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Discussion Starter · #3 ·
I'm making a conversion harness that will plug into the existing 9005 and 9006 pigtail on my USDM car, go through an SPDT relay and plug into an H4 bulb.Exactly what the author of the thread was trying to do. The relay is used to shut down the low beam circuit when the high beam circuit comes on. The problem I'm having is that I only see one way this will work with an SPDT relay. Towards the end of the thread a suggestion is made which causes the author to modify his diagram. I'm interested in the original and modified diagrams but the pics are gone :)
And so are the users, or so it seems anyway, not getting responses.
 

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To draw a schematic for your harness start by drawing what you would do for a simple adapter harness but don't connect the low beam. The grounds from both the 9005 and 9006 connections will connect to the common ground of the H4 socket. The 9005 power will be connected to the high beam filament.

Now add the relay. The coil will be wired in parallel with the high beam filament (across the 9005 socket). The power from the 9006 socket connects to the relay common connection. The H4 low beam filament connects to the relay normally closed (NC) connection.

When the low beams are turned on power is fed through the relay contacts to the H4 low filament. When the high beams are turned on the relay energizes, turning off the low beam.

A couple notes to add. I expect the reasons the low beams are turned off in the H4 setup is twofold. Having both filaments on for extended periods could over heat and possibly melt the housing. Having excessive foreground light will interfere with distance vision so there may be a lighting performance advantage to operating EDM lights the way they were designed to work.

JDM RHD lights are not legal in the US for a reason. They will direct excessive light into the eyes of oncoming drivers. This is why any car properly imported to the US for regular road use is required to have LHD headlamps fitted.

EDM lamps call for an H4 bulb. JDM lamps call for an H4H bulb. H4 and H4H bulbs have the filaments in different locations so they are not at all interchangeable. Trying to interchange these bulbs will result in very poor lamp performance because the focus will be way off.

Mike
 
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