I’m getting erratic indications on my engine cluster electrical instruments (fuel gauges and oil temperature gauges). The readings change when I turn things on and off, including the Pitot heat. What could cause this?
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There are at least four areas on these airframes where grounding/bonding leads are critical to proper instrument performance. One symptom, if you have a heated Pitot tube, is “needle jumping”, especially on the left fuel gauge, when the Pitot heat is turned on and off.
Here are the key bonding points:
Wing fuel sender unit to fuselage airframe (both sides).
Left wing to fuselage structure (if you have a heated Pitot tube).
Instrument cluster to instrument panel or sub-panel.
Instrument panel to fuselage structure.
Each of these points has been addressed by ages-old Beech Service Bulletins (two of them are SI0679-395 and SB2006). In my opinion each of these points should be bonded with at least a twelve-gauge aircraft-grade bonding wire. Note that the fuel sender bond needs to be from the sender itself, to the airframe; not just to or from the sender mount plate.
While the currents involved should not normally require twelve gauge wire, these airframes are all aging. Normal current paths are sometimes obstructed by high resistance. As a result, current can flow through unexpected paths. It is safer to use the larger wire, including in the case of a nearby lightning strike (to equalize or bleed off induced charges in parts of the airframe).
If your erratic indications are limited to your oil temperature reading, check all the connections between the gauge and the sender on the engine. The connection at the sender, in particular, has been found to fail on one or both pins, after so many years of vibration. The sender itself rarely goes bad (but they can). I would not replace the sender until all the other steps have been taken, and after the gauge has been shown to read correctly when a correct known resistance value has been substituted for the sender.
If the gauge readings are pretty consistent, but are consistently wrong, you most likely have both gauge and sender problems. Many of the electrical gauges are still the original parts, and have never been re-calibrated. And there aren’t very many auto and marine fuel senders that are 25-45 years old and still working.
Two key points. The fuel gauges are supposed to read Full any time there are 20 gallons or more in the tanks. That also means that they will read half-full at ten gallons. They are supposed to touch the red Empty line before the engine quits. The latter is the only calibration actually required by the FAA for certification.
For sender and gauge repairs, please do a BAC search on ‘fuel gauge’, and read the highlighted FAQ regarding repairs.