There are two kinds of wireless tire pressure monitoring systems: direct and indirect. Both of them can tell you the tires situation real time: underinflated, correct and overinflated. Also they can monitor the temperature data of tires to avoid the overheated issue. When the sensors detect the incorrect tire situation, they would start to warn the driver avoiding the small inconvenience becoming a nightmare.
Indirect tyre pressure monitors:
Generally direct tire pressure monitoring system provides more detailed and accurate data and better warning to drivers than indirect tire pressure sensors. The indirect tpms has an indirect monitoring method: it monitors the tire pressure by checking the changes of rotational speed of tires. If one of tires becomes underinflated, the rotational radius would become different. The load will fall gradually onto the underinflated tires, the rotational radius of this tire get smaller. Thus the speed of this tire is getting slower. The car's antilock braking system's wheel speed sensors will detect the difference and warn the drivers of the lower tire pressure alert by showing the alert on dashboard display or anything else. The connection is : under inflation of one of tires-----the load this tire carries becomes more------- rotational radius changes-----speed of tires becomes slower------- sensed by wheel speed sensors by comparing it with other tires—alert generated.
Weaknesses of indirect tyre pressure monitor:
Indirect tire pressure monitoring system doesn't offer the data like pressure reading and temperature, also not warn you of which tire is not correct in tire pressure. Thus if the four tires drops the almost same tire pressure, the indirect tire pressure monitoring system would be blind in this situation. Indirect being blind is not meaning this is normal, actually it is also dangerous since it's related the under inflation for 4 tires! Also indirect tpms has a very high rate of false alert of underinflated tires. One user's car usually generate the lower tire pressure, but when he went to the repair shop or 4S shop for checking , the guy told him the tire pressure is correct after careful checks. These false alarms bother his daily life. It becomes beeping if lower tire pressure alarm occurs. But he is not authorized to turn off the beeping alert. The only way to turn off the underinflated alert is to go to the authorized 4S shop. The professional staff canceled off the alert after conventional check. Also the alert are controlled by the central server, can't be dismissed by anyone else. These complicated formalities really take time. At last he decided to take the indirect tpms away from his car.