Tire Pressure Monitors (?)

Toyota.

If it is like my 2007 Highlander, they use the ABS system to monitor wheel speed. If the average speed of each wheel is not the same, they figure one tire is low on air and illuminate the yellow dash light.

Vito

If the valve stems are rubber, the tire pressure monitor (TPM) system uses the signal from the antilock brake wheel rotation sensors to monitor pressures. If the valve stems are metal, it uses a sensor mounted at the end of the valve stem inside the wheel.

How does the sensor work when it uses the antilock brake wheel rotation sensors?

Jeff

The tire that's low has a different rotational speed than the others. The speed sensors that monitor brake lock will show this difference to the computer while you're driving. If all of the tires are low, the speed will be the same on all four corners, therefore there will be no low tire detection.

See Jeff Strickland's explanation.

Thanks for the reply.

The latter would sound like the most direct (explicit) approach. For example, if all tires equally low, the computer would light up the warning lite on the instrument cluster, right?

The valve-stems on my RAV4 are metal. How does the vehicle's computer interface with the sensors? Are there batteries and transmitters in the stems?

I seem to recall reading that the Federal Motor Vehicle Safety Standards (FMVSS) are written so that all vehicles will eventually have a direct tire pressure monitoring system (TPMS) using the sensors mounted on the valve stems. As you surmised, even if all tires are equally low, the direct system will still illuminate the warning light. The sensors have batteries and transmitters that have to be registered with the vehicle's TPMS, and some vehicles also monitor the spare air pressure.

The advantage of an indirect system that uses the ABS sensors to monitor wheel RPM is that it is less expensive and easier to implement since no additional sensors are needed. The disadvantage of the indirect system is the problem with all tires equally underinflated and less accuracy.

The disadvantages of a direct system is that the metal valve stem needs a gasket that is another potential source of leaks, the valve stem must have the valve stem cap installed to prevent corrosion of the sensor, the system is more complex and therefore a greater source of potential problems, fix-a-flat in a can products will mess up the sensors, and the wheels need more weight to balance since the sensor is heavier than a valve stem.

I tried checking one of the wheels on my RAV4 by letting air out to produce a remaining pressure of only 18 PSI. When I turned the key switch on, the tire pressure warning light came on as usual, but after the checkout period, it went out. Does this seem right? Or do the wheels need to rotate to get the sensor closer to the receptor? Or none of the above?

The wheels probably need to rotate a few times for the warning light to illuminate.

My SO 's RAV4 and both of my Sister's RAV4's (2007 and two 2008's) all have the valve stem sending units. The spares have one also.

I thought the metal valve stem was a 100% predictor of valve stem tire pressure monitors until I had problems with a tire pressure sensor on my Ford truck. It has rubber valve stems so I assumed it either used the wheel speed sensors to determine that a tire had low pressure or it had a sensor strapped to the center of the wheel. In fact, it has valve stem tire pressure sensor, but they have rubber stems. They look just like the regular rubber valve stems.

Ed

Try reading the info here:

I learned something new today!