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Servo Valve Diagnosis:
During the manual sweep test, not only should the valve sweep evenly, but it is sometimes helpful to put
your hand on the servo and feel the little ticks as it rotates.
The ticks should be even without any stutter. If the stutter happens at even increments, like every 4th one,
then it is likely that it is a motor problem, or every 16th one, then it is likely an electronics problem. If the
stutter is random, it's probably a pot signal issue. Also, we’ve included a table of the resistance values that
should apply to a good servo motor assembly. It is a quick way to see if there is truly a physical problem
with the servo. The most common problems are high resistance where there should be open circuits in the
motor, which can indicate water in the motor. Any open circuits in pins 1-6 that have a value in the table
(other than ∞) are a sign of a burned out coil or damaged cable.
The following resistance checks can be performed on the 902S servo valves using the cable connector
and its pins. If your valves use an extension cable, then the values below will be correct ONLY if you
are measuring with the extension cable connected to the valve. Basically, you want to perform these
measurements at the cable end that plugs into the port on the outside of your sensor box.
Servo Valve Connector Resistance Check in ohms (Ω)
Description
Pin #
1
2
3
4
5
6
7
9
10 11
Coil C
1
∞
∞
∞
1.5
∞
2.8
∞
∞
∞
Coil B
2
2.8
∞
∞
1.5
∞
∞
∞
∞
Coil A
3
∞
∞
1.5
∞
∞
∞
∞
Shield
4
∞
∞
∞
∞
∞
∞
Center Coil A-B
5
∞
1.5
∞
∞
∞
Center Coil C-D
6
∞
∞
∞
∞
Coil D
7
∞
∞
∞
Potentiometer Signal (wiper)
9
X
Y
Potenti (servo CCW)
10
Z
Potentiometer – (Servo CW)
11
Notes:
1. All measured values should be within 10% of values indicated in table.
2.
∞ indicates infinite resistance, e.g. open circuit.
3. Pin 8 is not used
4.
X should be ~3400 Ω when servo valve is fully open (and properly indexed), at 0% on controller
5.
Y should be ~2300 Ω when servo valve is fully open (and properly indexed), at 0% on controller
6.
Z should total ~5600 Ω, no matter where the valve is positioned
7. Table connections taken from 1200A-0814-3A-120 schematic
7.0 Troubleshooting
