Can I use a horn relay for fog lights?

[corvette8n]

On my stock 6 volt system can I use a horn relay to run my fog lights?. Is there really a differance between light and horn relays. I have an extra horn relay laying around.

[ford38v8]

No problem I can see.

[Joe Immler]

Yes. My '41 had a horn relay for fog lights. I removed the lights for better looks.

[JSeery]

Horn relays are designed for short period of time usage as are starter relays (which are also sometimes used). I use a relay that looks like an original starter relay but designed for prolonged usage. Using the short term relays for a long term application seems like a miss usage (from a design prospective) to me, BUT many have done it and it seems to work!

[cmbrucew]

Originally Posted by JSeery

Horn relays are designed for short period of time usage as are starter relays (which are also sometimes used). I use a relay that looks like an original starter relay but designed for prolonged usage. Using the short term relays for a long term application seems like a miss usage (from a design prospective) to me, BUT many have done it and it seems to work!

Has to do with the coil windings. Heat build-up with prolonged usage.
Bruce

[koates]

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Horn relays usually have three terminals instead of four on a normal light relay. The horn relay coil winding on one end is internally connected to the battery supply terminal. The other end of the coil winding is connected to the "S" or switch terminal which connects to the steering wheel horn button where it switches to ground. You could wire fog lights through a horn relay, but the fog light switch would have to ground the relay "S" terminal instead of supplying power like in a normal four terminal light relay. Regards, Kevin.

[ABento]

Use a constant duty solnoid, it will look like the starter one but it is built for constant duty, NAPA has them

[JSeery]

Originally Posted by ABento

Use a constant duty solnoid, it will look like the starter one but it is built for constant duty, NAPA has them

Yep, but many don't.

[corvette8n]

Thanks I'll check with my local NAPA, even if they don't have it they are pretty good at ordering stuff.

[40cpe]

Those constant duty solenoids are relatively expensive. What do they do for a lighting circuit that a $4 relay won't do?

[drolston]

Not sure why a relay is needed for fog lights, which do not draw that much current. The headlights draw more and they do not use a relay; - just a switch on the dash. Why not just power the fog lights through the fog switch on the dash. Need a wire from the headlight terminal of the circuit breaker unit to the switch, and a wire from that switch to the fog lights. Use a wire the same diameter as headlight wire. Be sure both fog lights have a good ground. You avoid the current drain of the constant-use relay, and have one less component to go bad.

[JSeery]

Originally Posted by drolston

Not sure why a relay is needed for fog lights, which do not draw that much current. The headlights draw more and they do not use a relay; - just a switch on the dash. Why not just power the fog lights through the fog switch on the dash. Need a wire from the headlight terminal of the circuit breaker unit to the switch, and a wire from that switch to the fog lights. Use a wire the same diameter as headlight wire. Be sure both fog lights have a good ground. You avoid the current drain of the constant-use relay, and have one less component to go bad.

Same reason you would ever use a relay. The current required to keep a relay latched is very small. You can almost always wire items directly, in fact you could wire the horns directly (Ford did some years I believe). As time has gone on more and more things automotive use relays.

Just checked, Ford didn't start using a horn relay until 38 according to the wiring diagrams.

[Frank Miller]

A relay is where a smaller switch turns on a bigger switch. I do not see why a horn relay would not work as long as it is rated for the current of the lights. Horn relays I believe were used to eliminate voltage drop through a marginal horn switch. I know ModelA owners use them to get full voltage to the motor. Momentary relays are usually ones like starter solenoids, which is a relay but has to overcome spring pressure to engage. The OD solenoid does not engage a switch but utilizes a pull in and a holding coil which requires less current.