Tech Tip: Understanding Tire Pressure Monitoring Systems – UnderhoodService

Tech Tip: Understanding Tire Pressure Monitoring Systems

Are a good number of the vehicles on the road today equipped with underinflated tires? Look down at the tires on the car in front of you in traffic, and the answer may be obvious. The National Highway Traffic Safety Administration conducted an inspection of 6,240 vehicles within a 14-day period in August of 2001. They did this at service stations with the cooperation from motorists who stopped to refuel.

By Glen Beanard

Are a good number of the vehicles on the road today equipped with underinflated tires? Look down at the tires on the car in front of you in traffic, and the answer may be obvious. The National Highway Traffic Safety Administration conducted an inspection of 6,240 vehicles within a 14-day period in August of 2001. They did this at service stations with the cooperation from motorists who stopped to refuel.

They found that out of those vehicles tested, 27 percent of the passenger cars had one or more tires substantially under inflated. They also found that 33 percent of light trucks and SUVs had one or more tires that were also substantially under inflated. They further found that more than 90 percent of those stations were equipped with air pumps, however, 10 percent of those didn’t work.

To make matters worse, less than half of the stations with a working air pump had a tire pressure gauge. The ones that did have a pressure gauge, the gauge proved unreliable. Almost 20 percent of those stations had a gauge that would over-report air pressure by 4 psi or more, and nearly 10 percent would over-report by 6 psi or more. So, even if the motorist tries to do the right thing and check their tire pressure, they are likely still driving on improperly inflated tires.

Under-inflated tires can cause more than just poor fuel economy, pulling to one side, and uneven tire wear, they can result in death. An under inflated tire will cause excessive flexure of the tire. Excessive flexure will often result in fatigue failure of the sidewall (a blowout). Over inflation is just as bad as it can make the tire more vulnerable to pot holes and curbs. A blow out at the wrong time can result in the worst possible ending. About 8,000 blowouts every year result serious injury or death, according to the NHTSA.

What Are They?
OK, we get it! Stop with the numbers already! I think we all can agree at this point that an improperly inflated tire is an unsafe tire. I think we can also agree that the motoring public needs a little help when it comes to making sure they have the right amount of air in their tires. That’s where these Tire Pressure Monitoring systems, or TPM systems, come into play.

Since the TPM systems are being pushed as mandatory starting with the 2006 models and are already available on many common vehicles, we need to understand these systems now.

There are two different types of these systems currently in use. One measures the pressure in the tire directly (direct TPMS). The other checks tire pressure indirectly (indirect TPMS) by testing the effects on the tire from the pressure change.

Indirect TPM Systems
Add one programming “tweak” to the ABS module, a reset switch, a warning light, and splash in a little wiring to tie things together, and you are left with one indirect TPM system. Different makes have their own little twists on this system, but the basic descriptions and operations of them are very similar to one another.

This type of TPM system has two modes of operation while driving: calibration and detection. After tires are rotated or replaced, the “reset” button must be pressed to clear the prior saved calibration from its memory.

By watching the wheel-speed sensors, the ABS module “knows” how fast each wheel is turning. When the vehicle reaches a predetermined speed (or speeds) for the first time after resetting the system, it enters its calibration mode. In this mode, it “learns” how fast the individual wheels are spinning.

It takes a sampling of those measurements and saves those values in memory. It cannot detect a low tire in calibration mode. Every time after that, where it reaches those same speeds, it enters its detection mode. In detection mode, it compares the current speeds to the values saved in calibration mode.

If the ABS module “sees” a tire’s rolling speed change with respect to the other tires, it illuminates the “Low Tire Pressure Warning Lamp.” The tire pressure drop that is typically required to turn on the light is 12 psi. The speeds that the different brands use for calibration and detection modes vary some, so be sure to check these specifications in your information system for make-specific referencing.

The indirect pressure system does not “know” the actual tire pressure. Since it is not measuring pressure, but rather wheel speed, certain problems can (and do) arise with it. It may set a false warning light or otherwise not function properly under any of the following conditions:
1. Uneven treadwear that is typical to a misaligned front end due to varying rolling rpm from the wheel assembly.
2. Low traction areas can cause wheel slip that temporarily effects rolling speed of the wheel.
3. Use of snow chains may affect it.
4. Module entering “calibration mode” on tires that are not properly inflated.
5. Some custom wheel/tire combinations may render the system ineffective.
6. Tires that are replaced or rotated without resetting the system will surely cause the customer to return immediately to your shop.
7. Some of these systems will not turn on the warning light if more than one tire is low.

Also, since the indirect TPM system does not directly test the air pressure in the tires, it can’t detect the spare tire’s condition until it is put into service. For some of these reasons listed here, the indirect method does not satisfy the NHTSA’s requirements and will likely no longer be on new model vehicles after 2006. Diagnostics of these types of systems should be no problem for any shop that is currently performing diagnostics and repair to anti-lock brake systems. Such a shop should have the scan equipment needed that supports anti-lock brakes systems, an oscilloscope (DSO), and an information system to print up wiring diagrams and flow charts from.

