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.
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 will report a warning on a cold morning when the previous afternoon was quite warm.
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 160,000 kilometers 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.
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.
