Piper Navajo Chieftain PA-31-350 Service Manual page 926

NAVAJO CHIEFTAIN SERVICE MANUAL
(f) Install the fabricated patch cable and check the volt ohmmeter readings at the patch
cable junction box with the pulse generator running or the simulator installed.
(g) If this test is positive, a bad aircraft harness lead may be present. Check the harness
with a megger ohmmeter.
(h) Check the resistance and continuity through the pulse generator harness. Pins D and E
of the small plug attached to the computer should be approximately 200 ohms.
(i) Check the direction of the pulse generator by operating it from both ends and observe
if lights in the strobe sensor illuminate. (Use the volt ohmmeter to check light
illumination. A decrease in resistance should be read when the lights glow. Connect
meter at Pins A to C or B to C of the large plug). (The strobe sensor can also be
disassembled for a direct visual check).
(j)
To ascertain that the pulse generator is supplying timing information to strobe sensor,
disconnect plug PL2 from the computer chassis and measure the AC voltage across
contacts E and D while operating the master engine at near cruise RPM. A reading of .2
volts AC ± . volts is considered normal. If no voltage is supplied, replace the pulse
generator.
2. Computer amplifier:
(a) If the system operates in the manual mode but not in the phase mode, listen to the computer
for an audible relay click when the system is switched from the manual mode to phase
mode. The relay is on the bottom deck of the computer assembly. The use of a 2 amp meter
and test plug will simplify this check. (Refer to Figure 14-51.)
(b) If no AC voltage is supplied to the strobe sensor as outlined in step I d, it is suspected that
the lamp amplifier in the computer chassis is defective and the computer should be
replaced. (Refer to paragraphs 14-143 and 14-144.)
(c) Inspect the inside of the computer to ascertain that the circuit boards are secure and
that the tang plugs are free of corrosion. Also check for any loose wires.
3. Strobe sensor.
(a) Connect the fabricated strobe sensor tester (Refer to Figure 14-53) to the strobe sensor
plug in the right nacelle.
(b) Slowly rotate the strobe sensor core (if the tach shaft is disconnected) or propeller and
watch for a drop in the photo conductor resistance. Check both A and B lights. The
resistance should be 200,000 ohms with the lights covered and 180 ohms with the
lights uncovered.
(c) If the latter test (step b) proves negative, the sensor unit must be disassembled for a
visual check of lamp illumination. If the lamps do work the trouble is in the photo
conductors. To be sure, aim the photo conductor cells at a source of light and move a
finger or some object back and forth in front of the conductors. If the tester does not
indicate a drop in resistance when the conductor cells are exposed to the light, they are
inoperative and must be replaced.
(d) Connect the strobe sensor simulator. (Refer to Figure 14-54.)' Move the potentiometer
and listen to the computer. The motor should run at approximately 7 volts.
(e) Adjust the potentiometer just short of motor operation. Move the pressure-drain
switch from ON - pressure to OFF and then ON again; the current on the governor coil
should be .05 amps.
(f) Move the switch from ON - drain to OFF and then ON again, the current on the
governor coil should be .05 amps.
CAUTION
The pressure or drain should not be adjusted unless it is absolutely
necessary and the system still does not synchronize.
Reissued: 9/23/80
4110
ACCESSORIES AND UTILITIES