How would you wire this headlight kit?

[tdskip]

I have a headlight relay from another, cough cough, car and I want to utilize it for my '74 911.

I believe this use box layout (from a '77) is the same regarding headlights.



This kit is for an Alfa 105 series car;



Basics should be identical of now to connects to the actual lamps and grounding and battery - the question is how to connect it to the fuse box. The relay kit has a three prong connector.

I believe I need to wire both low beams (14 and 15) from the fuse box to the kit, and same for the both (16 and 17) to the high beams. That leaves me one un-utilized prong on the three prong connector - is that for power to the relays?

[Tremelune]

You're gonna need to cut 'em and stuff them in on one end of the fuse for highs and lows. The wires on the fuses will trigger a relay (one for highs, one for lows), which will then allow the power to be drawn directly from the battery instead of through the headlight switch in the dash.

It's worth noting that each of the two red wires that connect directly to the battery on that harness should be fused, and they do not appear to be.

The gauge of those wires looks small for powering 200W (2x100W) high beams, but it's possible that one wire is for one side of the car, as opposed to one pair of beams.

How would this be wired up on the Alfa?

[tdskip]

Thanks Tremelune, appreciate the response.

The kit says "plug the three prong into the harness", but of course the Alfa doesn't have that plug either.

[Tremelune]

Another way of thinking about it is that what used to power the headlights now "powers" the relays. You essentially want to put the relays between the fusebox and wires that go to the headlights.

It looks like they want you to plug the stock H4 adapter that attaches to the bulb into the new harness, and then plug the new harness into the bulbs using (what should be) the same style of H4 harness.

If this is confusing, maybe plug the new harness onto the bulbs and see what you have left over. The leftover stuff is what you want to attach to the fuses, either by cutting and attaching them, or by using the "old" headlight wires pulled from the buckets.

A lot of this comes down to how long the various wires are and if they'll reach. It's easy to adjust this, but would involve cutting, stripping, and crimping/soldering.

[tdskip]

Thanks - makes sense and that helps.

[tdskip]

Wait - think I am goofed up still - could you check this flow please?

Current from headlight switch
|
Fuse terminal
|
FUSE
|
Fuse Terminal
|
Relay

Like that? So basically the relay picks up the current after it goes through the fuse (diverting it from the original circuit to instead run through the relay)?

[Tremelune]

Well. Yeah. The current from the headlight switch should go to the relay. The fuse is mainly involved out of wiring convenience, as it is no longer protecting the wiring that goes to the headlights. It does nothing but harms nothing.

Related threads:

Headlight relay wiring on early 911

JWest headlight relay kit question

79 Headlight Relay Wiring

[tdskip]

Thank you very much. I should fuse the power going to the relay, correct?

[Tremelune]

I would fuse the two red wires going from the battery to the relay with whatever rating is max for the wire size:

FUSES

Two 100W filaments could draw 17A (200W / 12v). Eyeballing those wires, they look like 14awg max to me, so I wouldn't put anything higher than 15A fuses in there...You might find the wires are too skinny for 100W high beams, but it's hard to know without specs.

[tdskip]

Thank you again for all the information-you've been extremely helpful.

[T77911S]

Originally Posted by Tremelune

Another way of thinking about it is that what used to power the headlights now "powers" the relays. .

exactly.
that's how I changed the wiring in my 930. added 2 relays, one for low and one for hi,