TROUBLESHOOTING THE CESSNA CAPACITANCE FUEL QUANTITY INDICATING SYSTEM

The quantity gauge can be very useful in determining and isolating problems. Certain problems produce definite indications. For instance:
  1. A short between Hi-Z and Lo-Z will cause the gauge needle to register off scale "low" regardless of fuel level in the tanks or signal conditioner adjustments.
  2. A short between Lo-Z and ground or Lo-Z and shield will cause the gauge needle to register a constant zero regardless of fuel level or adjustments.
  3. Moisture in the probes or more importantly inside a connector plug will cause the gauge needle to register off scale "high" regardless of fuel level or signal conditioner adjustments. This same condition will occur if Lo-Z is shorted to Lo-Z 2.
  4. An open Hi-Z or Lo-Z will cause the gauge to read less than actual fuel quantity, but the gauge will respond to changes in fuel quantity and signal conditioner adjustments.
  5. Grounding of Hi-Z will cause an approximate 1/3 to 1/2 gauge indication, again, unaffected by changes in fuel level or signal conditioner adjustments.
  6. Poor shielding, loose terminals, corroded connectors or intermittent shorts will cause the gauge needle to fluctuate.
To isolate the problem, start at a point as far from the signal conditioner as possible and work inboard toward the signal conditioner. Disconnect the probes one at a time noting whether or not a change occurs on the gauge... if it does, you have found the defective part of the system.

CMF776 CONTROL MONITORS (TROUBLESHOOTING)

If one channel works and one channel doesn't the quickest way to figure out if it is the control monitor or something else that is defective in the aircraft is to switch the two HIZ connectors around.
If the problem moves to the other side it means the control monitor is defective.
If the problem stays on the same side it means the problem is elsewhere on the aircraft. It can be the indicator, a tank unit, or aircraft wiring.


BEECH -47 FUEL QUANTITY INDICATORS

We have noticed an increase in warranty rejections on the Beech -47 type of fuel quantity indicators. I believe that most of the problems could very well be caused by shipper's rough handling (this is a very delicate instrument). To help fix this problem we have started double boxing all variations of this type of fuel quantity indicator.
Also if pointer sticks, before swapping sides, clean glass thoroughly. Static build-up can cause pointer to stick or hang up.

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