pressure to the affected wheel.

The Modulator is connected directly to the brake servo, and receives the full brake pressure applied via the servo from the driver. This pressure is diverted through three separate channels one to each front wheel and one circuit to the rear, so that although there are sensors for each rear wheel both rear brakes are modulated as a pair. On a given signal from the module, the modulator operates the solenoid valve and release the brake pressure from one or more wheels. The ABS modulator actually contains its own pump which rapidly increases the pressure inside a wheel cylinder, depending on the pressure applied to the brake pedal. In this way extremely powerful braking effort is applied at a frequency approaching twenty times per second.  People who do not understand ABS often state that ABS does not make braking distances shorter but merely allows for steering while braking, and this is simply not true - ABS will bring a car to a halt in a shorter distance regardless of the type or condition of the road, in dry, wet and even on ice, because it keeps the tyres in a cycle near the point of maximum retardation with an efficiency beyond any human capability. It is for this very real safety advantage that from the 1st July 2004 it will be unlawful for a new car without ABS to be on sold in the UK.


Fault Finding, Teves MkIV

There is some mystique about the ABS system, but it is simple and robust. The Module very seldom causes trouble, unless it is spiked by careless welding or electrical procedures. Modulators have been known to fail, but faults are much more likely to be with connections or sensors. Before resorting to an expensive ABS diagnosis there are simple checks that can be made which are often all that is required.

The ABS light might appear to be useless as a diagnostic tool, but it can tell us two things of vital importance in fault-finding. It should come on when starting and go off after the few seconds of self-testing. If the ABS light stays on then it indicates a hardware failure rather than a signal loss from a sensor. Experience shows that the fault is most likely to be a faulty sensor - they are reliable units but they can fail.

If the light goes off, but lights again within a few moments of moving off, it often indicates that a sensor which passes the start-up test is now failing to send an adequate signal. This is most likely to be dirt collecting in the ABS sensor area or corrosion of the sensing surface and is preventing the sensor from detecting the sensing ring, but it could also be a loose connection which is vibrating or damp and interrupting a signal. If the ABS light comes on briefly on damp mornings it is most likely to be dampness affecting a multiplug, and experience suggests it is most likely to be at the front. During the second part of the moving test, the ABS control module fires each modulator solenoid in turn and checks the pressure. A problem with a solenoid will illuminate the ABS light.  This test may be detected occasionally if you are applying light pressure on the brakes at parking speeds, by an odd sensation on the pedal under your foot.

While the ABS light stays on, the ABS module is not operating at all, but normal brake pressure will still be applied from the brake pedal. If the vehicle is equipped with Traction Control the TCS shares the same light as the ABS and a fault in either module will keep the light on.

Connections

The Modulator is connected via a multiplug to the Module and this can be checked quite easily. On some vehicles an additional connection is made to the Modulator low down on the drivers side of the engine compartment at the bottom of the McPherson strut turret. This connection is in the road spray and baked by the heat of the manifold and should be the first connection to be checked, particularly if the ABS light shows intermittently - although some owners of later cars have not been able to find this connection.

Pull off the connections to the modulator and examine each for wetness or corrosion. Use waterproof grease and remake the connection. After each check, try the ABS system again on the ignition switch - if the ABS light then goes out you will know which connection was at fault. Return to that multiplug, clean the inside and pins as much as possible and use waterproof multiplug grease when reconnecting.

Sensors

The rear wheel sensor wiring come through the wheel arch through a grommet into the passenger compartment, beneath the rear seat cushion and under the carpet. Once the seat has been lifted the connections can be checked, but being in the dry they very seldom give trouble. It is a simple matter to connect a voltmeter to each sensor and check for a resistance. Compare them - but generally if a sensor has failed it will show as open circuit, and if this one sensor is replaced it is very probable that normal service will be resumed, so if an open circuit is found that sensor must be replaced wherever it is.

The connections for the front sensors are in the wheel arch, clipped up to the inner wing behind the MacPherson strut. The multiplug can be checked there too and the sensors checked for a resistance at the same time. Once again check the system with the ignition key after each operation.

If work has recently been carried out on a wheel hub in which you suspect that a mechanic has been hitting the hub with a hammer, check that hub first. ABS sensors are vulnerable to hard shocks and you may well find that the sensor has gone open-circuit. This may not happen at once, but often a couple of days later, when a sensor, weakened by the shock of hammering, then goes open-circuit over a bump or ramp in the road.

