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Question: I had a Mini in my garage with a tyre pressure monitoring system (TPMS) fault. Without the correct equipment, I had to send the vehicle to the main dealer. He diagnosed an ABS sensor fault. Can you explain how an ABS sensor can affect the TPMS system? And can you recommend a tool which means I won’t have to take the car back to the main dealer in future?

Lets look at the last part of your question first.  The simple answer is yes there is a tool, TRW Easycheck. The full function tool comes with: Brake (ABS & EPB); Climate; Scan (EOBD); Service; Supplemental Restraint System (SRS), Steering Angle Sensor (SAS) and TPMS. This function will allow you to read and clear fault codes on TPMS systems and re-code valves back to the main ECU in the event of a new valve being fitted or when the wheel positions are rotated on the car. An additional Valve Activator tool may be required on some systems, and of course TRW has this available as an optional extra.

Now to the first part of your question. There are two distinct systems for TPMS, direct and indirect. Direct systems use physical pressure sensors inside each tyre/wheel and send information from inside the tyre to the TPMS ECU and instrument cluster. These systems can identify pressure irregularities in all four tyres in any combination.

They are designed to cope with both ambient and road-to-tyre friction based temperature changes. The alarm trigger points are usually programmed in the ECU based on the vehicle manufacturers recommended cold inflation pressures.

Indirect systems do not use physical pressure sensors and this is the type of system fitted to the Mini you mentioned. Indirect TPMS measures the “apparent” air pressure by monitoring individual wheel rotational speeds and other signals available outside of the tyre itself. Most indirect systems work on the basis that an under inflated tyre will have a slightly smaller diameter than a correctly inflated tyre, and therefore rotate at a higher speed to cover the same distance as a correctly inflated tyre. All this is realised in software within the ECU using the ABS wheel speed sensors. Therefore indirect systems can only really warn of under inflation after the event. Another disadvantage is that whenever the tyre pressures are changed (i.e. routine pressure check and inflation) the system must be re-calibrated by the driver, usually by pushing a button on the dash. The calibration must only be performed when all tyres are at the correct pressure, otherwise the system will not report correctly.
 
Hopefully you will now see the connection between ABS sensors and TPMS.

With TRW’s easycheck service tool, you would have been able to read any fault codes in both the ABS and TPMS systems, diagnose which wheel sensor was faulty and complete the whole job yourself without having to rely on the main dealer. Don’t forget, TRW also has an ever increasing range of ABS sensors available in the aftermarket.

IMPORTANT NOTE: Repairs to motor vehicles should only be carried out by appropriately qualified motor mechanics. Please note that specific makes and models of motor vehicles differ, and that the advice given in this website may not be appropriate in all cases.  TRW KFZ Ausrüstung GmbH will not accept any responsibility or liability for loss or damage howsoever caused as a result of reliance upon the advice given in this website, save to the extent that such liability may not, by law, be excluded."