Headlights & Relays

[S Schroeder]

Hi Everyone. I just purchased some 55/60w H4's for my 1972. I am nervous to install them after reading different postings regarding the dangers of not using relays. The reason I purchased the low watt bulbs was to avoid any wiring hassels. Do I need relays or am I ok? If I do need them does anyone know which ones to buy and (more importantly) have wiring instructions? I would rather pull a motor than fight an electrical problem! Thanks, Scott S.

[Dave at Pelican Parts]

I would think that you're OK with the 55/60s. One way to be sure is to install them, turn them on, and then feel the wires that go to them. If the wires are warm at all, you should get the relays.

Installing them is relatively straight-forward. [Side note: I don't know the real EE-type terms for the various relay connections, so the names below are my own.] You run a big-diameter wire (8-12 gauge would, I think, be enough) from the battery (through a fuse--IMPORTANT!) up to the front trunk. That wire then gets hooked to the "input", which is terminal 30 on a Bosch relay. There are two "switching" lines, terminals 85 and 86 on Bosch relays. One of them (doesn't matter which) gets hooked to a ground, and the other gets hooked to the old headlight power wire. Then the "output" terminal (Bosch #87) gets a pair of fat wires hooked to it (10-12 gauge, maybe?) that go to the lights.

You can run both high and low beams off this circuit, but you need another relay. Just hook the "input" of the second relay to the "input" of the first--again, with the heavy-gauge wire. The wire that used to feed the other beams substitutes for the "old headlight wire" above, and the "output" goes to where other-beam wire used to on the headlights.

A relay works as an elecrically-operated switch. The two "signal" wires move the switch from open to closed when there is enough of a voltage difference between them. When closed, the switch connects the "input" to the "output", and when open, the switch connects the "input" either to nothing, or to an alternate "output" terminal (that's the #87a on the Bosch relays).

Sizing the fuse takes a bit of thought. Let's see, W = I * V. Solving for I (current), we get I = W / V. For a system that is nominally 12V, the 60W lights will flow ~5 Amps each. But a 10-Amp fuse would leave very little margin for spikes or errors, so a 15A fuse might be more appropriate. Fuses, BTW, should go near the battery whenever possible.

You may need a heavy-duty relay to be able to run both lights off of just one. I don't know what the capacity of the round Bosch ones are, but as noted you'll need one that can handle at least 10A continuously, and more would be a good safety margin.

If you are running both high and low beams off the same fuse, you'll want about a 25A fuse, since it is concieveable that both high and low will be on for a moment when you switch them.

Some might object to the use of a single wire and single relay from the battery, but if the wire is large enough there is little or no practical difference. And the simplicity is a desirable benefit. If you can't find a ~15A relay, find some that are ~7-10 and wire them in parallel--that is, hook the "inputs" together, hook the "signal" wires together, but hook up the "outputs" separately, one to each light.

--DD