Tire pressure monitoring systems
A tire pressure monitoring system (generally referred to as TPMS or TPM) is relatively basic in terms of the number of involved components. The pressure sensor transmitters (one transmitter mounted inside each wheel) monitor inflation data and send an FM radio signal to the system’s antenna and receiver, which then sends a digital signal to an ECU. Note that some vehicles may be equipped with one central antenna, while others feature individual antennas in the wheelwells at each corner.
A ”direct” tire pressure monitoring system (where a dedicated system exists that monitors actual tire inflation pressure) is generally calibrated to alert the driver when one or more tires have lost at least 20% to 25% of the programmed/recommended inflation pressure.
An ”indirect” system relies on tire diameter changes (via ABS wheel speed sensor readings) to alert the driver of tire pressure changes. The indirect system is long-antiquated. All new vehicles (2007 and newer) are federally mandated to feature TPMS in vehicles of 10,000 GVW or lighter.
If a “fault” signal is processed by the ECU, the in-dash tire pressure warning light will illuminate. Generally speaking, if a tire pressure problem is indicated, the warning light will illuminate constantly. If the light blinks (in most OEM systems), this indicates a system fault that must then be diagnosed with the proper diagnostic tool.
It’s important to note that whenever sensors are moved to new locations (during wheel rotation, etc.), that the ECU must be re-programmed (re-set/re-learn) in order to maintain correct location information for the system ECU. Otherwise, each sensor may transmit correct inflation data, but the ECU will then assign the pressure data to the wrong corner(s) of the vehicle. For example, if a vehicle is equipped with dash information that identifies each specific wheel location, you may see a warning that the left front tire is low, when in fact the low pressure problem may be found at the right rear (because the wheels were rotated and the sensors were never re-set). Each sensor has a unique ID imbedded in its pulse signal. The ECU receives this pulse signal and assigns (remembers) the sensor’s wheel position.
Three basic “styles” of TPM sensor/transmitters are available (NOTE: The correct OEM term for the sensor is ”wheel mounted sensor”): banded, snap-in and clamp-on. Banded sensors are affixed to the inside of the wheel using both a positioning adhesive (peel off) and a large diameter worm-drive clamp (the wheel valve is the standard valve style that has been in use for decades). Both clamp-on and snap-in sensors feature the valve stem as an integral part of the sensor. A clamp-on sensor mounts via the wheel’s valve hole and is secured with a mounting nut and is sealed with a separate grommet. The snap-in style features a traditionally mounted rubber valve style.
NOTE: It’s important that you’re able to quickly identify the presence of a snap-in style TPM sensor. The valve stem caps are longer than a traditional stem cap, and (with the cap removed) you’ll notice a beveled brass surface at the base of the cap mating area.
The banded TPM sensor (again, a sensor that is secured to the rim well with a worm-drive band clamp) is still featured on some Ford models, but by and large, all other makes/models utilize either a clamp-on or snap-in sensor style, where the valve is an integral part of the sensor.
Depending on the maker’s design, these wireless pressure sensors transmit their data to an ECU using either 315 Mhz or 434 Mhz FM signals, usually in the 125 kilohertz range (most domestic-brand vehicles will utilize 315 Mhz, while most import-brand vehicles will use 434 Mhz. An exception is Nissan, which uses the 315 Mhz signal).
Three primary sources for OEM TPM systems include Schrader International, Borg Warner Beru Systems GmbH and Pacific Industrial Co. Ltd.
Schrader systems are found on Ford, Chrysler, GM and Nissan/Infiniti applications. Pacific systems are found on certain Lexus vehicles, and Beru systems are used on Mercedes, Porsche, BMW, Audi/VW and Land Rover models.
The vast majority of OE TPM systems are stem-mounted as opposed to band-mounted. The primary reason involves diagnostic capabilities. If the sensor is band-mounted and deeper inside the wheel, it’s more difficult for the signal to pass to the diagnostic tool, whether the system uses an RF signal or a magnetic signal.
Stem-type sensors are light, weighing in the neighborhood of about an ounce, so this won’t be a problem regarding wheel balance compensation.
