When the National Highway Traffic Safety Administration (NHTSA) mandated the use of tire pressure monitoring systems (TPMS) on all vehicles under 10,000 pounds gross vehicle weight rating (GVWR), I knew that passenger and light truck tire service would change forever.
From the numerous TPMS relearn procedures to the hundreds of different sensors and seal kits that have flooded the market, retail tire dealers are still trying to catch up and adapt to the changes that have accompanied this type of technology.
For vehicles over 10,000 pounds GVWR, there are no federally mandated TPMS requirements, and it doesn’t appear that anything is on the immediate horizon.
While the trucking industry is well aware of the importance of proper inflation pressure, there are a number of factors that are working against the widespread implementation of TPMS.
First, in order for traditional TPMS to work in tractor/trailer applications — notifying the driver when a tire is below a certain threshold — there must be universal protocols so that every make and model of tractor can communicate with every make and model of trailer.
Second, tire and wheel positions in large fleets number in the millions, so the economic impact of mandating a $50 sensor in each assembly could put some of them out of business, especially if implementation would be retroactive to existing equipment.
Finally, truck drivers are already required by federal law to check the inflation pressure in every tire as part of their daily vehicle inspection.
As it stands today, TPMS is widely available in the trucking industry, but not widely used.
The Technology and Maintenance Council of the American Trucking Associations has developed engineering guidelines to list the operational parameters of TPMS in an effort to help alleviate some obstacles to implementation and ensure universal acceptability among manufacturers.
[PAGEBREAK]
However, at the original equipment level, availability is still quite limited. But there are several aftermarket TPMS systems that can be installed on every wheel position. All require a secondary display on the truck’s dashboard, along with additional wiring.
These systems use a traditional sensor on the end of the valve stem inside the tire or a special valve cap that can read and transmit inflation pressures to the cab. So, the technology is in place, but lack of demand for traditional TPMS has resulted in little or no market penetration.
A look at ATIS
That said, there is one type of TPMS in the trucking industry that continues to grow in popularity without any regulations or encouragement from the government. In fact, the concept is so simple and economical that more fleets are making it standard equipment on new trailers.
Automatic tire inflation systems (ATIS) use air pressure from the braking system to supply air to the tires, mainly on trailer tires, which are traditionally the most poorly maintained tires in any fleet.
A special control box also can detect a flat tire and warn the driver via an external light mounted where the driver can see it. There are still controls in place to ensure that the air pressure on the trailer tires does not go below the point that the brakes can be operated. The concept is so simple, it’s really a no-brainer.
There are two types of ATIS on the market. The pioneer in this segment is Pressure Systems International (PSI), which has developed a system where the hollow axle of the trailer is converted into a pressure reserve for tires.
The other type of ATIS does not pressurize the axle. It transfers air to the wheel ends via a series of hoses that connect to the air tank. This system goes by the name Tiremaxx.
Here’s a detailed look at each system:
1. PSI systems can be identified by a through tee in the middle of the hubcap. The existing trailer air supply flows to a control box and then to each axle. A special press plug is installed on both ends of the axle to form an airtight seal, which creates the air reserve, and a metal tube called a stator screws into a hole in the middle of the press plug. The stator has a recommended torque of 25 ft.-lbs. to create an airtight seal. The through tee screws into the stator at a recommended torque of 45 ft.-lbs. and is aligned so the ends are aligned with the inner and outer valve stems.
[PAGEBREAK]
Once the through tee is in place, hose extensions that connect to the valve stems can be put in place. The important thing to remember is that the valve core in the end of the extension hose that attaches to the through tee is a special valve core with a crack pressure of just 3 psi.
If these valve cores are replaced with standard valve cores, the PSI system will not work properly. The best practice in this case is to never deflate a tire by removing the valve core on the end of the extension hose that connects to the through tee. However, it is perfectly acceptable to check trailer tire inflation pressures by disconnecting the extension hoses at the through tee and then placing the air gauge on the end of the hose.
After checking the inflation pressure, the extension hoses should be hand-tightened on the through tee. When connecting the extension hoses to the valve stems, they should be tightened an additional half turn with a 7/16-inch wrench after hand-tightening.
The PSI ATIS has another feature called ThermAlert, a special thermal screw in the axle plug that melts once it reaches 280 degrees Fahrenheit. After the screw melts, the air will rush through the hole, which will give the driver and any service personnel in the vicinity an audible warning that something is wrong. If a technician approaches the trailer axle with a PSI ATIS and hears something that sounds like a major air leak at the wheel end, the vehicle should be serviced with extreme caution. If smoke is coming from the wheel end, the best practice is to call emergency services; a wheel end fire cannot be put out with fire extinguishers.
2. Tiremaxx is similar to PSI ATIS in the sense that the axle tube provides space for the air from the trailer to reach the wheel end. However, Tiremaxx uses a series of hoses to transfer the air to the tires. Four hoses are linked to the control unit and then fed through the axles to each of the wheel ends, where they are attached to rotary unions. Each rotary joint includes a fitting where the tee fitting attaches to the hose extensions that connect to the valve stems. Like the PSI ATIS, the valve cores on the ends of the extensions where they attach to the tee fitting have special valve cores, so they should not be replaced with standard valve cores.
When tightening the tee fitting, technicians should use a torque value of 120 to 140 ft.-lbs. with two wrenches so one wrench can hold the jam nut on the rotary joint adapter stationary while the torque wrench is being used to tighten the tee fitting.
Besides the hoses that transfer air directly to the wheel end instead of pressurizing the axle, the most significant difference is the location of the tee fitting, which is on top of the hub cap instead of in the center.
A special guard protects the fitting from being damaged by a wheel during the removal or installation process, but technicians should still exercise caution. The guard should not be used as a step for the wheels or anything else. Finally, it’s a good idea to check all of the connections with a leak detector mist.
Servicing the ATIS on most trailers doesn’t require a lot of training or special equipment. The most important things to remember are taking care of the valve cores in the extension hoses where they connect to the hub and making sure the tee connections are airtight before returning the vehicle to service.
CTD contributor Kevin Rohlwing is the senior vice president of education and technical services for the Tire Industry Association.
