Using relays

[Chris390]

Ok. I want to build my own harness for my truck since I have done everything else on it. And I was wondering what all do I need to use relays on besides the headlights? And how many do I need to use for the headlights? 1 per light or 1 for the both?

[basketcase0302]

Everything LOL! From a HVAC control freak who managed buildings via laptop and tablet the more the merrier:

[Chris390]

Do I need a relay for the gauge lights?

[Chris390]

And what are the lil fuses right next to the relays? And what r they for?

[Ranchero50]

.5 x 1" metal boxes with 2 studs are automatic resetting circuit breakers. In some applications they are better then fuses.

Ford used aux relay's on the heavier trucks with more parking lamps. The headlamp switch can't handle a lot of amps and if memory serves, some have a CB built in.

[69Ford]

One for High beam One for Lowbeam. If one goes out you still have headlights. LMC truck sells the harness easy to install.

[Nitekruizer]

You could probably use relays to control everything from your headlights to your electric choke, if you want.

Some things that you may want to use relays to control would definitely be headlights, especially if you've upgraded to Halogen bulbs. You would control both Low Beam circuits with one relay and both High Beam circuits with another relay. If you want, you could even control each headlight separately with two relays for each bulb, one relay for the low beam filament and one relay for the high beam filament. Some other things you could control with relays would include any high powered audio systems, off-road lighting or trailer lighting. Even the Blower Motor circuit could be rewired with relays and, of course the horn needs a relay, whether it's the original relay or replaced with an aftermarket relay.

Remember that even if you use relays, all of the current must flow through the wire that runs between the Alternator's output terminal and the positive battery cable at the Starter Relay. You don't want to overload that wire, so you may have to upgrade to a heavier wire and some type of Fusible Link or master fuse between the alternator and battery, especially if you ever go with a higher amperage alternator.

[basketcase0302]

Jamie's right about the circuit breakers. I'd always found them a heck of a lot easier than searching for fuses for our trucks which are getting harder and harder to find. Here's where the relay and the circuit breaker is located guys on the F-250 / F-350 models with cab lights. If you have one you've probably heard it click a million times and never realized it was there, (under the PS dash / just above the PS fresh air vent).
I had "dedicated circuits" (1-relay / 1 circuit breaker / dedicated grounds) for the following on my 71' DRW SBF aluminum flat bed.
1) Hi beam lights
2) Lo beam lights
3) cab lights
4) trailer "running" lights
5) full west coast side mirror heat

The idea of "the more the merrier is that this is actuaslly a much simpler means of wiring to troubleshoot, (as well as not affecting other areas of the trucks lighting when there is a wiring short or issue to seek out).

edit: forgot I wired the heat on the west coast mirrors through one relay.