The direct type pressure sensors are powered by a lithium battery, with an estimated life of five to seven years, depending on your information source (I’ve heard estimates as high as 10 years). However, since the use of these sensors is relatively recent, we really don’t have enough real-world experience to more accurately estimate battery life. The sensors are designed to “wake up” via centrifugal force (once the wheels rotates at about 20 mph or so). In order to help extend battery life, the sensors “go to sleep” when parked. In operation, the sensor produces a pulse signal in timed intervals (depending on the specific system) of every 30 seconds or every 60 seconds, etc. This also aids in extending battery life.
Are TPM systems really necessary?
Some might say that these systems are a waste of money, arguing that if people would simply check their tire pressures on a regular basis, there would be no need for an on-board monitoring system. But, therein lies the rub. Most people don’t bother checking inflation pressures, at least not as often as they should.
We can debate the issue, but the reality is that the use of TPMS has been federally mandated, so we don’t have a choice. They’re here to stay, and we need to deal with them.
Even though diehards may dislike the notion of additional monitoring systems, tire pressure monitoring systems are a great idea, even for those of us who know better and check pressures routinely. For example, pressure monitoring systems are an outstanding add-on for trailers. Simple aftermarket systems (sensors and monitor) are readily available (and are easy to install) that will adapt to older vehicles and/or any trailers. After all, being alerted that a trailer tire is experiencing a pressure drop can avoid a potentially costly and tragic trailering mishap.
Indirect tire pressure monitoring
The indirect system is no longer (as of 2007) used in production vehicles. As a relatively inexpensive method of altering the driver to a low tire pressure situation, some OEM vehicles obtain this information not through a dedicated TPM system, but rather by using information already monitored by the anti-lock braking system. This is referred to as an ”indirect” TPMS. Since a “low” tire will rotate at a different rate than properly inflated tires (due to a difference in rolling tire diameter), this difference in wheel rotation is sensed by the ABS. A signal is then sent to alert the driver via a warning light. It’s then up to the driver to check all four tires to determine which is under-inflated.
This approach has its problems, since faulty alerts can easily result if the wheels slip on an icy road, or if inside and outside wheels rotate at different speeds during sharp turns, where the ABS monitoring senses a difference in rotation and interprets this as a low pressure condition. In addition, if all four tires are equally under-inflated, the ABS won’t trigger the pressure warning, since all wheels are rotating at the same speed.
Indirect tire pressure monitoring systems generally are designed to illuminate the warning light when a single tire’s pressure has dropped by about 30% (in reference to its official recommended inflation pressure listed on the placard and in the owner’s manual).
As an example of initializing on an indirect system, where a tester tool is not necessarily required, we’ll cite a late model Toyota. First adjust each tire’s inflation pressure to the recommended pressure. With the vehicle stopped, turn the ignition switch to the ON position. Press and hold the tire pressure warning reset switch until the tire pressure warning light blinks three times at one second intervals (on a 2006 model, this switch is located under the left side of the dash). Drive the vehicle at 19 mph or more in order to complete the initialization of the skid control ECU. This takes about one half to a full hour.
If the tire pressure warning light blinks at 0.25-second intervals while the vehicle is being driven, the initialization may have failed. In this case, perform the initialization again. After initialization is completed, the skid control ECU monitors tire pressure by using the ABS/traction control wheel speed sensors.
Universal sensors — time and money savers
The OEs, in their usual fashion, have created an incredibly vast proliferation of pressure sensors (there are no industry standards, so everybody apparently felt the need to devise their own proprietary sensors in terms of data transmission protocol, with an estimated 75 versions currently in OE use). As a result, it’s impossible (or at least extremely impractical) for any shop to stock enough sensors to accommodate all makes/models/years (you’d go broke just in trying to stock these things in an attempt to achieve full coverage).
The solution: universal tire pressure monitoring sensors. These universal sensors allow you to basically grab a new “blank” sensor and program it for the application. Currently there are three sources for universal sensors: Dill Air Control Products/Continental VDO’s Redi-Sensor, Schrader’s E-Z Sensor and Myers Tire Supplies’ Sens-It.
While none of these universal sensors will accommodate 100% coverage, we’re told that they will cover roughly 85% to 90% of the current market. The makers are continually trying to obtain protocol information from the auto makers, so if certain application info hasn’t yet been made available to the aftermarket, it’s a waiting game for the sensor makers.
