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We are wiring my '51 and have a question about horn relays. The truck is 12v and the horn is stock. The hiring harness has no mention of a horn relay and the GM wiring diagram doesn't show one.

Am I correct in assuming that I don't need one? If I do need one which wire goes where on one like THIS.

Disclaimer: I'm electronically illiterate, so your patience is appreciated.

I am suprised the wiring harness has no allowance for a horn . Saves the voice and the finger when abusing some loser. 3 wire relays I think have 12v in from a battery source 12v out to the horn and the third connection goes to your column to earth the relay when pressing the horn button. Mounted in the engine bay you can get 12v from the alernator/generator stud or starter motor preferably 12v would come from a fused circuit inside the cab like radio or cigarette lighter check your loom for a fused circuit
regards
andrew

I didn't say it has no wires for the horn, it has no wiring for a horn RELAY and the factory wiring diagram does not show a horn relay.

Sorry for not making that clearer.

The "S" terminal comes from the horn button. The "B" terminal comes from the ammeter/power supply. The "H" terminal comes from the horn. I got this information off a 1949 Cheverolet A.E.A. tune up system card that I bought at a swap meet. The front side has tune up information and the rear of the card has a wiring diagram. Hope I could help. Don

Thanks Don, but the problem is that none of the terminals on the relay I linked to are marked and this still leaves the question - why isn't a horn relay on the original wiring diagram?

Woody - I think you've got more than one question ... if there's no relay shown on the 51 diagram, then it's no doubt because there wasn't one [or one was only used if there was an optional 2nd horn added?] ... the one sold as 47-54 is really for whatever year they started, possibly 54, but suppliers make assumptions - maybe the year range means that relay is for 6V, it does look different than the 55-59 relay. although that might just be cuz the later relay is configured for a harness plug rather than screw on spades

Bill

Woody, Classic Parts #53-312 relay is for 12V and is the one that I would use. If you look at it, the terminals are marked H, B, and S. As to your harness not having the wires I'd call the company who made it and ask them. I can scan a picture of my diagram and it would show you what to add if you want. I'm guessing that you have a wire from the horn button coming out at the end of the steering column.
As Bill says, the relay that Classic shows for '57 up is also a 3 terminal relay but uses push on wire connectors.
I just looked at my Chevrolet Truck Shop Manual 1948-1951 and it doesn't show a horn relay. It shows the wire from the horn button going directly to the horn. The other terminal on the horn shows it going to the ammeter for power. Also, figure 3-fuse box shows the bottom 2 terminals are for the horn. That is on page 12-4.
I would then assume that for a single horn no relay was used. If a person purchased the accessory horn set up from the dealer then they would add a relay?
If your using a single horn I would just pass on the relay, as long as your horn works ok with 12V put to it. Easy enough test with a battery and a couple of test leads.
Good Luck Don

On A-D trucks, the horn relay was used with the dual accessory "matched" horns (seashell style 47-55, available all years). The horn relay might have been used in 1953 when the seashell-style single/stock horn was used (mounted on the intake manifold). The horn relay was certainly used on the 54/55 trucks (stock seashell horn mounted on the front of the radiator housing).

A horn relay was not used on 47-52 trucks with the "trumpet-style" stock horn (but people might have retro-fitted a relay).

The horn relay is listed here for use with matched (dual) horns from 34-54.

I have the relay with a single seashell horn on my '53 bigbolt.
S=sound H= horn B= =battery
sound being the horn button inside the cab.

On my 51, the horn relay is on the firewall, near the horn. The feed comes directly from the starter junction and is quite short. I would say use a relay for sure.

We'll be following the factory wiring diagram (no relay) and then report back with the results.

However, if the truck explodes my report may be delayed.

Woody,

If you care...the use of a relay will result in a louder 'TOOT' and may prevent the horn button/spring/contact thingy from eventually going into self destruct.

Using no horn relay is fine but they did serve a purpose.

Stuart

Stuart,

I agree but as I stated there are no S, H or B on my horn relay that I can see. It's already attached so perhaps the terminals are marked on the back side. I'll pull it out and check but there is nothing marked on the "front" side.

I was hoping that someone had used one of these relays and could tell me which terminal is what.

Woody,

My handy/dandy picture shows this; Using the link you supplied of the horn relay, and in that same orientation, the far left tab is the horn button on the column, the center tab is BATT and the tab on the far right goes to the horn.

You can prove this with a small battery charger. Connect the positive cable to the center tab with a small wire. Connect the ground to the case of the relay with a small wire. Use a screwdriver or small wire to jump from the far left terminal to the case. This should make the relay go 'click,click'.

If the relay melts or your garage burns down, Denny Graham told me he would pick up the tab for repairs.

Stuart

Thank you Stuart, you win the prize for best answer! We'll be using it after all.

The six volt horn is very loud on twelve volts and I would guess that it draws even more current.

Connections: there are three terminals on the relay. With an ohm meter you will find continuity between two of them. One of these is the supply line and the other is the line for the horn button ground. When you apply power to the two terminals there will be a click and the third terminal will be connected to the supply line. You should be able to tell this using your volt meter.

Now wait a minute Stuart, "I ain't got no relay. I don't need no relays! I don't have to show you any stinkin' relays!"

The way I see it, if your horn or horns operated with no relay on 6v and didn't burn up the contact or wires then it certainly wouldn't be any worse with 12v applied. The resistance in the circuit doesn't change so all that means is your gonna be pulling half the current to get that cow out of the way that's in front of the bumper.
And wasn't the only reason they went with a relay in the first place was because they switched over to the new style horns which may have drawn more current and in some cases there were two instead of a singel horn.
Denny Graham
Sandwich, IL

You must be referring to the needs of your 1950 truck, right Denny?

Si, eso es gringo!

DG

I'm thinking if I just read all of czechman posts,I will have all my issues resolved beforehand.

He finds them before I get to them

And you might add that he shares his experiance with us by posting some great pictures in his album.

DG

Denny,
I think if the resistance stays the same and the voltage doubles you would end up with twice the current, correct? I mean I= V/R.

Fred

Oops E over R, not R ove E, must have been drunk or just the senility showing it's face. Sorry guys.
I'm just gonna bail on this cuz I got two funaces that taint workin', two trucks in pieces, a couple radios that need rebuilding, holidays and birthdays coming up and about another hundred things that need my attention. Been spending way to much time on the computer lately anyway and it's getting hard to keep my logic in line.
Thanks for correcting that Fred.
DG

Little late to game with this but I just finished this job last week.

When you find the wiring diagram for the relay the wires from your harness should be:

Wire from the horn button to the ground terminal on the relay.
Hot wire to the horn on the 12+ IN on the relay.
From the relay a 12+ wire to the horn.
Then a ground wire from the horn to good ground on the truck.

Wish I could help you with the relay diagram but each manufacturer/relay has its own. My choice for relays are the bosch type with five terminals and many applications.

Jim