Does anyone know how the tire pressure monitoring system (TPMS) works? Lately, I've been getting alerts from it even though my tire pressure is ok. At first, the alerts were persistent until I recalibrated it. The last couple of days, after I sat idling for a minute or two after starting my car, the alert would show up briefly then disappear. I'm curious as to how this system works and if, perhaps, a sensor is starting to malfunction.

 

That's what it sounds like. I believe it's on infra red or something like that. But a sensor going bad isn't unheard of, and will keep the traction control system turned off.

 

Also, check the tire pressure of your spare tire. It may have a sensor as well.

 

Your 2013 Accord comes with an indirect TPMS that doesn’t use tire pressure sensors mounted inside the tires. Instead, it uses the existing VSA wheel speed sensors to monitor and compare tire characteristics while driving.

This is part of the TPMS system description of models with Adaptive Cruise Control (ACC):

Low Pressure Detection Methods
The Indirect TPMS establishes a baseline tire pressure reading using the wheel speed sensors while, indirectly, monitoring tire pressure to detect pressure loss in any of the four tires.

The system uses these two methods to determine loss of tire pressure:

• Wheel speed differential
• Peak tire resonance.

Both methods make use of outside temperature sensor data to correct calculated tire pressures for ambient temperature variations.

Wheel Speed Differential
When monitoring tire pressure using the wheel speed differential method, the TPMS module compares the speed of the wheels in three different configurations:

• Diagonal comparison between the front and rear wheels
• Comparison between the two front and two rear wheels
• Comparison between the two right and two left wheels

Tire diameter shrinks as the tire deflates, causing the wheel speed to increase.

Consequently, if the difference in wheel speeds exceeds the threshold, the low tire pressure/TPMS indicator turns ON

Peak Tire Resonance
Peak resonance values are learned during calibration and become the reference values for pressure monitoring. When using the frequency resonance method, the TPMS module compares the peak resonance values between all four tires and the two front tires.

If the present values have dropped below a 20% pressure threshold, the low tire pressure/TPMS indicator turns ON.

Resonance frequency is corrected based on the outside air temperature sensor information, because resonance frequency is changed by the outside air temperature.

The TPMS module compares the peak resonance values learned at calibration. If the value drops below a 20% pressure threshold, the low tire pressure/TPMS indicator turns ON.

Temporary Suspension
Certain situations, such as when the vehicle is turning, accelerating, decelerating, or braking will temporarily prevent the system from monitoring tire pressures due to varying tire loads. The TPMS module uses other devices such as the lateral acceleration sensor, the yaw sensor, the steering angle sensor, the master cylinder pressure sensor and the PCM data to determine how the vehicle is being driven.

You can get the car scanned for VSA trouble codes with a code reader/scanner that has the capability to read to Honda ABS/SRS/VSA trouble codes (not all code reader/scanners has the software capability).

 

Wow. It's way more complicated than I suspected. I'm going to have to learn about what VSA wheel speed sensors are. It also sounds like these measurements occur while the vehicle is in motion. So, in theory, the TPMS alert should not happen when the car has just started and is just idling. Maybe that brief light that appears is merely an indication that the TPMS system is starting up.

 

The VSA speed sensor are similar to what used to be called ABS wheel speed sensors. Honda's VSA system controls more than just ABS.

Each wheel bearing has a magnetic encoder incorporated into them. The wheel speed sensors picks up the signals from the magnetic encoders.

 

Ah. My treadmill has a similar setup, I believe, to detect how fast it is running. There is a magnet on the pulley and a sensor located on the rail.

 

Yeah, it's similar to that but more sophisticated. Sorta like an induction sensor positioned close to a disk with a lot of little teeth.

Compared to older cars with only ABS, the sensors are pretty much the same concept. Just more precision because they are looking for very small differences in wheel speed. (ABS is looking for larger discrepancies in wheel speed.)