When reverse-engineering one part of a schematic, it can become essential to trace connections from there to other parts or pages of the schematic.
We can't do that with the snippet shown here (and even if you could provide the whole schematic, that might become a copyright problem). This limitation is affecting my answer (I want the whole schematic!) but here is my analysis, based on the provided small diagram:
- IMHO there aren't 2 switches or relays, as you speculate. There is one, two-pole (double-pole), three-position switch
S70021. The part number starting with
S is a good indication of a switch, not a relay (check the BOM / parts list for that component, for confirmation).
- When you have the manufacturer's part number for that switch, you might be able to find out more about its internal contact arrangement and hence what happens in each of the three switch positions. Don't limit yourself to reverse-engineering the schematic, if you can also use the BOM / parts list.
- The switch has 6 pins, labelled 1 through 6. The top pole is pin 2; the bottom pole is pin 5.
I'm not going to try to decode the likely connections in each of the three positions (I need more of the schematic to do that, and several cups of coffee) but we can make some deductions, for example:
- Power is supplied to the top pole of the switch, from the 28VDC ESS BUS II via circuit breaker
- In one of the three positions, that 28V is connected via switch pin 3 to position H in terminal block
- From there, power goes via position J in the same terminal block to somewhere I cannot see via
- Trace that power - is it related to the NVG part of the instrument panel lighting operation? Somewhere else in the schematic, the variable control for instrument panel brightness when using NVG (seen in position "LO" on that top picture) must be shown. That will give you a good clue, about whether you have traced the connections for the switch in position "NVG" or "ON".
What about the other connections? I would use the same approach as in the example above, to trace the wider power supply connections, for example:
- Where does the connection
LF1027A16 from pin 1 go to? Does that make sense in the NVG position, or only in the ON position?
- Follow the connections from switch pins 5 and 6, off the diagram given. In one (and likely, only one) of the switch positions, pins 5 & 6 are connected. Trace those connections to see whether that connection makes sense in switch position "NVG" or "ON".
What about the switch in position "OFF"? Since the switch has only two pins per pole for connections (pins 1 & 3 for the top pole, and pins 4 & 6 for the bottom pole) yet is a three-position switch, then it seems likely to me that in the "OFF" position, there may be no connection between the poles and either of the connections for that pole. This is common with "centre-off" switches, for example.
Due to the connection between switch pins 5 & 6 in only one switch position, and since the extra functionality (i.e. the variable light control) is probably for use in NVG mode, then that would be my starting hypothesis if I was reviewing the connections. Then try to confirm or deny that hypothesis, using the rest of the schematic. Does it make sense that those two points would be connected in NVG mode?
Hope that helps to show a general reverse-engineering approach. As I said, don't limit yourself to trying to interpret a single switch; use the rest of the schematic (and any other information sources like BOM, manufacturer's part number etc.) to help you.