Tires - Inflation Pressure Adjustment
PRODUCT GENERAL INFORMATIONPG005-02
Title:
ADJUSTMENT OF TIRE INFLATION pressure
Models:
'02 SC 430
April 4, 2002
Introduction
The purpose of this bulletin is to provide additional details regarding the tire pressure warning system on the SC 430. In addition, this bulletin provides guidelines for temperature compensation of tire pressure - taking into account changes in ambient temperature and actual tire temperature. This bulletin supplements TSIB PG018-01 and the Repair Manual.
Run-flat tires are designed with much stiffer construction, including the sidewall. This may cause the customer to believe that the tire does not lose air pressure, but it is still a normal condition for the tires on the SC 430 to lose up to 10 Kpa (1.5 PSI) per month. Seasonal temperature change can dramatically alter tire pressure. In addition, popular pencil type tire pressure gauges and service station air pumps have been proven on many occasions to be inaccurate and misleading.
Low Tire Pressure Detection Timing:
The tire pressure at each wheel is monitored all the time even if the ignition is OFF. If the tire pressure falls below 26 PSI while the ignition is off, the warning light will illuminate the next time the ignition is turned ON.
Tire Pressure vs. Tire Temperature:
Air in a tire expands and contracts as the temperature of the tire changes, changing the tire's air pressure. Tire temperature is dependent on "cold" tire pressure, driving distance and speed, ambient temperature and even road surface temperature. The "cold" tire pressure for the SC 430 is 230 Kpa (33.4 PSI), shown as 33 PSI in the Owner's Manual. When adjusting tire pressures for your customer, you have to consider all of these factors, plus the difference between the temperature in the workshop and the lowest outside temperature you expect to have in your area in the next few weeks.
"Cold" tire pressure, as recommended by the tire manufacturer (and shown on the tire pressure label in our vehicles), is generally considered to be the pressure in a tire that has not been driven on in the previous 4 hours, and has been parked outdoors.
Applicable Vehicles
^ 2002 model year SC 430 vehicles.
Warranty Information
Tire Pressure Adjustment
Recommended Tire Pressure Adjustment:
As noted above, there are many variables that can affect tire pressure in operation. However, you can use these guidelines to optimize the pressure when you set it in the workshop:
^ Use a high quality accurate tire pressure gauge, and check its accuracy regularly.
^ Consider the difference in the air temperature in the workshop and the lowest ambient temperature you expect in the next few weeks (especially in winter).
^ Use the chart (Figure 1) to compensate for temperature of the tires when you adjust the pressure.
Temperature Compensation Example 1
Assumed conditions:
^ The SC 430 has been parked overnight outside your shop (vehicle has "cold" tires).
^ Your workshop temperature is 20°C (68°F), and you expect the lowest ambient temperature in your area to be -10°C (140 F).
Calculations:
^ Compute the temperature difference between the seasonable lowest temperature and the workshop temperature: 20°C (68°F) - [-10°C (14°F)] = 30°C (54°F).
^ Using the tire pressure chart (Figure 1). find the intersection of the cold tire line at the point corresponding to 30°C (540 F), and read off the value on the tire pressure change axis. In this case it would be about 4.6 PSI.
^ Compute the actual tire pressure the tires should be filled to: 33.4 + 4.6 PSI = 38.0 PSI.
In this example the tires would need TO be filled to 38 PSI to compensate for the seasonal temperature difference.
Temperature Compensation Example 2
Assumed conditions:
^ The SC 430 has been driven to your shop on surface streets for about 30 minutes (vehicle has "warm" tires).
^ Your workshop temperature is 20°C (68° F), and you expect the lowest ambient temperature in your area to be -10°C (14° F).
Calculations:
^ Compute the temperature difference between the seasonable lowest temperature and the workshop temperature: 20°C (68°F) - [-10°C (14°F)] = 30°C (54° F).
^ Using the tire pressure chart (Figure 1), find the intersection of the warm tire line at the point corresponding to 30°C (54°F), and read off the value on the tire pressure change axis. In this case it would be about 6.7 PSI.
^ Compute the actual tire pressure the tires should be filled to: 33.4 + 6.7 PSI = 40.1 PSI.
In this example the tires would need to be filled to 40 PSI to compensate for the seasonal temperature difference and the heat in the tires from the short drive to your dealer.
Temperature Compensation Example 3
Assumed conditions:
^ The SC 430 has been driven to your shop on the highway for at least 60 minutes (vehicle has "hot" tires).
^ Your workshop temperature is 20°C (68° F), and you expect the lowest ambient temperature in your area to be -10°C (14°F).
Calculations:
^ Compute the temperature difference between the seasonable lowest temperature and the workshop temperature: 20°C (68°F) - [-10°C (14°F)] = 30°C (54° F).
^ Using the tire pressure chart (Figure 1), find the intersection of the hot tire line at the point corresponding to 30°C (54° F), and read off the value on the tire pressure change axis. In this case it would be about 9.0 PSI.
^ Compute the actual tire pressure the tires should be filled to: 33.4 + 9.0 PSI = 42.4 PSI.
In this example the tires would need to be filled to 43 PSI to compensate for the seasonal temperature difference and the heat in the tires from the long, high-speed drive to your dealer.
From these examples you can see that while the "cold" tire pressure specified for the tires is 33 PSI in the owners manual, you may need to inflate the tires to 43 PSI, or more, depending on seasonal weather conditions and initial tire temperature.