Driver Controls and Instruments Fuel Gauge CAUTION The fuel gauge indicates the amount of fuel in the fuel tank(s). Gel cell batteries can be damaged if the battery voltage is allowed to drop below 12.0 volts or if the charging voltage is more than 14.1 volts. Start the engine to recharge the gel cell before the battery becomes fully discharged. If an external charger is needed, disconnect the gel cell battery and use only an external battery charger that has been approved for gel cell batteries. Intake-Air Restriction Indicator An intake-air restriction indicator measures the vacuum on the engine side of the air cleaner at the air cleaner outlet. See Fig. 3.30 .
If the yellow signal stays locked at or above the values shown in Table 3.4 after the engine is shut down, service the air cleaner, then reset the indi- cator by pressing the reset button. Primary and Secondary Air Pressure Gauges These air pressure gauges register the constant pres- sure in the primary and secondary air systems. Normal pressure, with the engine running, is 95 to 125 psi (655 to 862 kPa) in both systems. A low-air-pressure warning light and buzzer connected to both the primary and sec- ondary systems, activate when air pressure in either system drops below a minimum pressure of 62 to 68 psi (427 to 469 kPa).
When the engine is started, the warning light and buzzer remain on until air pressure in both systems exceeds minimum pressure. An air pres- sure gauge that registers pressure in both the primary and secondary systems may be installed on some vehicles. Intake-Air Restriction Engine Model * Service at or Above Mercedes-Benz MBE900 22 inH 2 O Caterpillar 25 inH 2 O Cummins 25 inH 2 O Detroit Diesel 25 inH 2 O * Turbocharged engines must be checked at full load and governed engine speed. Table 3.4, Intake-Air Restriction NOTE: Avoid opening the air cleaner and disturb- ing the seals or filter element until the yellow sig- nal stays locked at or above the value shown in Table 3.4 .
When this occurs, the air cleaner needs 3.43