What can you tell me about when the low voltage and high voltage indicators should come on, for the later 28V Duchess aircraft with the combined engine instrument cluster?
Search strings: over-voltage, over voltage, under-voltage, under voltage, high voltage, low voltage, voltage indication, voltage sensor, voltage sensing.
Technical Editor:
The following information is from Beech Technical Support. The under-voltage light should come on when the voltage monitored at pin 11 of the engine instrument cluster decreases to 23.75 +/- .5 VDC (reference Shop Manual/Maintenance Manual Section 24-30-00, page 3). The over-voltage light should come on any time that bus voltage is applied to pin 12 of the instrument cluster. This voltage (at pin 12) should exist only when the over-voltage relay detects the bus voltage exceeding 32.0 +/- 0.3 VDC, and places a voltage on the wire going to pin 12 of the engine instrument cluster.
Editor’s Note:
At the same time that the OV relay places voltage on Pin 12, normally via the OV relay’s white wire, the relay will also either interrupt the regulator’s feed voltage, or trip the field circuit breaker (by internally grounding the regulator feed circuit). An over-voltage light will normally be accompanied by the alternator going offline, and a tripped 5A field breaker. While the 5A circuit breaker may be marked ‘Alternator Field” or ‘Alt Fld’, it is not actually in the field circuit between the regulator and alternator field. It is actually in the circuit that feeds voltage to the voltage regulator (via the OV relay); the regulator sends power to the alternator field winding, as required to maintain the correct alternator output voltage.