This sensor is akin to a universal TV remote control unit, in that it’s designed to cover all (available) protocols in a “shotgun” approach. The sensor is already pre-programmed with all system protocols. It sends out all frequency protocols (multiple data commands in a continuous wave), allowing the vehicle’s ECU to grab the one that fits. These sensors are pre-programmed, and are not programmable (similar to a read-only CD that can’t be over-written). Basically, install and go.
The up-side: The sensor is already programmed with multiple protocols, so no programming is needed.
The down-side: You can’t update it with new programming in the future (if a newer/different protocol is required).
This sensor was jointly developed by Schrader and Bartec USA (Bartec has long been involved in TPMS, making test/verification equipment for OE vehicle production, and now offer hand-held units for shops).
The sensor is “blank” (like a blank CD), requiring a Bartec TPMS tool for programming. The tool reads data from the original sensor (reading the sensor’s “fingerprint”). A new EZ-Sensor is then placed in the tool’s cradle. The tool then imprints the original sensor’s info (its fingerprint) into the new universal sensor, in effect, creating a replicate (clone). If you screw up during the imprinting process, you can start over, since the EZ-Sensor is “re-writable.” In order to implant the original sensor’s info onto the new blank sensor, the Bartec tool is required. However, down the road during future service, any TPMS tool may be used if vehicle ECU relearning is needed.
NOTE: Myers Tire Supply is currently running a promotion. With the purchase of the 27002A Bartec 400 TPMS Service Tool Kit, a free Wheelrite TPMS Mechanical Tool set is included, featuring a Toyota reset tool, a special 3/8-inch drive torque wrench with 11mm and 12mm sockets, a pre-set torque limiting torx tool for GM sensors, a pre-set valve core torque tool, multi-purpose grommet tool for removing 11mm and 12mm clamp-on sensor nuts and for installing non-beveled grommets, a TPMS valve holding tool and a digital air gauge.
The up-side: The sensor is easily programmable and can be re-programmed at whim.
The down-side: This sensor is dedicated (for programming) for use with only the Bartec tool, and the tool must be updated annually at a cost of about $300/year (in order to update the programming capabilities to meet ever-changing OE vehicle protocols).
This system is so cool that my eyebrows twitched with excitement when I saw a demo.
The sensor is blank (again, like a blank CD). Programming ID is accomplished via the Internet by going to the Sens-It Web site. A docking station pad connects to your PC via a USB cable. Simply lay the blank sensor onto the pad, go to the Web site, and follow the simple prompts. Enter the vehicle’s VIN to obtain the correct protocol info, assign a wheel location (a simple drag and click deal on the site’s illustration), and hit a button. The info uploads, and presto — the new sensor is now programmed for the vehicle. Here’s the really sweet aspect of using the VIN as the reference — during a production run of the vehicle, the OE may have changed the pressure sensor model from one run to another. By using the vehicle identification number, you end up programming the new sensor with exactly the same data as the original sensor that was installed during vehicle production.
The up-side: The sensor is easily programmed (and can be re-written up to about 15 times). The programming base (this plugs into the PC via a USB cable) runs a mere $199. In addition, protocol data is continually updated on the site (hourly), so you get the latest info at the moment instead of waiting for an annual update. Plus, the programming and updates are free. If you know how to use a mouse to click and drag, you can program this sensor.
The down-side: Your shop must have a PC and access to the Internet in order to program the sensor and to obtain the continually-available free updates. As far as I’m concerned, this isn’t much of a drawback. If you don’t have Internet access at your shop, get out of the dark ages and get with it. Even if you don’t have a PC back in the shop area, simply use the boss’s computer. Programming only takes a few minutes.
Service and maintenance: Read this!
Traditional valves (prior to TPMS) are routinely replaced during any tire change. Stem rubber, the grommet area, and valve core seals age and become brittle or weakened. Besides, valves are cheap, so it just makes sense to install a fresh one when the opportunity arises.
