High/Low Beam Relay Install

Every car has an electrical system. Here's the place to learn all about it.

Moderator: 2088 bob

Post Reply
User avatar
kangaboy
Posts: 746
Joined: Wed Jan 22, 2003 3:01 am
Location: St. Louis, MO

High/Low Beam Relay Install

Post by kangaboy » Tue Jan 05, 2016 10:36 am

My computer is being a jackass, so I can't load my picture.
I have 12v going to a 6 panel fusebox, that will power my high and low beams via a pair of relays that get swithced on/off from the headlight and high beam switch.
My question is, does it matter if the 12v power is fused before or after the relay? I only ask because on my Megasquirt and Hard Start relays, the 12v that go to the MS and the Starter have the fuses after the relays. With the headlights, I am wanting to do the opposite, and fuse the 12v prior to the relay.

User avatar
Dale M.
Posts: 1606
Joined: Mon Oct 05, 2009 10:09 am
Location: Just a Little Bit West Of Yosemite Valley

Re: High/Low Beam Relay Install

Post by Dale M. » Tue Jan 05, 2016 10:47 am

It does not matter, personally I try to install fuses where it protects more of the circuit its powering (closer to source) ....

Dale
Lives his life vicariously through his own self.
1970 "Kellison Sand Piper Roadster"

User avatar
kangaboy
Posts: 746
Joined: Wed Jan 22, 2003 3:01 am
Location: St. Louis, MO

Re: High/Low Beam Relay Install

Post by kangaboy » Tue Jan 05, 2016 11:48 am

Thanks Dale, that's what I figured, but I just wanted some confirmation.

User avatar
Marc
Moderator
Posts: 23732
Joined: Thu May 23, 2002 2:01 am
Contact:

Re: High/Low Beam Relay Install

Post by Marc » Tue Jan 05, 2016 3:25 pm

You can make a case for placing the fuses at either location. Realistically, where do you expect a short to be more likely to occur in a headlamp circuit - at/near a lamp or at the dash?

By fusing upstream (closer to the source) of the relay, the relay itself is "protected", yes, but actually may have to handle more current momentarily under certain fault scenarios. Make sure that the relay is rated generously, say for twice the amperage of the fuse.

If the fuse(s) are located downstream of the relay, the critical section of the circuit is the wire(s) between them - a short there will smoke the relay and/or the switch and related wiring. But a short at one headlight will leave the other one operational (assuming you fuse them separately like the factory did).

To cover all the bases, one would need to run the supply power to the relays through circuit breakers/fuses, then fuse each lamp filament separately.

User avatar
Dale M.
Posts: 1606
Joined: Mon Oct 05, 2009 10:09 am
Location: Just a Little Bit West Of Yosemite Valley

Re: High/Low Beam Relay Install

Post by Dale M. » Wed Jan 06, 2016 10:23 am

Marc wrote:You can make a case for placing the fuses at either location. Realistically, where do you expect a short to be more likely to occur in a headlamp circuit - at/near a lamp or at the dash?

By fusing upstream (closer to the source) of the relay, the relay itself is "protected", yes, but actually may have to handle more current momentarily under certain fault scenarios. Make sure that the relay is rated generously, say for twice the amperage of the fuse.

If the fuse(s) are located downstream of the relay, the critical section of the circuit is the wire(s) between them - a short there will smoke the relay and/or the switch and related wiring. But a short at one headlight will leave the other one operational (assuming you fuse them separately like the factory did).

To cover all the bases, one would need to run the supply power to the relays through circuit breakers/fuses, then fuse each lamp filament separately.
Valid point on fusing each lamp separately...

Dale
Lives his life vicariously through his own self.
1970 "Kellison Sand Piper Roadster"

User avatar
kangaboy
Posts: 746
Joined: Wed Jan 22, 2003 3:01 am
Location: St. Louis, MO

Re: High/Low Beam Relay Install

Post by kangaboy » Wed Jan 06, 2016 11:42 am

Here is my set up. It will work, but it isn't incorporating SOME of the best practices.
The only thing that interests me about it now, is that originally both left and right lights were fused with an 8amp fuse. Now I will have both left and right lights on the same fuse (I understand that I will lose both lights with a blown fuse). Should I put a larger rated fuse in now since both left and right are drawing through it? Or will the circut still only be drawing around 5 amps?

Image

User avatar
Marc
Moderator
Posts: 23732
Joined: Thu May 23, 2002 2:01 am
Contact:

Re: High/Low Beam Relay Install

Post by Marc » Wed Jan 06, 2016 2:59 pm

The total current through a parallel circuit is the sum of the branch currents, so the fuse is handling twice as much now. You'll need to step up the fuse amperage, to what depends on what lights you're running and what level of overcurrent protection you want. I wouldn't go less than 150% or more than 200% of "normal" circuit current in most cases.

A 6014 sealed beam is rated (at 12.8V) for 50W on low, and 60W on high beam.

P=IE (voltage times current equals wattage). Can also be expressed as I=P/E.

50/12.8=3.9 - each lamp draws 3.9 amps on low, so an 8A fuse will be on the verge feeding two under normal operation...a 12 to 16A fuse would be better.

On high beam you'd want a 14 to 19A fuse for the same 50% to 100% overcurrent limit.

If you'll be running "high-output" lamps, obviously the current will be higher; for example, 90/100W H4s would draw ~15A per pair so you'd use about a 25 or 30A fuse.

manx1969
Posts: 20
Joined: Sun Jan 03, 2010 3:35 pm

Re: High/Low Beam Relay Install

Post by manx1969 » Thu Jan 12, 2017 2:31 pm

it seems that this will only work with a floor high beam switch not a column blinker/high beam as it click and hold as its only monetary unlike the floor switch how do I get around this problem with generic relays

User avatar
Marc
Moderator
Posts: 23732
Joined: Thu May 23, 2002 2:01 am
Contact:

Re: High/Low Beam Relay Install

Post by Marc » Thu Jan 12, 2017 5:03 pm

The question nearly answers itself. In your last post, the diagram clearly shows the control power for the load-reduction relays as coming "From Dimmer Relay".

"Generic" relays are designed for load reduction. To get a "latching" feature that doesn't need current applied when at steady-state condition, you need to use a relay designed for the purpose - like the stock hi/lo relay - upstream of your load-reduction relays to perform the function of the foot-operated dimmer switch using the momentary steering-column trigger.

You could simply place a SPDT switch on the dash and use that to select which load-reduction relay is "on line".

Post Reply