Adding a horn

[dgrinnan]

My 47 project did not have a horn or any of the parts.  I have source an original horn assembly.  The button and the components (rings, springs, etc...) including the wire tail.  I also have a horn.  Does this require a horn relay? 

[LazyK]

Simple answer, yes a relay is required

[Young Ed]

1 hour ago, LazyK said:

Simple answer, yes a relay is required

OEM for trucks is no relay. 

[Plymouthy Adams]

with these parts getting a bit harder to find replacements, I say protect the entire horn circuit with the relay removing the heavy current from the grounding ring/contacts and let the cheap replaceable relay take the brunt as it is designed just for this.

Edited January 10, 2023 by Plymouthy Adams

[Los_Control]

I want to follow along here @dgrinnan ... I am curious because I know all horns have relays from the factory?

 

Here in Texas you need a working horn to pass safety inspection.

 

I helped a friend awhile back to get his horn working on a chebby silvarado truck for inspection.

I bought a $3 plastic horn, a $3 push button to mount to the dash.

I asked Charlie at my parts store ... "Do I need a relay?" ... Nah will be fine he said.

 

4 years later the horn still works fine with no relay.  ..... How often & how long do you use a horn?

 

Is the relay a 6 volt thing?

I know modern 12 volt also uses a relay .... is it really needed?

 

Last time I used the horn on my chebby truck was to pass safety inspection ... a short 1 second blast to show it works. 1 year ago.

I do need to go in for my annual safety inspection this January "now" Give my horn the usual 1 second blast .... 

 

At what point does a horn relay become required?

Normal use on a 12 volt vehicle they work fine. ..... I'm thinking if you actually need a relay to cover the horn circuit .... you may also need good combat training if you plan to lay on the horn long enough to burn it out?

[Plymouthy Adams]

depends on the manner the horn itself is made...

[kencombs]

I think every load over 'x' amps should be on a relay.   For me x is 5 amps.  Of course one could make an exception for short duration loads, but even that can cause pitting and burning of the switch that carries the load.

 

IMO, the desire to reduce material cost is the biggest thing driving car makers to use relays.  They are cheaper than heavy gauge wire.  Put the relay close to the load and use light gauge wire to trigger it.    I'll do that, but no CANBUS stuff for me, that is just too far up the evolutionary chain.

[PT81PlymouthPickup]

My 39 PT81 did not have a relay when new. Perhaps by 47 they may have? But, I believe Young Ed is correct. No relay. A wiring diagram specific to your truck would be most useful. On my truck, power it's routed via the amp gauge through the harness to the horn and then back into the harness to a connector which connects to the ground wire that goes up through the steering column to the horn button. I kept mine wired this way without a relay. I converted to 12 volt many years ago. I'm still using the oem 6 volt horn and it works flawlessly. However, it rarely gets used. If I were to use it daily, a relay might make sense. I doubt you'll be using it much in a 1947 truck? The neat thing is; my 6v horn will scare the bejesus out of anyone I honk at. ?  By installing a relay, the load will be carried through the contacts in the relay, that protects the horn button contact from arcing and as Ken suggests could shorten the life of the horn button switch. More importantly, whatever you decide, make sure there's a fuse in the circuit. When you start making changes like I did (12v conversion, stereo, halogen headlights, signals, alternator, electronic ignition) it got really challenging for me. Altering the harness and routing wires correctly and making it look right was no easy task. Did I mention, I had little idea of what I was doing. For me, it was the most frustrating thing of the entire build. But that's just me.

[billschwindt]

Oem single horns did not have relay!