Recently, I received a call from a technician who was about to go crazy from a TPMS problem. He did everything the right way using the correct service information to learn a new sensor. The customer said the light was on for a long time and one tire was going flat. One sensor had failed.
This service procedure required a test drive so the vehicle could learn the sensor IDs and position on the vehicle. He had checked the TSBs and even checked that the sensor IDs matched the ones in the TPMS module. All of the information was correct, even with the IDs in the module. He was on his third test drive and the light would not go out.
I had to ask what other things were wrong with the vehicle and if there were codes elsewhere. He said that the left rear had an ABS wheel speed sensor fault for erratic performance. On this vehicle, the wheel speed sensors and steering angle sensor had to be operating to perform the reprogramming procedure for a new sensor. The system uses the rolling circumference of the tire to calculate the pressure inside the tire.
When you think you know everything about TPMS systems, there will be a car or truck that pulls into your bays that will challenge all of your knowledge and diagnostic skills. It may not be a case of a mouse chewing a wire or poorly installed accessory—but rather how the manufacturer engineered the system.
Direct or Indirect
You would think you could simply classify TPMS systems into two distinct categories of direct and indirect vehicles. But, there is a third type of system. BMW and Mercedes-Benz both use direct sensors in the tires, but also look at wheel speed sensor data as part of a flat tire detection system. The combined system can detect a blowout faster.
On most vehicles, there is a single antenna for the four sensors. The antenna may only be for the sensors, or it could also be used for the keyless entry system. Antennas can be hidden behind fender liners and embedded in windows.
Some vehicles have a dedicated module and antenna(s). And, some have four antennas in the wheel wells. Some Chrysler systems have three dedicated antennas, with the ignition switch module antenna acting as the fourth antenna. You probably won’t know that until you pull up the wiring diagram.
When a TPMS sensor transmits, it is a very dull, but fast (and important) conversation. It typically starts with an identification number that tells the module connected to the antenna to listen up. This code says the sensor belongs to the vehicle. The next part of the transmission contains a command or status update on the tire pressure and temperature.
Just when you thought sensors do not receive signals, you find a system that has transmitters in the wheel wells. Instead of waiting for the sensor to transmit on its own, a transmitter in the wheel well will send a ping to a sensor at around 125Hz. The signal tells the sensor to transmit a signal, even if the vehicle is stationary.
TPMS Mapping, Modules and the Light
Once the antennas, sensors or wheel speed sensors have detected a low-tire condition, it has to be communicated to the driver. This is done with a light in the dash and/or a message in the driver information display. The information can take several routes and pass through different modules.
Finding how the sensors communicate with the light is a critical step for situations where the system is not functioning or is causing DTCs in other modules. Sometimes, the root cause of a TPMS problem is not the TPMS module or sensors, but it could be the network or the modules on the network.
I believe in the “Test Before You Touch” method as a way to perform triage when assessing the initial customer complaint. This procedure can rule out inflation and dead sensor problems. But, if the problem can’t be resolved, it requires looking at the service information and TSBs.
Your TPMS tool is not just for relearns and programming sensors. Many of these tools can pull codes from the TPMS module through the OBDII connection. They can also be used to detect and measure the strength of radio signals coming from sensors and the environment around the car.
Many TPMS systems are more than a decade old. With older vehicles, there are more curveballs. During the first years of the TPMS mandate in 2007 and 2008, many manufacturers were still trying to integrate TPMS sensors, antennas and modules onto pre-existing platforms. These single-model exceptions are where you might have to be a diagnostic switch hitter. Bottom line: Never assume that all TPMS systems operate the same way.