headlight relay or any relay explanation

[Jim100]

could someone explain in dumbdown fashion the headlight relay bit? Thanks,
jim

[EGroscup]

I will give it a shot. A relay is designed to be a remote actuated electrical switch. They are often used to switch on electrical loads that use a lot of current. The starter has a big one as the little ignition switch in the dash would never be able to flow enough current to make the starter run. You twist the key and if everything is right, a small amount of current actuates the starter relay or solenoid which is a big heavy duty switch that flows enough current for the starter motor. same principle for headlights. When our trucks were new, the lights worked well. Over time resistance can build up in the electrical system, causing weak lights. This usually happens at connections, the wires themselves can last a very long time. Think about how many connections are in the headlight circuit. Batt. + to the fuse block, the fuse connections,from the fuse block to the switch, connections at and in the switch, connected to and in the dimmer switch, through the bulkhead connector on the firewall, and then on the light.Now we must follow the ground circuit back to the batt.- Lots of places for loss there, the body to frame connection is super important here. For proper charging the engine to body connection must be good also. A lot of connections with a little loss each adds up fast. A relay can be used to get around some of these problems. A relay does not need much current to operate it so using the existing wires from the dimmer switch to turn it on and off will work. The trick is finding a power source for the relay to turn on and send out to the headlights. Generally one would run a new large wire from the batt.+ or the next + connection at the starter relay and some sort of circuit protection (fuse,circuit breaker, or fusible link) to the headlight relay. What should happen is when you turn on the headlights, the headlight relay will be told" HEY TURN ON!"by the low current coming through the old system. The relay should snap into action sending the power from the new wire that doesn't have 500 corroded power robbing connections to the headlights and all should be good. I'm sure that there are others here that can expand on this and even whip up a diagram to help clarify. Feel free to ask more questions if need be. Hope this helps, Erik

[junior]

GO TO 60-66 GMC GUY.I KNOW,IT'S A GMC TRUCK SITE,BUT THEY HAVE A GREAT ARTICLE ON HEADLIGHT RELAY SWITCHES.SHOWS YOU HOW TO HOOK THEM UP. I AM SURE YOU WILL FIND THE INFORMATION VERY HELPFUL.GOOD LUCK.

[EGroscup]

Thanks for the backup Junior. 6066 is over on the ATHS forums also. A neat place full of vintage big trucks. American Truck Historical Society. Erik

[elacruze]

Stock, your headlight switch supplies power to the lamps.
A relay separates the switch from the power.
Your switch activates the relay instead of the lamps.
The relay supplies the headlights with power from the battery instead of the switch.

[Jim100]

Thanks. Now I get it. So would a relay still be worth it if I replaced all my wires with a new harness?

[junior]

I replaced every single wire on my truck with new,thick 12 gauge wires.The headligts are nice and bright.I did not use a relay switch only because the truck is never driven at night.

[40YearOldFury]

A relay is a good Idea no mater if you rewire the entire truck or not. The full power will not be passing through a little 40 year old dash switch then. It modernizes the wiring. New vehicles use relays with any equipment that pull higher amounts of power. When I rewired my son's 240z we even ran relays to supply his fuse block in acc, run, start, and constant. Just good wiring practice. I know it seems like adding relays complicating things but I won't run a huge draw circuit without them. (driving lights, stereos, power windows, door locks)

[Jim100]

Thanks 40 yo fury. That makes sence. I will get some lined up for headlights and fan? jim

[40YearOldFury]

The fan is a great Idea. I have a fan out of a Lincoln March VIII or something like that planned for my truck and it pulls 50A on high speed!...Wonder if My alt will handle it??? hmmmm?

[surfnuke9]

The fan out of a Mark VIII is a great fan, pulls over 4500 cfm at full speed.

A Lincoln Mark VIII fan will pull as much as 100 Amps on startup. The one in My 68 Pontiac pulls around 40 Amps at full speed.

A variable rate electronic fan controller is the ticket on those. It slowly ramps up the speed of the fan to full in order to better control startup current. It also modulates the speed such that it isnt always running at full capacity.

I smoked several 40 and 50 Amp relays before realizing how big of a current draw it was.

[40YearOldFury]

Yeah, I am going to use my DCC controller. http://www.dccontrol.com/fancontrol.htm I have been more than pleased with it. It uses the two speed capabilities and has the input for the AC control as well. What controllers have you used?

[mercado]

Is a ballast resistor sort of like a ignition coil relay? can you go to parts store and ask for a relay? I mean, is it like a small box that you plug both connections into?
Thanks.

[Hobcobble]

Is a ballast resistor sort of like a ignition coil relay? can you go to parts store and ask for a relay? I mean, is it like a small box that you plug both connections into?
Thanks.

Hmmm Not sure what it is you're after specifically.
If you go to a parts store and ask for a "relay", you'll likely
wind up with the starter relay that is located on the left inner
fender..... if that's what you're wondering.
John

[mercado]

Do you know where it would be on a a100 van 318, in the fuse box?

[Marc]

The purpose of the ballast resistor is to LIMIT the amount of electricity going to certain ignition parts at certain times. It is not a relay. :) It is a resistor.
These items are being viewed with too much significance. Just look at their names. RESISTOR. it's purspose is to "resist" electricity. RELAY. think of a relay race. The baton is the electricity. :)

[mercado]

Ahh, i get it now, thanks!
To check if a relay is bad, do i just do a continuity test between the blades?

[Yellow71]

could someone explain in dumbdown fashion the headlight relay bit? Thanks,
jim

Headlight, Horns, starters...etc...are high current items...

Switches are normally low current items...

A switch is used to energize a circuit that has low current flow, but it is enough current to energize the solenoid in a relay, which causes the contacts in a "Normally Open" relay to close.

The contacts in the relay act as a switch, and that "secondary" side of the relay can electrically handle a higher amount of current flowing to the headlight, horn etc, due to a low amount of resistance in the headlight or horn etc.

Another nice characteristic of switching with a relay...you can isolate, say the interior, from high current which is a higher fire risk, especially when the switch starts getting older, and therefore has more resistance, and more heat is dissipated by the switch when current flows through a higher resistance in the circuit.

[Yellow71]

Ahh, i get it now, thanks!
To check if a relay is bad, do i just do a continuity test between the blades?

You should have a measurable amount of resistance between the two sides of the windings or coil side of the relay.

Resistance between the contacts of the relay will be Infinite for a normally open relay, and zero for a normally closed relay...

both conditions reverse as soon as the relay is energized. You have to test the contacts by energizing the windings.