So I have a headlight system, we converted from the halogen to the Xenon style

Halogen truck operation:
The BCM on this truck provides power to the low beam. When the high beams are activated the low beam power circuit is shut off, and power is sent to the high beam circuit only

Xenon Truck operation:
The BCM powers the low beam circuit constantly(the ballast has an independent power supply). When high beams are activated, the low beam circuit stays on, BUT power is also applied on the "high beam" to open the shutters

What I need to achieve is having the high beam circuit on the Halogen truck to send power to low beam circuit(or power with a relay or transistor). But without back feeding power to the now currently not active low beam circuit on the BCM. The transition has to be almost instant as if the ballasts suddenly loose their "on" input signal they start to act FUNKY.

Low beams on: acts normal
Switched to high beams: power or a signal is sent from the high beam circuit ONLY to keep lows activated, as well as power highs
Switched back to low: power/signal from truck returns to lights for low beam circuit, high beam has no power

I desperatly tried re coding the BCM, but it refused to swap its operating procedure.

I have spent hours googling, tried many different circuit lay outs in two different simulators, but just can't seem to get the result I want. I'm fairly certain I am over thinking this. I do not wanna just use a few diodes, since each it will see over 3 amps, and with the collapse the the shutter coil i don't wanna blow a little diode out. I first attempted to use 2 pmosfets as "reverse current protection" but since its not actually reverse current and the drain side of the mosfet is floating, it never opens, so current would still back feed

 

Use big Diodes mounted on an Aluminum Heat-Sink, that is NOT mounted under the Hood..
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Diodes can work and if the voltage drop is not an issue it is the best choice. BUT if that is a problem, then use two diodes to control a relay feeding power to the low beam headlight. the diodes will only need to be adequate for the relay col current.

 

Or as @MisterBill2 suggests, using the two inputs to activate a relay can mitigate the size and heat of large diodes on heavy heat sinks. The "Out" would be the coil input. The relay contacts would be constantly powered on the NO (Normally Open) contacts and Common (C) would go to each light.

 

The other benefit from using a relay is that if a diode or the relay fail you are back to the way it worked before.

 

Two different methods I came up with. The relays I use take about 240mA to energize the coil. I NEED the coil to stay energized when switching, though this diagram shows the high beam as one switch, its two FETs. Would putting a cap over the coil of the relay hold it energized for a few millisecond?

 

Acutally there is a 100ms overlap. Red is low beam, blue is high beam

 

There is no need and no benefit provided by either the 50K resistor not the transient protection diode, shown in post #6. That is because the "shutter" device is already connected across the relay coil, as well as the headlamp bulb is also across the relay coil.
The religion of a diode across every relay needs to understand that not every instance requires it. This is one of those instances.