Inflation

TIP: This is good information to share with owners.

Tire inflation is the single most important, and most often overlooked, part of tire care.
Operating a vehicle with just one tire underinflated by 20% (approximately 8 psi or 15 kPa) can reduce that tire's life by more than 9300 miles (15000 km) and can increase fuel consumption by 4%. With today's emphasis on fuel economy, this is a worthwhile tip.

Owners should check tire pressure at least once per month, when the tires are cold (driven less than 3 miles). They should be encouraged to observe the tire placard label, usually located on the driver door edge.
It displays the Maximum Vehicle Capacity Weight, the Original Equipment Tire Size and the cold inflation pressures for front, rear and spare tires.

TIP: The inflation pressure specification embossed on the sidewall of the tire is the MAXIMUM operating pressure, which is not necessarily the right inflation pressure for the vehicle.

TIP: Air pressure in a tire varies according to the outside temperature and the speed and distance the vehicle has been driven. Typically tire pressure increases about 1 psi (7 kPa) as temperature increases 10° F (6° C).

Tire Pressure Monitoring System

In accordance with Federal regulations, all 2008 GM vehicles sold in the US are equipped with a tire pressure monitoring system.
While some earlier TPM systems calculated tire pressure using wheel speed signals from the ABS system, the currrent TPM system uses a direct-measuring sensor in each wheel. Each sensor has a unique ID code that is transmitted along with pressure data.

A warning symbol is displayed when pressure in any tire drops to or below 75% of the recommended pressure. On many vehicles, the DIC displays actual tire pressure readings for added convenience.

TIP: Due to the temperature/pressure relationship explained earlier, if the TPM indicates low pressure on a cold tire, and turns off when the tire warms up, the pressure is marginal and needs to be inflated to the placard specification.

Bulletin 07-03-16-004A, revised in June 2008, provides an overview of TPM system operation, along with some informational Frequently Asked Questions.

Be sure your customers know how to interpret the tire pressure light. The light remains on solid to indicate a low tire pressure condition. The light blinks for about a minute and a Service Tire Monitor message displays when a malfunction occurs.

TIP: It is not necesary to perform sensor learning when the problem is resolved simply by correcting tire pressure.

TIP: TPM sensors are not compatible with commercially-available tire sealants. However, they have been tested with and are compatible with GM inflator kits and the sealants they use.

Tire Rotation

The importance of tire rotation is emphasized in the Owner Manual. Generally, the rule of thumb is to rotate tires every 5,000-8,000 miles (8000-13000 km), or whenever uneven wear is noticed.

On vehicles with a compact spare, a 4-wheel rotation scheme is required.

On front-wheel and all-wheel drive vehicles, move the front wheels straight back to the rear, and cross the rear wheels from side to side while moving them to the front.

On rear-wheel drive vehicles, move the rear wheels straight forward, and cross the front wheels from side to side while moving them to the rear.

TIP: Check tire pressures and correct as necessary after rotation.

TIP: After rotation, it is necesary to relearn the tire pressure wheel positions using the pressure-change method or the J-46079 tool. See SI for details.

IMPORTANT: Be sure to torque every lug nut to specification following the recommended sequence.

With Tire Pressure Monitor systems (TPM) being installed on nearly all new vehicles, here are some timely service and care tips. Follow the proper procedures when handling these sensors.

Damage

Analysis of returned sensors indicates physical damage due to improper installation, over- or under-torque of the valve core, and damage to the schrader stem from inflation air chucks or when using tire changers.

A TPM sensor may need to be replaced for multiple reasons: wheel replacement, faulty sensor, faulty schrader valve, or valve damage.

When removing a sensor from a wheel assembly, make certain that the threads of the stem are in good condition.

Inspect the stem for cracks or any indication of a bend, as this could cause a leaking valve.

When removing the valve stem core, use the proper core removal tool.

When installing a TPM sensor, it is critical to use the correct installation torque for the valve stem. A stem or core can be damaged both by over- or under-torquing. Calibrated torque wrenches are available for both the stem nut and the valve core.

The TPMS Service Tool Kit JSP-96254 (fig. 3) is available through the GM Tech Tools program. 1.800.GMTOOLS (1.800.468.6657) or www.gmtechtool.com. The valve stem torque wrench is included in the kit. The valve core torque wrench LIS-18810 is included in the kit and is also available separately.

Over-torquing the core, using a convenional core wrench, can crack the stem housing, which compromises the seal and creates a leaking stem.

The GM torque specification for valve cores is 4 in lb (0.45 Nm). This torque ensures that the core has seated properly in the stem and is properly sealed, without being over-torqued.

It is also important to avoid applying lateral pressure to the head of the valve stem with the air chuck while inflating tires. Excessive lateral pressure can cause the stem to crack or break off, or cause the core to become damaged and begin to leak.

Be extra careful because these stems are made of aluminum and are fastened solidly to the wheel. They are not as flexible as a regular rubber valve stem and cannot absorb the lateral pressure.

Excessive pressure can also be caused by a tire changing event were the changer mount head or tire lever strikes the valve stem.