Did the 48 F-1's come with a horn relay from the factory? My new wiring harness from Sacramento vintage ford wires directly to the horn. My truck has an old relay on the inside driver fender well in front of the radiator. Some I have asked said they didn't have one, same say they did.

 

If you have the single horn mounted next to the radiator, the wiring connects directly to the horn. There is no relay. The dual horns used on the '51-'52 trucks had a relay.

 

With no relay do the headlight wires also hit the top of the horn instead of the relay?

 

It's easy to wire in a relay, and smart too. Horns draw a fair bit of current. If you've switched to 12V, the standard cube relays are generally good for 30A - enough even if you're running your 6V horns on double the current! They sound good and loud too!!

 

Double the voltage, but half the current. My '50 F-6 had no relay but I'm installing one. I only want minimal current going through the horn button contacts.

 

FAA-13853-A .. 6V Horn Relay / 1948/55 trucks.

Pic: Scroll down to 13853. Parts catalog makes no mention of the number of horns used with the relay.

 

The shop manual gives more detail

 

Actually, if your resistance stays the same (ie - you don't change your horn), double the voltage also doubles the current. Ohm's law: I = V/R

Using 15 amps as indicated in Ross' post above, we can work this backwards to find the approximate resistance of the horns:

R = V/I or 6/15 = 0.4

Plugging that back into the original formula but with 12V, we get:

I = V/R or 12/0.4 = 30 amps

 

Excellent! I stand corrected. That explains why you can get a voltage reading with a DMM, but yet cannot pass any current through corroded contact points.

 

And now I wonder why all this time we've heard that a 6 volt system has to have heavier gauge wires because the current is double that of a 12 volt system when in fact, it is exactly half that of a 12 volt system. Can anyone explain this?
Here's Ohm's Law:
Ohm's Law

 

Joe, that is true when you are dealing with lights, where whether or not it's 6 or 12 v, the Wattage is the same. Because wattage is VxI. True for some other accessories, too.

 

I hate to disagree but double the voltage equals half the current. It all equates to wattage - Volts x Amps = Watts. That is why 12 volt wiring is half the size of 6 volt, and 220 volts amperage is half the amperage of 120 volts.
Bruce

 

I should also have added that you should always use a horn relay to minimize the current draw on the horn button contacts. The same goes for any high current draw, headlights etc. If you look at what Detroit does today they run almost every thing in light gauge wiring and use a relay to handle the higher current flow.

 

You have heavier gauge wire to reduce resistance.

It depends - AC is different than DC, but ohms law is pretty clear (and doesn't universally apply to AC). In DC circuits, double the voltage = double the current (all else being equal).

What is confusing is that a 6V heater motor and a 12V heater motor will draw about the same current if driven by the designed voltage. If you run a 6V heater motor on 12V, it will spin twice as fast and use twice the current. Wattage doesn't really come into play because you're comparing two different systems...

To further confusing things - anything with a coil is subject to magnetic inductance as well - so while Ohm's law works, it's not useful for exact numbers out of the equation. The principles still apply, but it's not a true direct relationship necessarily. This is why motors don't play by the same rules as light bulbs.

 

Yes. That makes perfect sense. Thanks.