Do you rotate tires? Replace them? Servicing the TPMS is going to become an increasing part of those basic services you offer.
Systems designed to warn the driver of a severely underinflated tire are not new. They made their first appearance back in 1986 on the Porsche 959. However, it was the Ford/Firestone tire disaster of 1999 that led Congress to pass the TREAD Act, requiring Tire Pressure Monitoring Systems (TPMS) on all vehicles with a gross vehicle weight rating of less than 10,000 pounds. Phase-in started in the 2006 model year, and full compliance was to occur by the 2008 model year.
This means that more and more of the vehicles coming to your shop for even routine service will require the tech to have some knowledge of how these systems work and how to service them. Even a simple tire rotation, if done improperly, can result in the TPMS warning light illuminating.Common Service Information
TPMS comes in two forms — indirect and direct. Before I go into the details of the two, let me give you a few tips that apply to all.
To start, there are no general guidelines on servicing these systems. That's why the first addition you should make to your shop is a quick reference guide to aid you in identifying what vehicles have TPMS, what type of TPMS it is, what the service requirements are and what the reset procedures are. Make this information available to both your technicians and your service writers so TPMS-equipped vehicles can be identified right from the start.
Make it a habit to look for the TPMS indicator on the instrument panel during the Key On, Engine Off bulb check. If the light appears, the vehicle is equipped with this system. Then see if the light remains on after start-up, indicating there is a problem you and your customer should know about before routine service starts. Typically, it's just an underinflated tire and that's the first thing to check. But it might indicate a fault in the system itself. Catch it before you start to avoid a conflict.There are a variety of TPMS diagnostic/reset tools with a number of options available on the market. No doubt you'll need one, but consider your needs versus the tool's features before investing. A few vehicle models require OEM level access to the control module in charge of the TPMS when replacing a sensor, but some of the TPMS tools available include this feature as well.
(See the listings at the end of this article for a few Internet resources you can use to learn more.)
TPMSes are required only to warn the driver if a tire is underinflated by more than 25 percent and might not provide warning in the case of rapid air loss. Changes in temperature affect tire pressures and can cause a false TPMS warning to set. And just because the car has a TPMS doesn't mean you shouldn't still check the tires for proper inflation, wear or damage. Educate your customer on the importance of maintaining their tire pressures, and how to do it.Indirect TPMS
An indirect TPMS uses information from the existing antilock braking system (ABS) wheel speed sensors to detect a low tire. An underinflated tire will have a smaller circumference than one that is properly inflated, and in a straight line it will turn at a different speed from the other wheels. Indirect systems use an algorithm added to the control module's programming that typically compares diagonally opposed tires to the other pair of tires, and a few measure the actual resonance of the tires in determining one with low air pressure.Because of the method used, no additional components are needed. There are a few drawbacks. One, the system cannot tell there is a problem if all the tires are underinflated. Two, the control module first has to learn what is right before it can determine what is wrong. This is done by putting the system into learn mode (some service information calls it initialization), with the tires already adjusted to the correct cold inflation pressures. Then the vehicle is driven a specified distance, allowing the TPMS module to record known good values. The TPMS monitoring may be suspended due to road surface irregularities affecting the wheel speed sensor outputs. Next, ABS system faults can also suspend the TPMS system. Look for ABS related codes when diagnosing indirect TPMS system problems.
Here are a few more service tips for you. Because this system works by comparing wheel speeds, and that's affected by the rolling diameter of the tires, mixing tires can affect the ability of the system to operate. That includes installing one new tire on the same axle as an old one, rotating the tires, installing the spare or installing tires not OEM-approved.Initialization usually is accomplished by first making sure all the tire pressures are properly adjusted, then placing the module in learn mode by operating a dedicated switch or even the exterior light switch. A few require an OEM level scan tool or TPMS tool with this feature. Some aftermarket scan tools are also now equipped with this function. In all cases, a review of the applicable service information is important before you perform any tire related service to avoid problems for you and your customer.
