S-SERIES INSTRUMENTSAIR PRESSURE GAUGEOperationThe air pressure gauges are the mechanical type andoperate on the Bourdon tube principle.When air system is pressurized, air enters the airpressure gauge and exerts pressure on the Bourdontube. As pressure increases, the Bourdon tube tends tostraighten out and thus actuate the sector and piniongear (Figure 22) to which it is attached. This causesindicator to move across dial in an upscale direction.When pressure decreases, the Bourdon tube relaxesand pointer moves in a downscale direction. A steadilyapplied air pressure holds the Bourdon tube and pointerat a fixed scale reading corresponding to appliedpressure.Figure 22 - Air Pressure Gauge DetailsRemoval1.Detach instrument cluster and cluster bezel.2.Unscrew air lines from fittings on rear of the two airgauge inlets on back of instrument cluster.3.Remove gauge mounting screws and demountgauges from instrument cluster (Figure 23).4.Separate individual gauges from bracket.Figure 23 - Removing Air Pressure GaugesTesting1.If a suspected gauge has been reading high anddoes not return to zero when disconnected, Bourdontube has been damaged. Service by replacing with anew gauge.2.If gauge has been reading lowor does not read atall, examine the inlet connection for dirt which mightrestrict air from entering gauge. Make a trialconnection and if gauge still does not registercorrectly, replace gauge.NOTE - Always make a comparison test with a gaugeknown to be reading correctly before discarding aquestionable gauge.InstallationInstallation of air gauges is the reverse of removalprocedure. Be sure to use non-hardening sealing com-pound on fitting threads to prevent leaks. Checkinstallation so as to prevent kinks in air lines.CTS-4140Y - Page 13
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