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I have trouble understanding how the battery master switch activates the battery contactor in this schematic from the Cessna 152 manual. This is the model from 1979 or later with an Alternator Control Unit.

Cessna 152 wiring diagram Image source: Flight Literacy

I believe that the battery contactor is a relay that connects the top and right contacts (as in the image), when the left contact is powered.

Initially, all switches are off, the alternator is not turning, and there is no oil pressure:

  • The left contact of the battery contactor is connected to ground via the battery master switch in off position.
  • The battery contactor is not powered. It disconnects the battery from the connection through the Ammeter to the primary bus.
  • The battery voltage is distributed only to the clock and the oil pressure switch.
  • The two diodes close to the battery contactor are in reverse bias and therefore not conducting.

When I change the battery master switch to the on position, I expect that the battery contactor is closing the contact, and connects the battery via the Ammeter to the primary bus.

  • That requires power on the left contact of the battery contactor.
  • The battery master switch connects this contact (via "Alternator field circuit" breaker and "Pull off ALT" breaker) to the primary bus.
  • The primary bus is not powered yet.
  • In the way I read this schematic, the battery contactor cannot be activated. Nothing happens.

What am I missing?

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I think two of my premises were wrong.

  1. The battery contactor is activated by connecting the left contact to ground (instead of 24 V as I thought). It works different from the starter contactor (which has a permanent ground connection).
  2. The battery master switch toggles between ground [on] and floating [off] (instead of primary bus [on] and ground [off] as I thought).

Like this, the plane will be able to start!

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The left connection on the contactor is brought to ground on the right, BAT, side of the master switch. That allows current to flow between the center contactor terminal and the left contactor terminal.

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