Rover 75 &  MG ZT  ABS

DIY Fault Finding & Fix HowTo

 

 

Introduction:  

 

Unlike  earlier MG-Rover models the Rover 75/MG ZT   uses a Bosch ABS system with active (Hall Effect)  wheel sensors,  previous generations  of cars from MG-Rover  and other manufacturers  used inductive sensors.

Another change from earlier models is the toothed ABS ring on the front CV joints and rear hubs has been replaced by ferromagnetic  rings built into the  “active” wheel bearing oil seals.  The clearance between the wheel sensor and the ferromagnetic ring on the wheel bearing is critical excess clearance due wheel bearing wear can trigger the system to log an ABS fault.

 

Because of  these changes fault finding on this ABS system without dealer level diagnostic equipment requires a more careful  approach than with earlier types of  ABS as  both the Hall Effect sensors and  active  wheel bearings are more easily damaged. Hall Effect  sensors on this model  should not  be tested by measuring resistance  as a  simple resistance test with an average Digital Multi Meter may burn out the sensor.

 

To avoid wasted time, unnecessary  expense and frustration please  read this  HowTo through to the end before testing replacing and components.  

 

Start-up  Self Test

  1. (1)Start-up test:  at ignition switch on the ECU  in the  ABS  Modulator performs a basic self test during which the yellow ABS  illuminates. During  the start-up test each sensor is tested electrically    and  the ABS modulator  is tested both electrically  and hydraulically. If the ABS light remains on it most likely   points to an electrical fault  or  less likely an ABS  modulator or hydraulic fault. 

  2. (2)In normal operation each wheel sensor  is supplied   with a 12v supply, if a sensor fails start-up self test   the 12v  supply to that sensor is automatically shut-down by the ABS ECU. 

  3. (3)If  this first stage of the self-test is passed the ABS warning light extinguishes  and the next stage of self-test continues when the vehicle is in motion. 

  4. (4)If the  ABS fault warning light extinguishes  as normal but then re-illuminates after driving a short distance (over 7.5 mph) then it usually suggest  to the ABS  having detected implausible  signal from one of the wheel sensor. 

  5. (5)An implausible signal from a  wheel sensor would usually be either a problem with  an active wheel bearing  or intermittent sensor or electrical connection. 

  6. (6)Another common cause of  the ABS illuminating on an implausible wheel sensor error is mismatched tyre sizes, of course this will not show up on an oscilloscope test. 

Important Clues on Where to Start

The ABS wheel sensors signals are passed by  by  the ABS  Module ECU  to other systems on the car this can give pointers to where start test individual sensors.

 

  1. (1)Speedometer  is driven by  the signal from the right front wheel sensor,  if the speedometer works normally the fault is less likely to be in the right front wheel sensor or associated  parts. 

  2. (2)On KV6 models with cruise control the speed signal  to the cruise control system is taken from the front left wheel speed sensor,   If the cruise control works normally the fault is less likely to be in the right front wheel sensor or associated  parts.  The engine ECU also takes its' speed signal from the left front sensor. 

  3. (3)SatNav system takes data from both rear wheel sensors,  if ABS  and  SatNav faults appear at the same time it is likely to be a problem related to a rear sensor. 

Before testing individual wheel sensors  it  is worth removing the car's battery and battery box and unplugging the multi-pin connector from the ABS  Modulator  and checking for water ingress and corrosion, this is a known problem on Rover 75's  particularly pre-Longbridge cars.  

Premature ABS Activation

Premature  ABS activation or unexpected  pulsing felt through the brake pedal  can be  caused by  problems with the the ferro magnetic rings in the wheel bearings  unlike older system where  a clean up and visual  inspection of the toothed reluctor rings with  active wheel bearings will often reveal the cause of the problem the only way diagnose this type of  problem is to examine the signals from each sensor with an oscilloscope while the vehicles is driven preferably at constant speed.   However with this type of fault it is worthwhile checking  all the electrical  connections and bleeding  the brake hydraulics.

 

 

 

 

 

 

 

 

 

 

Testing  an Active Wheel Sensor

The Hall Effect  wheel sensors  should not  be tested by measuring resistance  as it is likely to burn out the sensor.

Each sensor  is connected to the ABS modulator by two wires, one is a  nomminal 12 volt supply the other    the signal  return to the  ABS modulator. Following a visual inspection of  the wiring and connectors the first  test is  to measure voltage present  between  each  of these two  sensors wires and a good ground on the vehicle body.

Because the  wheel sensor connectors are very difficult to back probe to get usable test connections  it is more convince   to break into the wiring  of the vehicle  either by making tap connections or  making a special breakout connector  using plug & socket parts salvaged from a scrap vehicle.

ABS  Wiring Colour Codes

Wheel Sensor

12v Supply

Signal Output

Notes

Front Left

Red, Black stripe

Red

 

Front Right

Yellow

Yellow,  Black stripe

Also Drives Speedometer

Rear Left

Green,

Green, Black stripe

 

Rear Right

White

White, Black stripe

 

 

 

 

 

      Wheel Sensor DMM Test.