However, when it comes to TPMS (clamp-on or snap-in styles), many techs may be afraid to tamper with them, so as a result they are often ignored. We need to recognize that in terms of sealing rubber components, nothing has changed — grommets, core seals and cap seals are still prone to gradual deterioration and should be replaced when needed. Service kits are available. For clamp-on styles, this involves replacing the valve core, valve grommet, valve nut and valve cap. TPMS valves require a nickel-plated core. The use of a non-nickel plated core can lead to electrolysis (galvanic corrosion), which can result in the need to replace the entire sensor assembly. The nut used on clamp-on sensors is treated with a bonded lubricant that aids assembly, allowing the proper torque value when tightening the nut (which seats the grommet to the wheel). The valve grommet is subjected to compression force as well as high wheel temperatures. The need for periodic replacement should be obvious. The valve cap provides protection from moisture and other road contaminants, as with any wheel valve.
If the sensor is the snap-in type, the rubber valve body can be un-clipped from the sensor and replaced easily (new rubber body, core and cap). When it comes time to mount another tire, make a point to replace these components.
As we all know, designed torque values are assigned to every threaded fastener, regardless of size, application or location. Well, tire valve and TPMS components are no exception. The valve core should be tightened to a value of 4 in.-lbs. Granted, most folks simply snug ‘em down until they “feel” good, but especially with a TPMS, you need to make certain that you’re following the correct procedures (if for no other reason than to protect yourself from a liability standpoint), so invest in a dedicated core tool that features a pre-set torque function. Stop laughing — this is important.
On clamp-on sensors that feature a Torx screw (securing the valve to the sensor), the torque value will generally be about 11.5 in.-lbs. (check with a service manual). On clamp-on style sensors, the valve nut tightening value will vary depending on the specific make/model/year application, so check with the service manual (values may range from about 35 to 80 in.-lbs.).
In any TPMS, the ECU needs to be able to recognize each sensor. When replacing a sensor or multiple sensors, we need to make sure that the vehicle system will be able to “read” the new sensor(s). If using the Sens-It programmable sensor, if during programming, you provide the old sensor’s number and the vehicle VIN, you don’t need to perform any further re-learning. If you only have the VIN but don’t have the original sensor number, then you’ll need to perform a re-learn.
Some vehicles (depends on make/model/year), the system will automatically re-learn. In others, you need to perform the OE-specified re-learn procedure. These procedures vary widely, so you’ll need to refer to the OE service manual or to an independent data source. These procedures can range from the simple to the absurd (beep the horn twice, press the brake pedal four times, flash the headlights once, run a lap around the vehicle, drop your shorts once, blink your eyes twice, etc.). You get the drift. Simply follow the OE procedure. Many of the current TPMS tools allow easy re-learning via plug-ins to the vehicle’s OBD diagnostic connector.
In order to service any direct TPM system, a testing tool is absolutely required. At the OE level, these calibration/resetting/diagnostic tools range from hefty hand-held units to large stationary pieces of equipment. For aftermarket use, easy-to-use hand-held testers are readily available. You don’t have a choice, by the way. If you plan to stay in the tire and wheel business, you must be able to reset and troubleshoot TPM systems. If you haven’t purchased a tester yet, you’re wasting valuable time.
Service cautions and tips
Exercise care during tire demounting to avoid sensor damage caused by a tire iron. In other words, don’t blindly dig into the wheel cavity. If the wheels have OE stem-mounted sensors, these units will be located very close to the rim, so again, be careful.
For tire demounting, some OE service manuals advise first deflating the tire, then removing the sensor’s retaining nut (clamp-on style) and dropping the sensor inside the tire. This keeps the sensor out of harm’s way during bead breaking and demounting. However, instead of allowing the sensor to drop and bang around inside the cavity, Myers Tire Supply offers a handy short-cabled T-handle tool that secures to the valve stem, allowing you to move the sensor well out of harm’s way while preventing it from a harsh drop.
If the sensor is visible (when replacing a sensor, when a sensor is already in place and a tire is about to be mounted, etc.), be sure to note the ID number on the sensor. This number will be required for input when initiating or resetting.
Remember to write the number on a piece of paper before mounting the tire.
Pay strict attention to torque specifications with regard to installing either a clamp-on or snap-in type TPM sensor. Over-tightening can damage the sensor and/or the valve core.
