hi/low beam relay problem w/ picture

Trying to install aftermarket lights on my car. The middle two lights are supposed to be on for low beam, and all four for high beams.

Unfortunately, I have a limited electrical experience. Presently, all four light up when switched to both high and low. I have made a wiring schematic to show how I have it wired (shown below) to help with diagnosis.

I believe it has to do with the low beam wire sending power back to the relay. Can i solve this with a diode?

-Sean

The problem is the way the relay is installed - a diode won't help you.

What's going on is when you turn on your high beams you are energizing the relay (connecting Pin 30 to Pin 87) which in turn is powering your low beams.

You need to keep the high beam and low beam circuits isolated - use a separate relay for high beam and low beam.

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Kevin Pierson

I believe you want the low beams to be on with either the Low or High beam input, and the high beams to be on only with the High beam input.

You cannot use the high beam input to send power to the low beams, you need to use another fused power source from the battery and isolate the circuits (break the connection between pins 30 and 85)

You can use a diode between the high beam input and low beam, you don't even need a relay in that case. But the diode drops the voltage a bit (around 0.7 v) that will reduce power to your low beam. Not something you want.

Also take note to wire the relay with the positive side on pin 86 and ground on pin 85. If there is an internal diode in the relay, it will blow otherwise.

After second though, your circuit should work with one minor modification; connect the low beam to pin 87, not 87a. You should leave pin 87a disconnected. This will cause both the low and high beams to receive power once the relay is closed. When the relay is open, no current will flow through the relay and everything will work normally.

You need to be careful about overloading circuits if you are using the OEM light feed to power additional loads. It is very possible that adding a second headlight will cause your headlight fuse to blow. The additional lights should be powered through a fused source connected to the battery.

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Kevin Pierson

The simplest is to use one SPST relay for hibeam and one for low beam.
They can have their own supply from the battery (each via a self-resetting circuit breaker) but be triggered from the original headlight sockets.

Usually it's the outer 2 beams that are on for low beam (that's the usual regulation - not inner beams).

And for low beam on all the time (even with high beams - ie, the lows are dedicated low with np high-beam filament), the low beam relay is triggered by the "beam" power - ie, the wire that goes to the dipper switch. The dipper than acts as a simple on/off for the high beam (not a changeover between hi & lo beams).