Instruments
Standard instruments are present on every vehicle.
Optional instruments, typically located on the auxil-
iary dash panel or right-hand control panel, are not
found on every vehicle. Instruments are listed here in
alphabetical order to make the information easier to
find.
Air Intake Restriction Gauge
The air intake restriction gauge indicates the vacuum
on the engine side of the air cleaner. On standard
installations, it is mounted on the air cleaner. As an
option for easier viewing, an air intake restriction indi-
cator (see
Fig. 4.6) can be mounted on the auxiliary
dash panel.
NOTE: Rain or snow can wet the filter and
cause a temporary high reading.
Air intake restriction vacuum is measured in inches
of water (inH
2
O). For vehicles equipped with a
graduated indicator or a restriction gauge on the
dash, check the gauge with the engine off. If the yel-
low signal stays locked in the red zone once the en-
gine is shut down, or is at or above the values
shown in
Table 4.2, the air cleaner element needs to
be replaced.
Vehicles may be equipped with a go/no-go restriction
indicator without graduations (see
Fig. 4.7) instead of
a graduated indicator.
Air Intake Maximum Restriction Values (inH
2
O)
Engine Make
Pre-EPA07 (ADR
80/02) Engines
EPA07 (ADR 80/
03) Engines
Cummins 25 25
Detroit 20 22
Mercedes-Benz 22 22
Table 4.2, Air Intake Maximum Restriction Values
If air restriction exceeds the maximum allowable
value, operate the vehicle for one more day, making
sure not to run the engine over rated rpm. Refer to
the engine operation manual for more information on
rated rpm for your engine.
If air restriction exceeds the maximum value again,
replace the air cleaner. For instructions, refer to
Group 09 of the Century Class Trucks Workshop
Manual.
Application Air Pressure Gauge
An application air pressure gauge registers the air
pressure being used to apply the brakes, and should
be used for reference only. The gauge will not regis-
ter air pressure until the foot brake pedal is de-
pressed or the trailer hand brake is applied.
10/10/2001
f610568
Fig. 4.6, Air Intake Restriction Indicator
04/08/2005 f090431
Fig. 4.7, Manual-Reset Air Restriction Indicator, Go/
No-Go
Instruments
4.7
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