Some TPM systems will share the keyless entry’s receiver. This means that the remote key fob may be needed for specific procedures. Also, be careful to avoid “playing” with the key fob remote during any TPM sensor procedures. Don’t replace wheel sensor parts haphazardly. Some sensors feature aluminum caps and nickel-plated cores. The end of the valve stem may serve as the sensor’s antenna. Only replace cores or caps with the correct originals.
Use only a high quality pressure gauge when filling or checking tire pressure on any TPM system! The sensors found on direct TPM systems are very sensitive and precise. Using a dime-store gauge (or a mis-calibrated air gauge at a service station or car wash location) can lead to slight over or under-inflation which may be enough to cause a system warning light to activate. Quality pressure gauges have always been necessary, but the use of TPMS creates an even greater need for accurate readings. NOTE: Resetting a tire pressure monitor system is required if any of a number of procedures are performed, including tire pressure correction(s), tire/wheel rotation, tire/wheel replacement, TPMS sensor replacement, receiver antenna replacement, TPMS control unit replacement or loss of vehicle battery power.
Among the various sources of TPMS-related information, an excellent reference is the Tire Industry Association’s Domestic and Import TPMS Relearn Chart. This extensive (and large) booklet provides OE re-learn info/procedures, sensor manufacturers, sensor part numbers, service packs, sensor nut torque, Torx screw torque, worm gear clamp torque and wheel lug nut torque values, all listed per vehicle make/model/year. As a plus, the entire brochure features slick plastic surfaces, so your greasy fingers won’t smudge the pages.
This very informative brochure may be ordered by calling (800) 876-8372, ext. 106, or by visiting www.tireindustry.org (see the Training tab).
(all part numbers and prices per Myers Tire Supply)
Sens-It (distributed by Myers Tire Supply)
P/N Item Price
21191 Sens-It Programming pad $190.00
21192 Ball joint Sens-It sensor $56.50
21193 Snap-in Sens-It sensor $56.50
21194 Metal ball joint valve for Sens-It $4.50
21195 Snap-in valve for Sens-It $2.30
P/N Item Price
21620 EZ Sensor $79.00
21621 Optional aluminum valve for EZ-Sensor $6.50
27002A Bartec Tech 400 TPMS tool $1,899.95
27009 EZ-Sensor training kit for Bartec 400 $79.95
P/N Item Price
21630 Redi-Sensor 315 mz $67.60
21631 Redi-Sensor 433 mz $67.60
Universal aftermarket retro-fit TPMS
P/N Item List Sale price
21145 Dill four-tire TPMS system for trailers $322.50 $249.95
21168 Orange 4-tire TPMS for cars & trucks $286.66 $189.95
Aftermarket universal TPMS sensors
Dill Air Controls Products (Redi-Sensor)
1500 Williamsboro St.
Oxford, NC 27565
Myers Tire Supply (distributor of Sens-It)
1293 South Main St.
Akron, OH 44301
205 Frazier Rd.
Alta vista, VA 24517
TPMS diagnostic tool &
Technical information sources
44231 Phoenix Dr.
Sterling Heights, MI 48314
Mitchell 1 (info. in booklet form or software)
SPX/OTC (tool and info.)
655 Eisenhower Dr.
Owatonna, MN 55060
Spectrum Composites Inc.
1090 Doris Rd.
Auburn Hills, MI 48326
Makers of oem TPM
(brands most widely used at OEM level)
Schrader Electronics Ltd.
11 Technology Park
Belfast Road, Antrim
N. Ireland (UK) BT41 1QS
+44 (0) 28 9446 1300
Produces systems for GM, Infiniti, Jeep, Lincoln, Mercury, Mitsubishi, Nissan, Saab, Suzuki
D-80333 Munich, Germany
+49 89 636-00
Produces systems for Chrysler/Dodge, Jeep, Mazda
+49 7141 132-0
Produces systems for Mercedes, Porsche, BMW, Audi/VW, Land Rover
Pacific Industrial Co. Ltd.
Ogaki, Gifu, Japan
Produces systems for Lexus/Toyota
Kelsey Hayes Co.
49081 Wixom Tech Dr.
Wixom, MI 48393
Produces systems for Acura, Honda