Electrical – car Ignition switch relay maintain closed after remove power

relay

I have a car ignition switch that has 4 settings and is +12v DC:

  • 0=off (no voltage)
  • 1=accessory (only some items have power) (no voltage will go to pump)
  • 2=on (all items have power)
  • 3=start (momentary switch for start power)

I have a pump I want to wire up in the following way:

  • When the switch is on 3 (momentary), the pump gets power and turns on.
  • The same switch then moves to setting 2, where the pump remains on.
  • If the switch is turned to positions 0 or 1, the pump turns off and
    will remain off until the switch is returned to position 3

    • (the pump will NOT turn on in position 2 alone, but will remain on
      in position 2 AFTER position 3 has activated the pump).

I believe this can be achieved with two SPST relays, but can't seem to figure out how to do so.

Is this a good way of doing it:
enter image description here

Best Answer

schematic

simulate this circuit – Schematic created using CircuitLab

This will almost work. The idea is that the both the motor and the relay is powered as long as the relay is closed. So we've made a latching relay. In position 3, everything is powered, and hopefully it will stay that way as the switch changes to position 2.

There's nothing magic about the ignition switch. Position 0 has nothing connected to it, and position 3 just springs back to 2 when you let go. Position 1 may be wired such that it is on in position 2, but since we don't use it, it doesn't matter.

The problem is that the relay will drop if it breaks the connection while switching from Start to Run. If Run stays energized between position 3 and 2, you're golden. Alternatively, the motor may supply power (as a generator) briefly during the switch return. A third possibility is to put a capacitor across the relay coil, though a fairly large one would be required.

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