MOTOR TRUCK SERVICE MANUAL
INSTRUMENTS
During engine operation oil enters the connecting
Before starting the engine with key switch off, the
tube and exerts a pressure on the inlet of the Bourdon
gauge will show the condition of the battery. The battery
tube. As the oil pressure increases, the Bourdon tube
section of the gauge is subdivided into three colored
mechanism within the oil gauge tends to straighten. The
segments:
indicator pointer, which is attached to the Bourdon tube
GREEN - a well-charged battery
by the linkage, then rotates across the face of the dial in
YELLOW - a low battery charge
the upscale direction. When the oil pressure decreases,
RED - a very low battery charge
the Bourdon tube relaxes and the pointer rotates in the
downscale direction. A steadily applied oil pressure
With the engine running at operating speeds the
holds the Bourdon tube and the pointer at a fixed scale
gauge will show the condition of the generating system.
reading corresponding to the applied pressure.
The generating section of the gauge is divided into two
colored segments:
1. If the gauge is reading high:
GREEN - generating system working properly
RED - voltage output too high
a. Disconnect the gauge from the system.
Constant reading in either RED area indicates that a
b. If pointer does not return to the lowest scale
complete check of the battery and generating system
reading (for instance, the pointer stays at 40 psi),
should be made.
the Bourdon tube has been damaged. Service
by replacing with new gauge,
Terminal identification is as follows, Fig. 21:
2. If the gauge is reading low:
1. The ground terminal is tin plated.
a. Examine the connecting tube for dirt restricting
2. The ignition terminal is the first terminal
the oil from entering the Bourdon tube.
counterclockwise from the ground terminal (looking
at back of gauge).
b. If connections and tubing are secure and free
from dirt, the gauge is damaged. Service by
3. The ignition terminal and the tie-in terminal are
copper colored and are joined by a calibrating
replacing with new gauge. NOTE: Be certain
resistor.
that engine oil pump is delivering correct
pressure before discarding gauge.
Air Pressure Gauge:
The mechanical air pressure gauge operates on the
Bourdon tube principle as does the mechanical oil
pressure gauge. The air pressure gauge, as is implied,
uses air as a pressure source rather than oil. For a
description of the operation, refer to the mechanical oil
pressure gauge.
For servicing procedure, refer to the mechanical oil
pressure gauge.
Fig. 21. Battery-Generating System Indicator Gauge
Battery- Generating System Indicator Gauge:
Connect a known voltage source (battery) to the
voltmeter terminals.
The battery-generating system indicator gauge
indicates the condition of the battery, alternator and the
The voltmeter should indicate the voltage supplied by
voltage regulator.
the source (battery) .5 volt.
The gauge is divided into two sections, one marked
If this test proves UNSATISFACTORY, service the
"BATT" (battery), the other marked "GEN" (generating
voltmeter by replacement.
system).
If test is SATISFACTORY, the problem is in the
wiring; CHECK CAREFULLY.
576
