Rob
I just bought a 2002 Toyota Sienna and for the first time, I've been exposed to tire pressure electronics. Not knowing what I was in for, I took your advice and read "the dust-covered little book in the glovebox." I can't believe the information in it!
I'm sure that to some people the techie jargon makes sense, but I was lost. A vehicle that looks after its own tire pressure has to be a good idea, but I would like to know how it works. How can my van check tire pressure on wheels while they are turning?
In simple terms, can you tell me how this works?
Dwayne
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It's all electronic smoke and mirrors Dwayne, but I mean this in a good way. Tire pressure monitoring systems - TPMS for short - are becoming more popular. There are a couple of excellent reasons for this: safety and fuel economy.
Anyone who remembers the Firestone/Ford Explorer debacle in the late 1990s will be easy to convince that maintaining proper tire pressure is a good thing. Thousands of Ford Explorer occupants were injured and hundreds killed in rollover accidents blamed on poorly designed Ford Explorers equipped with defective Firestone tires. More than 6.5 million tires sold as standard equipment on Explorers and other SUVs were eventually recalled.
Part of what came out of that issue was a United States government mandate on Sept. 1, 2007 that forced all auto manufacturers to install TPMS on every vehicle sold in the U.S. that had a Gross Vehicle Weight Rating of 4,535 kg (10,000 pounds). Those of you who have a vehicle equipped with TPMS will have noticed a new warning light or computer monitoring screen located somewhere in the instrument cluster.
The simplicity (or complexity) of these indicators also serves to illustrate the two system designs that are in use. They are Indirect Sensing and Direct Sensing. So Dwayne, we now get to the bottom of how these things work.
In a nutshell, both designs sense tire pressure but in different ways.
Indirect sensing systems typically use existing Anti-lock Braking System (ABS) hardware, with modification made to the operating software contained in the ABS control module (computer). The key to this design function is monitoring the rotational speed of each wheel through the existing wheel speed sensors (WSS), located at each wheel.
The pairing of a tone ring and a sensor measures the rotational speed of each wheel. If the pressures are same at all four wheels, the rotation speed will be the same. These sensors will send a consistent signal to the ABS module. If one tire loses pressure, the signal from that WSS will be greater. The circumference will be less, therefore this wheel will have to rotate faster to maintain the same road speed as the others.
Direct Sensing systems are more complex. This design relies on individual pressure sensors installed inside each wheel. The usual location is integrated with the tire valve. Depending on the manufacturer, these sensors can be attached to a band that is located in the drop-centre of the wheel.
A black section at the base of the valve stem contains a battery and a small radio frequency (RF) transmitter. This RF is picked up by an on-board antenna and monitored by a dedicated electronic control module. This signal is sent to the instrument cluster and in many applications, will actually read out the pressure inside each tire. Many also measure air temperature because, as temperatures rise, so does air pressure - which may have to be considered in calculations made by the control module.
That's the Reader's Digest version of TPMS, Dwayne.
However, as with any technology, there are issues:
Indirect Sensing
- What happens if all four tires lose pressure? There will be very little differences in rotational speed.
- How much pressure should be lost before a rotational speed difference can be noticed? This has been set at approximately 30 per cent of the tire's normal pressure.
- What if I have to replace one tire and it isn't exactly the same circumference as the remaining three?
Direct Sensing
- What happens when you have to replace a tire?
- Can I install winter tires on separate wheels? What about the sensors?
- Can I use gel tire sealants? What about balance beads?
- Does the tire shop have properly trained technicians to service TPMS?
- How long will the batteries last inside the sensors?
- Can the sensor/tire valve combo leak?
- If my instrument cluster doesn't display pressures, how will I know how much pressure a tire has lost? This limit for Direct Sensing has been set at 25 per cent of the tire's normal pressure.
Roughly 70-to-85 per cent of driving will take place on replacement tires. Consider this and it's easy to see that replacing a tire (or tires), is critical to maintain the integrity of TPMS.
As always, the best advice for your vehicle Dwayne, is to follow a regular maintenance schedule. This includes tire pressure checks - even if the warning lights or indicators are not flashing. Do not rely solely on TPMS.
Ensure your vehicle is serviced by a trusted shop with properly trained technicians familiar with TPMS. It might sound trivial but even fitting the right valve stem caps can make a difference to the proper operation of TPMS.
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