(TPM) Tire Pressure Monitoring

Checking tire pressure is one task that vehicle owners rarely perform. In fact, many people don’t regularly check their tires at all. Often times, a passenger, or a stranger in a parking lot, will notice a near flat tire. By that time, the tire pressure is well below where it should be. Low tire pressure can result in poor handling, abnormal tire wear and decreased fuel economy.

Public Service Announcements on TV and radio have touted the benefits of properly inflated tires for years, but to no avail. That has left the job to modern computer technology. The Tire Pressure Monitoring (TPM) system helps remedy this situation by alerting the driver when tire pressure has fallen below a certain point. Advanced systems allow the driver a quick way to check the actual pressure in each tire. Let’s examine how this technology works and then we’ll take a look at diagnosing some basic problems.

BASIC OPERATION

Although there are two basic types of Tire Pressure Monitoring systems (magnet systems and non-magnet systems), there are some common operational traits. Let’s examine these traits. First of all, each system has a pressure sensor combined with a radio transmitter mounted on each road wheel (we’ll simply call this the sensor). The sensor transmits a signal, representing the tire pressure, to a combination receiver and control module, which we’ll call the receiver.

Both systems have threshold pressures that trigger and cancel warnings. For example, a particular vehicle has a low threshold pressure of, say 25 psi, that will trigger a low pressure warning. If the signal received is 25 psi or lower, a low pressure warning will be displayed inside the vehicle. In order to cancel the warning, the tire pressure must be increased beyond the threshold. Instead of 26 psi, which is 1 psi above the low threshold (and would not have triggered a low warning), the pressure might have to be increased to 28 psi, which is 3 psi above the low limit. The same logic applies to the high threshold pressures.

When a warning is detected, either low or high pressure (but much more likely the warning will be low), some type of graphic is displayed inside the vehicle. The type and location, and whether it is accompanied by a chime, depends upon the application. The display might be a simple icon on the instrument panel, as shown in Figure 1. On the other hand, it could be a graphic display which specifies the location and pressure of each tire, such as that shown in Figure 2.

While there are similarities and differences in components and operation, the same is true for diagnosis and service. All TPM receivers monitor the system for proper operation and record trouble codes if problems are detected. But it should be noted that the troubleshooting procedures differ from system-to-system. Also, the location of diagnostic menus on scan tools can also vary.

TPM DOS AND DON’TS

Checking tire pressure regularly and maintaining proper tire pressure should be a standard practice whether a TPM system is installed on a vehicle, or not. The TPM system is just an aid.

Tire pressure should always be checked cold (stationary for three hours or driven less than one mile). Always use a tire pressure gauge. Do not use the graphical display in the vehicle, if so equipped. Often times, the display does not show the instantaneous changes in pressure detected by a gauge.

Remember, a drop in temperature reduces tire pressure. On TPM-equipped vehicles this might trigger a warning. This will most likely be the result of an extreme ambient temperature change on a vehicle in which the tire pressures are near, but not at, the low pressure threshold. For instance, the vehicle driver will report a warning on a cold morning when the previous afternoon was quite warm. Be sure the system is operating properly and the tires are properly inflated.

Only replace TPM components after a thorough diagnosis. And when it comes to parts, be sure to use only those specified for the system being serviced. While this is basic common sense, it is especially true with TPM systems, as there is much variation from system-to-system and vehicle-to-vehicle. Never swap parts between different systems!

Note: Aftermarket wheels and tires that are a different size from the original can prevent a TPM system from operating correctly.

The average battery life in a TPM sensor is 100,000 miles or 10 years. Batteries are not serviced separately. The entire sensor assembly must be replaced when the battery goes dead. Don’t forget that magnet and non-magnet sensors are different. The magnet sensor has a shorter threaded area on the sleeve compared to the non-magnet style. Also, the valve stem hole dead ends in the body, while its companion sensor has a through hole.

Replacing sensors unnecessarily can easily be avoided. This is time consuming and costly.

First, be sure to use the recommended procedures for breaking down the tire. Following such procedures will help you position the sensor correctly without damaging it.

Secondly, do not use tire sealants. On some systems, sealants can contaminate the sensors and interfere with the ability to sense and transmit accurate information. More importantly, the sealants can clog the sensors.

Our last precaution is about valve stem caps and cores. Those items used on TPM-equipped vehicles are specially designed. To avoid corrosion, never use a standard valve stem cap or core in a tire pressure sensor.

FINDING A BAD SENSOR

A vehicle with a non-magnet type TPM system is brought into your shop with the complaint that the warning light is illuminated. The car has no Electronic Vehicle Information Center (EVIC) graphics which would indicate which tire has low pressure. When the tire pressures were checked, all four were within specifications. So, what’s the problem?

Let’s start by checking for trouble codes with the StarSCAN.® Well, a stored code was found, but it did not specify the sensor location. The scan tool, however, did display pressures for individual sensors, as shown in Figure 3.

In order to find the bad sensor, lower the air pressure in each tire while observing the display. The bad sensor will not show a drop from the recommended pressure when air is released from the tire. Replace that sensor.

If you had a similar vehicle that did not display the location of the tire with low pressure, but did specify the number of tires with low pressure, finding the bad sensor is a bit easier. Once again, release air from the tires, one at a time, but monitor the message. If you lower the air pressure and the number of tires with low pressure does not increase, you’ve found the bad sensor.

Of course, on vehicles that specify the low tire location, it would have been easy to see which sensor was reporting an inaccurate pressure.