The hub bearings keeps the sensor ring and the sensor itself at a suitable distance. If a bearing is failing the shaft may be oscillating and this is likely to fail the moving test - the ABS light will come on after moving off. If during the checks a hub is found to be worn, particularly at the rear, then this wheel bearing should be replaced first since it is probably the cause of the ABS error.

Because the TCS uses the rear ABS sensors, if both lights illuminate, check the rear sensors first

Dirt/Corrosion

Particularly in the event of a ABS light coming on soon after motion has commenced, the sensors will each need to be removed and checked for dirt/corrosion or damage.

Front Sensors.

Raise the front of the vehicle, remove the front road wheels. The ABS sensor is set into the front hubs at 3 o'clock, behind the brake disk. Clean the area around the sensor so as not to admit dirt, remove the bolt and withdraw the ABS sensor. Check for corrosion/dirt or damage and replace. Torque the mounting bolt to 14Nm. Check the ABS light with the ignition key after each inspection.

Rear Sensors.

Raise the rear of the car and remove the road wheels. Remove the disk caliper by removing the two bolts at 10 and 2 o'clock into the rear of the caliper. They are protected with plastic plugs - don't lose these.

At 12 O'clock the sensor can be seen mounted into the rear hub. Carry out the same check for dirt/corrosion and replace. Torque the mounting bolt to 14Nm and replace the caliper, torquing these caliper bolts to 56Nm and do not forget to replace the plastic covers.

Note: some owners have reported that the ABS sensors have corroded into the hubs and cannot be removed for checking without causing damage. If this is the case it is probably more economic to have the ABS system interrogated by a properly-equipped local garage without going further. Typical rates for a reading for fault codes have been in the 40 to 60 area. Main Dealers are typically 90.

Our Vehicle Explorer software, together with a PWM/ISO combi lead can now read ABS diagnostics, thanks to work by Mark P and Alex Peper in the US.  If you already own a single lead, you may send it back to the US in return for a combi lead, and the latest version of Vehicle Explorer has been updated to recover Trouble Codes from the ABS module.
 

FAULT

SUGGESTED ACTION

ABS light stays on after self-test

Check fuses 16, 17 and ABS 27, 46 & 47
Check connections to the modulator
Check connections to each sensor
Check sensor for open circuit with voltmeter
Check ABS for operation after each

ABS Light goes off after self test, then lights up when moving In addition to the above

Check rear tyre pressures. Variance of more than 4 p.s.i. can cause ABS/TCS warning to trip, especially over rough ground.

Remove each sensor from the Hub and examine for dirt/corrosion, clean or replace as necessary.
Check ABS operation after each sensor.


It is quite likely that in carrying out these checks the fault will be found and rectified. However, if it is not then the car will need to be booked in to a local garage that can read Teves codes - it need not be a main dealer - so that the ABS module can be interrogated and the error code read, but at least the owner will have the satisfaction of knowing the the simplest causes will have been eliminated.  With the latest version of Vehicle explorer and
the PWM/ISO combi lead you can reset the module yourself.

I have carried out this procedure on two separate vehicles showing ABS faults, a Granada Scorpio and a Rover 420GSi and repaired both without resort to a main dealer. In both cases an ABS sensor had failed and was showing an open circuit instead of a resistance, and as soon as the offending sensor was replaced the fault light went out on self test. In an intermittent fault light on another Granada the problem was caused by a damp connection on the nearside front wheel sensor in the wheel arch.

Fault Finding, Teves Mk 20

Robert G found that even after repairing a fault the fault light stayed on. It appears that a fault code stored in the Teves Mk 20 has to be cleared and the system reset by an ABS Diagnostic system before the light will go out.  Neither a battery disconnect or a fuse removal will clear the code. This applies only to a start-up failure. If the fault light comes on when above 15 KPH (the moving test) then this fault will clear itself.

In view of this the above instructions need to be amended, only so far as ignoring the instructions to check the ABS light after each operation - it appears that with the Teves Mk 20 there is no point. Carry out the checks and repair any problems that are found and then make an appointment with a garage equipped with diagnostics and get them to reset the module, or use the PWM/ISO combi lead and reset the module yourself.

 

 

EricR

 

 

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