Direct TPMS
Direct TPMS uses dedicated pressure sensors in each wheel to detect problems and is much more accurate than indirect systems. These sensors can be one of three types: the band sensor that attaches to the rim center, the metal stem sensor that is positioned just below the valve stem or the rubber stem sensor, found on many GM models.These sensors send a signal to the control module every 30 to 60 seconds when vehicle speed is over a minimum threshold, typically 20 mph. When the vehicle is stationary, the sensors may continue to transmit for a set time. However, the transmission rate generally drops to once every 30 or 60 minutes, depending on make. Some direct systems use a vehicle display capable of telling the driver how much air is in each individual tire, and some only operate the warning light when a problem is detected.
The first step after identifying a vehicle equipped with a direct TPMS is to make sure the sensors all are working as they should. These sensors are battery powered (and no, the battery is not replaceable) and should last seven to 10 years. This is where your TPMS diagnostic tool comes in. Before performing any tire service, use the tool to activate each sensor and verify function. A few models also monitor the spare tire, so make sure you check the service reference to see if you need to include it in your initial inspection. If you find a bad sensor, advise your customer of the need to replace it.A heads up for you: I've seen customers install tire sealants in their tires when they have a flat and no other option in getting back on the road. Some reports say tire puncture sealants may mean bad news to these sensors. The sealant may clog up the sensor port, and prevent it from working. If sealant is found in the tire during repair, replace the sensor at the same time, whether it is currently working or not.
Depending on the system, tire rotations may or may not require that the module relearn the relative position of the sensors. Check your quick reference guide or your service information for the specifics on the vehicle you are working on. Tire replacement, however, is a different story. Here, care must be taken not to damage the sensor while removing the tire. Metal stem sensors are easy — just remove the locknut after breaking the bead and allow the sensor to fall into the tire.Direct sensors are precision devices and require care on installation. Always replace the seal, washer and locknut on a metal stem design and torque them to specification with a dedicated tool made for this service, even if you are reusing the sensor. The valve cores themselves are nickel-plated and are not the same as a traditional valve core. These, too, must be properly tightened and should not be exchanged with a non-TPMS core.
The sensor assembly on the rubber stem design is usually attached with a No. 10 Torx fastener that also requires a specific torque value. Too loose, and you may have a leak or a sensor that doesn't function properly. Too tight may lead to sensor damage. During installation, make sure the sensor is correctly positioned relative to the wheel. If not, the reed valve switch internal to the sensor may not activate at speed and get the sensor transmitting.When replacing an existing sensor with a new one, it will be necessary to initialize the new sensor. This may require an OEM-capable scan tool or aftermarket unit with this feature. Typically, the ID of the new sensor has to be entered, and you'll find this number on the sensor body. Some systems can relearn the sensors without entering this code. One OEM method is performed by placing the module in learn mode and adding/subtracting air from each tire in a specified order while the module looks for the signal to change. Another method is similar, but uses a magnet to trigger the sensors in order. Basic TPMS service tools generally come with the magnets you will need. Check your service information before proceeding with the repair to make sure you have what you need to successfully complete the job.
Pete Meier is an ASE CMAT, member of iATN, and full-time tech in Tampa, Fla. His experience reaches back more than 30 years, and his contributions to Motor Age reflect a wide variety of experience with almost every make and model. You can contact Meier directly at www.autoservicetech.com.
About the Author
Pete Meier
Creative Director, Technical | Vehicle Repair Group
Pete Meier is the former creative director, technical, for the Vehicle Repair Group with Endeavor Business Media. He is an ASE certified Master Technician with over 35 years of practical experience as a technician and educator, covering a wide variety of makes and models. He began writing for Motor Age as a contributor in 2006 and joined the magazine full-time as technical editor in 2010. Pete grew the Motor Age YouTube channel to more than 100,000 subscribers by delivering essential training videos for technicians at all levels.
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