Direct TPM Systems
The direct TPM system is the only method that currently satisfies the NHTSA’s requirements. It works by use of a radio transmitter located inside the tire. The radio transmitter is actually the valve stem’s base. The valve stem is part of the transmitter and serves double duty as an anchor and as an antenna.

So keep that in mind if you ever get one of these systems displaying “Tire Pressure Unavailable,” and you look down to see big thick dice, “8” balls, or flashing lights for valve stem caps. Those things that people like to, ah, “improve” there vehicle with, stand a chance of blocking the signal from the transmitter to the receiver.

If the vehicle is equipped with a spare (don’t take it for granted that all cars have a spare, look at a Mini Cooper), the spare will have a transmitter as well. The value of this system is obvious to anyone who has had to install the spare, only to find it flat.

The first direct TPM sensors (like found on some older Corvettes) were a rather large sensor (approx. 3 inches long, 2 inches high, and an inch wide). They were strapped to the rim by a huge hose clamp that wrapped around the entire rim inside the wheel in order to anchor the sensor to the rim.

The typical modern direct TPM sensor has a 10-year lithium battery that is not replaceable. It weights just over one ounce. It’s accurate to 2 psi of its reading. They’ve been tested to over 200 mph. They carry price tags in the low hundreds each. And, they don’t like a tire-tool invading their space behind the valve stem, so be careful during tire replacements.

Also, make sure your tire technician is provided with the proper tire changing equipment. You can’t afford for him not to be.

The transmitters are not constantly on. Instead, they only send a signal to the receiver about every 30 to 60 seconds at speeds of 15+ to 20+ mph. Then, when sitting still, they transmit a pressure reading about once each hour.

On GM systems, the signal is sent to the keyless entry receiver, then to the BCM. On Ford and Chrysler systems, it is sent to a designated TPM module. The system is watching for a typical tire pressure range between a 25 psi to 28 psi low and a high of 39 psi to 48 psi.

You will need to consult the manufacturer’s specific ranges when diagnosing these systems because they do vary from make to make. Should the module determine pressure in any tire to be too low, too high, or fail to receive a signal at all from the individual sensor, it will display the appropriate message on the driver’s information center, or illuminate the appropriate light. This includes the signal from the spare tire. These systems are often “aware” when the spare tire is put in service and some will display the message “Spare tire swap detected.”

On most of these systems, if the low tire message is displayed, the warning will usually clear after re-inflating the tire to the specified inflation and driving the vehicle. However, if the tires are rotated, most of these systems require retraining the sensors. Since the direct TPM system “knows” the placement of each tire (LF,RF,RR,LR and spare), the technician must retrain the module each time the wheel assemblies are rotated, or if the tires are replaced and the rims are not put back in the original positions. If the “retaining” is not performed, the TPM system will place “blame” on the wrong wheel in the event of a fault. The training events are similar between makes, but still require consulting your information source for details.

They all require use of a retaining magnet: Kent Moore tool J-41760 per GM and special tool 8821 per Chrysler. Retraining the modules involves placing the special-shaped magnet over the valve stems, one at a time in the specified sequence with the modules in “Retrain Mode.”

Again, check the manufacturer’s specific information in your data source, this is not a “one-size-fits-all” procedure. In all cases, do not attempt to retrain more than one vehicle in the shop at a time. The signals will interfere with each other.

Neighboring devises that create an electromagnetic interference, such as radio towers, TV towers, electric motors, appliances, cell phones, remote transmitters, etc., may temporarily interfere with the retraining process as well. These types of devises may also cause a temporary “Tire Pressure Unavailable” message to appear on the message center. Keep that in mind should you get one with such a complaint that only happens when the customer drives across a military base, or something like that.

Diagnostics for this type of system may prove more involved than the indirect TPM system. Unlike the indirect TPM system, where technicians actually have about two decades of experience diagnosing the quirks, the direct TPM system will require the technician deal with a wireless communication system. Again, a scanner with appropriate software may be required.

There are two tips I can give you to help identify vehicles with these systems on them. First, if it is factory equipped with Extended Mobility Tires (EMT, commonly referred to as “runflats”), then it will have a pressure monitoring system (such as Mini Cooper and some BMWs). The second tip is simply to look at the valve stem. If it is a metal stem, locked in place by a nut (like some high pressure valve stems), then you should play it safe when dismounting these tires and treat them with caution, as you are likely to find those to be pressure sensors inside.

Always consult your repair information system to get the specifications and the description and operation information for the specific vehicle you are servicing. For sake of space, and available information, I naturally couldn’t go into vehicle specific details for every single-vehicle on the road.

Be sure to inflate tires to the pressures specified on the vehicle’s data plate, not the pressure on the side of the tire. The number found on the side of the tire is often a maximum pressure. Since the same tire may be placed on several different vehicles, with several different weights, center of gravity, and uses, any tire pressure rating molded into the sidewall would be generic at best. However, the vehicle’s data plate has specifications for that particular vehicle including even the specific position on the vehicle (front vs. rear). Also, you should ask yourself: “Is it time to purchase a new tire pressure gauge?”

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