Tools  required   Digital  Multi-Meter,   Car Jack.

  1. 1.Jack up the car so that the wheel is clear of the ground and can be rotated by hand. 

  2. 2.Turn ignition and allow ABS system four seconds to complete self-test. 

  3. 3.Using  a  Digital Multi Meter (DMM)  set to the 20v DC range, connect the negative lead of the meter to a good earth ground on the vehicle body   then check the voltage on the 12v supply  wire to the sensor. If a  +12v  supply is  present then the sensor has passed power on self test.  If  the +12v  supply is not present then it points to a failure in the sensor or the wiring between the sensor and the ABS modulator ECU.  Connection problems due to water ingress by capillary action via  the wiring from the front ABS sensor are not unknown on the 75/ZT,  see notes in a later section. 

  4. 4.Now with the meter on the 2v dc range check  the voltage on the sensor output wire,  expect this to read between approximately +0.6 volts  and +1.7 volts. 

  5. 5.With  the wheel  clear of the ground rotate the wheel by hand while checking output voltage which should switch between approximately  between +0.6v and +1.7v  when the wheel is rotated. 

  6. 6.Repeat  for each in turn wheel. 

Passing this simple test will only show if the wheel sensor  is energised and switching in response to movement of the active  wheel bearing it will not indicate if the active wheel bearing is generating plausible signal.  As  the magnetic ring in the wheel bearing is relatively delicate and the clearance between the ring and the sensor is critical, failure  is relatively common.  

An oscilloscope is  the   only way to check  the quality of the signal from a  sensor,  the output  wave form should be examined using an oscilloscope  when the vehicle is being driven on the road.

 Oscilloscope Test on Signal from Wheel Sensor

This  test is to check the quality of the signal returned to  the ABS Modulators ECU  by  a wheel sensor.  Any defect in the wheel bearing magnetic ring could generate an irregular signal which will fail the plausibility test and toggle  the ABS light on as soon as  the vehicle  speeds exceeds 7mph.

As with the DMM test  the measurements are  with the ignition on and power on self-test complete, the oscilloscope connected between the the sensor signal output wire and a good earth the  vehicle body.

To obtain meaningful results I  would suggest the test is carried out when the vehicle is on the road driven at constant speed.

When the car  is being driven the wheel sensor output viewed  on  an oscilloscope should  be a nice regular square wave  between a low of  +0.6  and and peak of +1.6v.   I would suggest starting with an oscilloscope timebase setting of 0.1 seconds per division.

 

Replacing Wheel Sensors

Take care to only purchase OEM quality sensor either MG-Rover/X-Part  or Hella cheap unbranded sensors are an expensive lottery.

The sensors are held in by single 6mm diameter  bolts with  8mm hex heads, so once  you have identified a sensor you think is defective it should be simple to replace,  well no :-(  

The bolts usually come free easily enough  but the sensor bodies made  of a relatively fragile plastic are an extremely  tight fit  in the housing even when new.

The rear sensors are easiest to remove. If after soaking with WD40 they show no signs of budging   then simply remove the brake disc and wheel  hub to gain access to the working end of the sensor, then  use a 10 or 12mm diameter drift to punch it through from brake shoe back plate side.

The front sensors are generally much more difficult to remove and attempt to remove a sensor by pulling  with mole grips  is likely to result in the sensor breaking up leaving the stub firmly seized in the hole leaving  a choice of three options:

  1. 1. Carefully drill the sensor out  with a 12mm drill,  requires great care to avoid damaging the active wheel bearing  also results in  iron oxide, metallic dust  and plastic swarf trapped in the void between the CV joint and the wheel bearing which can often cause contamination problems with the active wheel   bearing. 

  2. 2.Undo the outer CV joint retaining nut,  bottom ball joint,  track rod end  and anti-roll bar link and pop the CV joint out from the hub.   This  allows the sensor to be drifted out. 

  3. 3.Replace the complete  swivel hub  with a scrapyard part  either with a known good sensor or fitted  a new sensor.  You can connect the replacement sensor parts up to the car and do a quick DMM  test and self-test on the sensor and active wheel bearing  before reassembly.  Because of problems with active wheel bearings replacing the complete swivel hub with a tested item has become my preferred option especially on higher mileage vehicles. 

 

Notes on Active  Rear Wheel Bearings  Replacement.

The rear wheel bearings come complete as  part  the wheel hub  assembly and is very easy to change however take care to carefully examine the replacement assembly before fitting.  

As the clearance between the wheel sensor and the ferromagnetic ring on the wheel bearing is critical any excess clearance or run out of the ferromagnetic ring is likely to cause problems  with the signal from the sensor.

 When you unpack the new part carefully  check that the ferromagnetic ring as been pressed in evenly  and  is sitting flush with the rim of the hub.

The front wheel bearings are a fairly major job during which you can run into niggling snags, to minimise time off the road I would recommend buying a second hand swivel hub and drive flange assembly  and either fitting as it comes  or  overhauling it with a new bearing  before fitting.