Lesson 2/Learning Event 2
If the repairs are beyond the unit level of maintenance, notify your
maintenance sergeant so that he can arrange for the truck to be
repaired by the support unit.
To isolate the fault in the hydraulic system, the repairer from the support
unit will probably use a pressure test gage set that is designed for this
task. In case you get to assist on the job, here is how it is done.
The highpressure line between the pump and control valve is disconnected at
the control valve. A tee fitting is installed on the control valve where
the line was removed. Then, the highpressure line is connected to the tee
fitting. At the center of the tee, connect the pressure gage using a short
hose provided in the test kit. The hydraulic system can now be operated,
and the pressures developed will be shown on the pressure gage.
Before making any pressure tests, fill the reservoir to the proper level
with OE 10 and operate the engine until the hydraulic oil is at normal
operating temperature. If a rapid pressure buildup occurs when the engine
is started, turn the engine off quickly. Look for plugged lines, fittings,
and oil passages before going any farther. A plugged or partially plugged
passage can cause pressure buildup that will burst lines and castings.
After the unit has reached normal operating temperature, turn the steering
wheel for a right turn with the engine running at 1,000 RPM. Continue
turning the steering wheel until the right turn stop is reached; then, hold
it tight against the stop for just a few seconds. The gage should read 750
PSI for trucks with gasoline engines and between 850 and 1,000 PSI for
multifuel and diesel engines.
To repeat this test in the left turn position, it may be necessary to have
an assistant hold a piece of iron, 1/4inch thick, before the front axle
left turn stop. If the hydraulic power piston is allowed to travel to the
end of the cylinder on a left turn, it uncovers a port that allows the
pressure to escape. The 1/4inch piece of iron prevents the piston from
traveling far enough to uncover the port.
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