TIRES AND PRESSURE - PART 2
TPMS: direct vs indirect systems
Below are the characteristics of both TPMS indirect and direct systems, especially in relation to wheel and tire maintenance
It has already been noticed how the TPMS control system has become a homologation requirement, therefore mandatory, for M1 class vehicles - passenger transport <3.5t.- and therefore:
-will be subject to an efficiency control in the periodic roadworthiness test of the vehicle;
-deactivations and/or alterations are not allowed. That would make the car no longer roadworthy and safe for road traffic, with a series of consequences (insurance, legal, etc.).
With that in mind, let us see some of the characteristics of the two set-ups, especially in relation to wheel and tire maintenance.
It does not measure the actual pressure (and temperature) of the tire, but through the already present ABS or ESP systems, it detects and compares the differences in rotational speed between the wheels. The change in rolling radius resulting from a loss of pressure (at least 20% - as stated by Rule 64) triggers a warning light that must be visible even in daylight.
No sensors are lodged inside the tires and normal snap-in or clamp-in valves are used. This makes the set-up simpler and does not require special precautions during routine tire maintenance.
Conversely, due to this configuration TPM Sensors:
-need a substantial distance driven before an alert will be generated;
-are less accurate;
-if all four tires are similarly low in tire pressure from neglect or other reasons, an indirect system will not trigger a warning alarm;
-any malfunction occurring after parking the vehicle will not be signalled when starting the vehicle again.
The system’s “reset” - which can be performed by the driver following the procedures highlighted in the vehicle’s manual – must be performed after every intervention of:
-removing/fitting one or more tires;
-correcting the tire pressure.
In this phase in fact, the tire pressure – resulting in the relative rolling radius- is taken by the system as "reference".
The procedure stages:
1- Car info
2- Vehicle condition
4- Starting the engine
Reset takes place while driving, while the wheels are in motion.
Consequently, there is a clear need for:
- correct tire pressure through the use of accurate tire pressure gauges;
- periodic monitoring of the tire pressure, as required by the manufacturers.
As already stated, these systems:
- are very accurate;
- indicate which tire is underinflated and by how much;
- have quick reaction times; the systems work immediately after starting the engine.
-are more expensive (valves, sensors, hardware, etc);
-require complex management (check-in and check-out);
-need greater precautions during maintenance on wheels and tires;
-always need tires equipped with sensors, i.e. including seasonal tire replacement for winter conditions;
Sensors are mounted on the inside of the rims, on the valve stem of each wheel, and constantly read the all related info (pressure, temperature, location, etc.) and report it to the vehicle’s receiver. They are battery- powered independent units, that can last several years, or at least beyond the 100thousand km. mark.
We will not tackle the details of the structural and functional characteristics here; these can be viewed looking at the technical specifications of the manufacturers / distributors.
Four different types of sensors are currently available on the market:
-OEMs: original equipment fitted during manufacturing. These are specifically programmed (with an identifying ID) for a specific vehicle model, wheel location, etc.
-clones: these are copies of the originals, for a specific vehicle.
-multi-function: already programmed for a number of different vehicles, reproducing frequencies, protocols, data, etc. The vehicle’s base station will then receive (and accept) the correct flow of information.
-clone-able: these have to be programmed for a specific vehicle, followed by a learning stage by the control unit, unless the vehicle automatically relearns the ID while being driven.
Manufacturers/distributors supply all the programming and diagnostic instruments with related instructions on how to create and clone a sensor.
Similarly to the types currently in use, that is, rubber in standard (up to 4.5 bar) and high pressure (up to 5.5 bar) versions, and metal for speeds over 210 km / h, direct TPMS systems use special valves:
-rubber valve stems, with an independent sensor unit
-metal valve stems, with a fixed angle or an independent sensor unit
Identification and maintenance
The identification from the outside of a snap-in TPMS is quite easy, since the valve caps are longer than on traditional ones, and removing the cap reveals a beveled surface at the base with the rubber coating that does not reach the thread.
You can completely replace the valve with its components every time you change tires, while maintaining the sensor in its place, with the proper tools and spare parts. For example, close attention to the proper tightening torque of the components must be paid, according to specifications.
Even metal valves maintenance is to be provided where necessary with proper replacement parts, keeping the sensor in its place if independent from the valve.
Episodes of corrosion could result from electrochemical phenomena (the contact of metal parts of different nature), whereby damages may occur during inspection phase or when replacing tires.
Since 2013, the TPMS valves are also incorporated in the ETRTO "Standard Manual" (European Tyre and Rim Technical Organisation). To facilitate the recognition of such valves, the general notes recommend to mark the stem externally, while the TPMS marking at the base of the stem is optional.
The system’s re-set should always be performed after replacing tires or wheels, after correcting the pressure, and when reversing the position of the wheels, front / rear (that is, if the unit is not equipped with an "Autolocation" system). It should be performed with the correct tire pressure, otherwise the reliability of the data cannot be guaranteed.
As seen, it is important for a workshop to test the vehicle (check-in, or "test before touch") before starting to work on it: visual valve control / absence of warning alarms / sensors in working condition.
Diagnostic tools enable the identification of the vehicle, the state of the system and needed programming of the sensors, etc.
Advanced detection of critical situations for valves and / or sensors will prevent future difficulties with the customer for the costs of any intervention.