The Stovebolt Site Forums The Shop Area The Electrical Bay Headlight + horn relay are they needed on 40

Hi, I have a 40 1/2 ton that I have restored. I made the wireing harnress by following what was already there. there was no horn or headlight relay mounted anywhere on truck. I did not know that they existed until I was reading a catalog of parts. I am presently useing the horn wired hot. The headlights are also wired hot with a fuse on the lightswitch. My question is do I need them and what do they do? If so how do I hook them up? Any help would be appriciated. Thanks Gerry

Gerry,

Welcome to the Stovebolt. Relays like you have described are used when you want to control a high current device with a not-so-high current circuit.

The horn relay for instance would be physically located near the horn. Big honking wires would go from a battery source to the relay, through a open 'switch' then to the horn. The button on the steering wheel could have smaller wires that merely signaled the 'switch' to close and honk the horn. Pushing the button on the steering wheel wouldn't pull lots of juice, just enough to close that relay.

The same operation applies to a headlight relay...a small control voltage signals a switch to close that makes the circuit for a high current device, like a horn or your headlight.

This wiring method makes the horn more loud, and can make the head lights brighter. Your truck didn't have them before obviously, but they could be added.

The existing button on the wheel and the headlight switch would control the relays. New, fat wire would go from a battery source to the relay, then on to the headlights or horn.

This is a generic explanation. I don't know how your rig is wired, but that could be addressed if you decided to do this.

I hope this wasn't overkill in the explanation department.

Stuart

Thanks, I understand, Did some trucks not have them as original equpment? Am I going to harm anything if I don't use them? I have the original 6 volt system. they could have been missing I am not sure. Thanks again.

This wiring diagram for the '40 does not show a horn relay. Apparently they were used in some years, and not in others.

http://chevy.tocmp.com/chevywiring/pdf/40truck.pdf

Gerry,

Relays weren't used until many many years later. I think the first showed up on Chevrolet trucks about late '52. Your truck has worked fine for 68 years without them. Adding them might make a slight difference in the horn and the headlights, but probably not worth the effort.

Stuart

The nice thing about using a relay (as has been noted) is that the amperage for the horn or headlights runs from the battery to the relay to the device and to ground.

The more direct the path, the better voltage delivered to the device.

If the amps have to go through the ignition switch, the headlight switch, all the wires and finally the headlights, each connection (ignition, headlight switch, etc.) has some voltage drop and the headlights themselves will see less than the 6 volts EMF from the battery.

In addition, each voltage drop is due to the resistance of the device or connection (maybe just a little bit), but resistance produces heat, and heat, well, go figure.

The downside of a relay is the potential for the relay failure. If the relay fails open, well, then no headlights or horn. But if the relay falis and closes, the headlights or horn will blast away until the battery runa down, and as the battery runs low on producing EMF (voltage), the amperage will increase trying to keep the device working, and the heat problem occurs for a whole 'nother reason.

Relays are a good idea, but they do have drawbacks too.

They didn,t come with relays but one on the horn is a good idea. Practically every vehicle built from the mid 50s on has one. Headlight relays were seldom if ever original equipment. I have them on my 70 & the lights are a little brighter especially with halegens.

Thanks for all the input. You have helped me alot

Head light relays were added when a truck was loaded up with extra lights that came on with the headlights. It was easy to overload the head light switch, which would then shut off all the lights after some indefinite time period, somewhat dependent on high/low beam useage. They would stay off until an internal breaker reset, also an indefinite time period- then you could go on your way. Until the cycle repeated. Head lights going out is unsettling enough when you are on a freeway with other traffic to light your way, but it really focuses your attention if it happens on a mountain road when the last thing you saw was a curve ahead.

I don't know if your truck has a circuit breaker in the switch, but newer switches have them. I seem to remember that yours has a fuse on the back of the switch.

Harvester