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How do tyre pressure sensors work?

Bradley Jando | Monday 16th August 2021 3:17pm

tyre pressure sensor connected to valve

Have you ever had an alert appear on your vehicleís dashboard that tells you that your tyre pressure is low? If youíve experienced this, then your vehicle will be fitted with a tyre pressure monitoring system (TPMS). These systems are very useful and can be found in all vehicles that have a manufacturing date of 2014 and later.

Within the TPMS is a sensor that is able to monitor tyre pressure. It can issue a warning on the dashboard screen if the pressure drops below a set amount. The TPMS should be serviced regularly to prevent any faults from developing, particularly as a faulty TPMS is an instant MOT test failure. These devices can be used to determine whether your tyre is frequently leaking air, either through the trim or because of a slow puncture. Solving this problem quickly could reduce the chances of an accident occurring due to a blowout or a flat tyre.

So how do these devices work and where can you find them on your vehicle?

How do car tyre pressure sensors work?

The two main types of TPMS are direct and indirect.

Direct TPMSs are the most common as theyíre more reliable. This system uses pressure sensors that are located within the tyre to track the pressure. The sensor is fitted inside the tyre to the back of the valve, which is where you apply new air when the tyre needs pumping up.

Because of their location, they can track the exact pressure within the tyre, making them accurate and reliable. When the pressure drops past a set parameter Ė usually 25% Ė a warning alert will appear on your dashboard. At this point, the car should be taken to a petrol station to have its tyres pumped up. Some TPMSs will specify which tyre is low so you donít have to manually check them all.

These sensors are fitted with an internal battery, however, battery failure is the main reason TPMSs stop working. The more miles you do, the quicker the sensor will run down. Unfortunately, for them to work properly, they need to be sealed units Ė so you canít simply replace the battery when it runs out.

Indirect TPMSs donít have their own sensor. Instead they use another sensor thatís located inside the wheel, usually the anti-lock braking systemís sensor. It works by tracking the tyres as they revolve. A tyre that is underinflated will have a smaller radius than a tyre thatís properly inflated, and therefore it will make more or irregular turns. This is how the sensor detects that the pressure could be low. This kind of system is less accurate but is still able to tell you when a tyre requires more air.

Regular signal from an indirect TPMS showing normal tyre pressure
Irregular signal from an indirect TPMS showing lower tyre pressure

These sensors can be very beneficial. If your car is frequently telling you that one tyre is low, that tyre may have a slow puncture or could be leaking air from the trim. You should take it to your local garage to have it looked at or get a replacement. Driving with under-inflated tyres could also have an impact on their lifespan. They may wear down more quickly and they could even have an effect on your steering and fuel usage. To prevent this, you should ensure that your tyres are pressured to the manufacturerís recommendation.

If youíve re-inflated your tyre to the correct air pressure and the warning alert is still on your dashboard, you may have a faulty sensor that needs replacing.

Where are tyre pressure sensors?

Direct TPMS sensors are located within the tyre, usually in the space between the tyre and the wheel trim. Indirect TPMSs use the sensor on the anti-lock braking system (ABS). The ABS sensor is typically located near the brake rotor. The wheel may need to be removed in order for you to access it.

When you get new tyres, you should make the garage aware that they will need to refit the sensors. You wonít need new sensors - the old ones can be refitted from the previous tyres.

How to remove tyre pressure sensor

Occasionally, an indirect TPMS sensor, or an ABS sensor, may need cleaning because of where it is located. First, you should check your carís handbook to find exactly where it is situated. Once you know where it is, youíll need to safely take the tyre off. Ensure that youíre using stable jack stands for this job, as you donít want the car to fall off them.

When the wheel has been taken off, turn the steering wheel so itís pointing in the opposite direction to the side youíre working on. For example, if youíre working on the passenger side, turn the steering wheel to the driverís side. An extra safety tip is to put the removed wheel underneath the car (usually under the axle) as a backup Ė if the jack stands do happen to fail, then at least the carís not going to crash fully into the floor.

Now that the tyre is off, you should be able to see the sensor more clearly. Itíll be bolted in place so you will need to remove these bolts using a socket wrench. Once all the bolts are removed, you should be able to remove the sensor easily. Using a soap and water solution, carefully clean the sensor with a cloth. Bolt it back into place and refit the tyre.

Replacing or removing the direct TPMS sensor isnít as easy. You may need a professional to do this job for you, so itís best to get in touch with your local Kwik Fit centre for reliable service and advice you can trust.

Tags : Tyres

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