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Tire Pressure Monitoring Systems

Many of you have been asking about TPMS (Tire Pressure Monitoring Systems), so I have asked Dave Giles for the benefit of his knowledge and experience on the subject.

John Cannell
Dave Giles

By Dave Giles, (photo left), and John Cannell (photo right)

Editor's Note: Dave Giles of D.A.T.S. Canada Ltd is the premier supplier of top quality automotive training in Atlantic Canada. John Cannell has been recognized as an authoritative expert of the automotive industry in Atlantic Canada and has been a long time editorial contributor to Auto Atlantic. 

This is the first part of a three part series on tire pressure monitoring systems (TPMS), with a technical tips column. Tire pressure monitoring has become a big topic throughout the entire automotive service industry. Introduced as an option in the early nineties TPMS has become a mandated requirement for 2007/2008 vehicles.   It is now obvious how important and complex these systems can be.  When tire pressure was cited as a possible culprit in the rash of SUV rollovers and tire separations, OEMs soon turned to TPMS as a way to protect  themselves from massive lawsuits. Engineers didn't take long to fast track and integrate tire pressure monitors into the world of vehicle technology.

In this first part we want to address the two types of systems and how a shop can implement TPMS into a pre check before any tire service is done. TPMS also can be found in two configurations. The first one is the non direct system, the second being the direct system.

The non-direct, indirect or Deflation Detection System (DDS) as it is sometimes called, uses wheel speed sensors. This method allows the EBCM (Electronic Brake Control Module) to make calculations based on wheel speed sensor input. If a tire is the wrong size or is under-inflated  the EBCM can make a calculation based on all 4 wheel speed readings. If the pressure in all four tires is incorrect but the difference is still within the programmed range, the ABS-based system cannot determine that inflation pressure is incorrect.

The NHTSA (National Highway Traffic Safety Administration in the USA) states that the system must be able to sense all four tires being incorrectly inflated. This type of non-direct system does not meet  current  NHTSA requirements.

Direct TPMS was introduced in 2002 using RF wireless sensors, General Motors being the first manufacturer to introduce the direct system. Direct TPMS monitors tire pressure through a transducer attached to the valve stem or attached to the inside of the rim. Pressure for each tire is monitored in real-time once the vehicle reaches a certain speed (usually 24 kmph). Dashboard readout shows pressure and warns of tires with low pressure. Each sensor on a direct system weighs about one ounce and is powered by a 3 volt lithium battery with a life expectancy of 10 years. Any wheel service including rotation, tire change, balancing etc. involves resetting and/or reprogramming the TPM system. We will discuss reset procedures and reprogramming methods in the next article.

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Tire servicing the older non-direct system is still the standard procedure as followed in the past. The wheel speed sensors are the main input for non-direct TPM systems and care should be taken not to damage them or shock them during tire rim removal.

The direct system tires will require certain precautions during tire service. These procedures will differ between manufacturer and types of sensor. For example, Ford uses a banded sensor where other manufacturers use stem sensors. Stem sensors are easy to identify by their aluminum stem as banded sensors are hidden in the tire and strapped to the rim. The banded sensors are the most challenging for technicians and the most commonly broken.

How do you prevent broken sensors? First is to identify the system type (indirect or direct.) A light on the dash is not an indicator of a direct system, so some investigation is needed. A Chrysler vehicle with a light on the dash indicates a TPM system but has  a standard rubber valve stem which would be an indication of an indirect system. This vehicle would not need any special service for tire removal as the indirect uses wheel speed sensors. If the vehicle has an aluminum stem with a TPM light on during a bulb check this would be an indication of a direct system. It's best when removing the tire from the rim with this type of sensor, to unscrew the valve stem/sensor retaining nut and allow the sensor to fall into the tire. When reinstalling the tire, install the sensor first with a new grommet and nut, then torque the retaining nut. At the risk of being repetitive, remember to take great care when removing tires from rims as the sensors can be easily damaged.

One of the best services that can be performed on a direct TPM vehicle is the audit. This allows the technician to check the operation of the sensors on a vehicle before a tire is removed. This will prevent any problems after tire service and helps keep customers involved with the additional service to be performed.

The critical information to be read in the audit, will be the battery level of the sensors, tire pressures (current), temperature, and sensor SIN numbers. The audit can only be performed by tools that will display this information both on the tool or printed form. The printout can be given to a customer to help build customer confidence that a complete service has been performed.

In the next issue of Auto Atlantic, we will discuss some of the reset and learning procedures required by the TPMS for non-direct and direct systems. With the proper information, tooling and training, Tire Pressure Monitoring Systems can be easily and successfully serviced by informed technicians.

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