Section 6.6: Tires, Wheels & Tire Pressure Monitoring Systems (TPMS)

Tires, Wheels & Tire Pressure Monitoring Systems (TPMS)

Tires and wheels are the sole contact points between the vehicle and the road. Modern automotive technicians must master tire construction, wheel dynamics, and the diagnostic service of Tire Pressure Monitoring Systems (TPMS) to ensure safety and performance.

Tire Construction and Sizing

Automotive tires are complex assemblies of rubber, fabric cords, and steel belts.

• Radial Construction: Virtually all modern passenger vehicles use radial-ply tires. The casing plies run at 90 degrees to the centerline of the tread, and steel belts are placed under the tread area. This design allows the sidewall to flex independently of the tread, reducing rolling resistance, improving fuel economy, and maximizing the tread contact patch.
• Tire Sizing Designations: A tire marked P225/55R17 97V is decoded as:
  • P: Passenger car tire (LT indicates Light Truck).
  • 225: Section width in millimeters (from sidewall to sidewall).
  • 55: Aspect ratio. The height of the tire sidewall is 55% of its section width.
  • R: Radial construction.
  • 17: Wheel rim diameter in inches.
  • 97: Load index (indicates maximum carrying capacity, in this case, 730 kg or 1609 lbs).
  • V: Speed rating (indicates the maximum safe operating speed, in this case, 240 km/h or 149 mph).
• Uniform Tire Quality Grading (UTQG): A federally mandated grading system marked on the sidewall:
  • Treadwear: A comparative rating based on wear under controlled conditions. A tire graded 400 lasts twice as long as one graded 200.
  • Traction: Graded AA, A, B, or C, representing the tire's ability to stop on wet concrete and asphalt.
  • Temperature: Graded A, B, or C, representing the tire's resistance to heat generation and dissipation.

Tire Inflation Dynamics

Proper inflation is critical to maintain the designed contact patch (the area of tread in contact with the road).

• Under-inflation: Causes the tire's center to collapse inward, shifting the vehicle weight to the outer shoulders. This results in rapid wear on both shoulder edges, excessive sidewall flexing, heat buildup, and increased risk of a blowout.
• Over-inflation: Causes the tire tread to balloon outward, forcing the vehicle weight onto the center of the tread. This results in rapid center tread wear, a harsh ride, and reduced traction.

Wheel Assembly, Balancing, and Runout

Vibrations can originate from wheel imbalance or dimensional irregularities (runout).

Runout Diagnostics

Runout is measured using a dial indicator mounted on a magnetic base.

• Radial Runout: Out-of-roundness of the tire or wheel. Measure radial runout on the center of the tire tread and the inside/outside wheel bead seats. Maximum allowable radial runout is typically 0.76 mm (0.030 in) for steel wheels and 0.50 mm (0.020 in) for aluminum wheels.
• Axial Runout: Side-to-side wobble of the wheel or tire. Measure axial runout on the wheel rim flange. Maximum allowable axial runout is typically 0.76 mm (0.030 in).
• Correction: If wheel runout is excessive, replace the wheel. If tire runout is excessive, perform match mounting (rotating the tire on the rim to align the high spot of the tire's runout with the low spot of the wheel's runout).

Wheel Balancing

• Static Balance: Equal distribution of weight around the wheel's circumference. A static imbalance causes a heavy spot that forces the wheel to hop up and down, creating a vertical vibration ('wheel tramp') felt at highway speeds.
• Dynamic Balance: Equal distribution of weight on both sides of the wheel's centerline. A dynamic imbalance causes a side-to-side wobble ('shimmy') that transmits vibrations to the steering wheel. Dynamic balancing requires placing wheel weights on both the inner and outer planes of the wheel rim.
• Road Force Variation (RFV): A variation in tire stiffness around its circumference. Standard balancers cannot detect RFV. A road force balancer applies a roller (typically simulating 635 kg or 1400 lbs of load) against the spinning tire to measure force variations. Match mounting is used to minimize RFV.

Tire Pressure Monitoring Systems (TPMS)

TPMS is a safety system designed to alert the driver when tire pressure drops by 25% or more below the placard inflation pressure.

Direct TPMS

Direct systems utilize battery-powered pressure sensors mounted inside each wheel assembly, typically integrated with the valve stem.

• Operation: Sensors measure pressure and temperature, then transmit radio frequency (RF) signals (usually 315 MHz or 433 MHz) to a receiver module.
• Maintenance: The sensor stem is made of aluminum or rubber. Aluminum stems are prone to galvanic corrosion if brass valve cores are installed. Technicians must only use nickel-plated valve cores and aluminum valve caps.
• Torque Specifications: The sensor retaining nut must be torqued to specification, typically 4 Nm (35 in-lbs). Over-torquing cracks the sensor body or aluminum stem; under-torquing causes a slow air leak.
• Service Kits: Whenever a tire is dismounted, the sensor's rubber grommet, sealing washer, retaining nut, and valve core must be replaced to prevent leaks.

Indirect TPMS

Indirect systems do not use wheel-mounted sensors. Instead, the anti-lock braking system (ABS) module monitors wheel speed sensors.

• Operation: When a tire loses air pressure, its diameter decreases, which decreases its rolling radius. Consequently, the under-inflated tire must rotate faster than the other tires to cover the same distance. The ABS module detects this speed difference and illuminates the low tire warning light.

TPMS Relearn Procedures

A relearn procedure writes the sensor IDs and positions into the TPMS module.

• Manual Relearn: The vehicle is placed in relearn mode (e.g., cycling the ignition switch and pressing the brake pedal), and the technician lets air out of each tire in sequence until the horn chirps, signaling the module has received the signal.
• Trigger Tool Relearn: A TPMS tool is used to transmit a low-frequency (LF) activation signal to each sensor in sequence (Left Front, Right Front, Right Rear, Left Rear). The sensor responds with its RF ID code, which the module learns.
• OBD-II Relearn: The technician triggers the sensors with a tool, reads the IDs, and writes them directly to the vehicle's computer via the OBD-II diagnostic link connector.