school me on relays

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street demon

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I've heard people putting them on the headlight harness and making the lights use less amps. What do they do and how do they help?
 
street demon said:
I've heard people putting them on the headlight harness and making the lights use less amps. What do they do and how do they help?
Click to expand...


What your doing is running a shorter lenght of wire to the draw. What that means is if you decide to install H-4 Halogens, by using a relay your suppling power through a remote swich (the relay) and your NOT making all that current run in through the bulkhead connectors and through the actual headlight switch itself, which will surely fail under that load. The relay lessens the amount of resistence in the circut.
Look at the attached diagram and see how the headlight switch now supplies power to turn on the relay, which feeds the headlights directly from the battery.
headlightrelays-vi.gif

JUST REMEMBER: You have a relay for the low beams, and a seperate relay for the high beams.

When you turn on your headlights terminal #86 will be powered up. Look at the diagram. One feed is from the low beams, the other for the high beams. Once #86 is powered it will close the contacts because #85 is grounded. Once the contact close, the power will now flow from the 20Amp fuses (circut breakers are better...and more expensive) to terminal #30. Because this in now closed, the current will be availiable at terminal #87 and now powers the lights. The heavy current draw only travels from the 20 Amp source (directly from the battery in the engine compartment) to the lights through the relay,which all should be mounted near the battery & lights. And NOT into the cabin throught the dash switch. This type of relay circut should be used for ANY high draw component, such as cooling fans, electric fuel pump, gonzo stereo systems, etc....

If you have any other questions, please ask!
 
The lights themselves will not use less amps. A bulb is rated with wattage and is fixed. The power goes thru the switch and can create heat because of resistance in the contacts and connectors. What you do with a relay is the switch will now switch the relay and the higher amperage draw of the headlights goes thru the relay direct from the battery.


Chuck
 
yup, the relay would cook before it starts your ammeter on fire or the bulkhead connector or your headlamp switch, or... well you get the idea.. ;)
 
player1up said:
yup, the relay would cook before it starts your ammeter on fire or .....;)
Click to expand...

The only way a relay cook would cook is if it was rated lower than the draw, which is one thing I forgot to mention. Make sure to get relays rated for the amperage your powering. A 30 Amp relay is overkill for Halogen headlights, but it's better to have a higher load ability. And use a heavier gauge wire as mentioned in the diagram.

Hope this helps.
 
Using relays and depending on the wire gauge, the lights will actually draw MORE than with the stock wiring. Mopar was notorious for using too small a wire and running the lights through the switch for MANY years. You'll notice the headlights are much brighter running off relays with heavier gauge wiring. That's because they are getting FULL voltage through a properly designed wiring system.
 
Thanks for the info. I'm replacing my under dash harness soon along with the headlight harness. Figured since I was going through the trouble I'd add some relays. Thanks again.
 
Okay, so this is a timely thread for me and I don't want to hijack it but I've got a related question.

I've got door pop solenoids that include wiring diagrams but no instructions concerning what gauge wire to use with the solenoids - other than they include short pieces of 10ga. I have no idea how much current the solenoids actually draw. The relays included are rated at 30/40amp. Most of the pre-wired 30a relay sockets I've seen have wires no bigger than 16ga. (I don't understand how they can safely pass up to 30 amps - but I'm not an electrician).

I plan to mount the receiver and relays in the trunk so I don't have all that crap under the dash and can fire the solenoids from there in case a remote craps out or the battery dies.

The other thing I don't understand, based on you diagrams, is that they have 12v going to both terminals 30 and 85. I'm guessing then that the receiver doesn't send voltage to terminal 86 of the relay, but instead, grounds the terminal when triggered.

I plan on running a 10ga wire from the fuse block (tap) to the trunk with an inline 30a fuse. Then run 10ga wires from the relays to each solenoid.

The question is, are 10ga wires to the solenoids overkill? Will smaller gauge relay socket wiring be damaged? I know a brief shot of current through a smaller gauge wire probably won't be a big deal, but if the relay were to stay closed for an extended period of time due to a malfunction, I don't want to have a meltdown.
 
Let me get this all straight. Instead of the wire running from the bat/alt. threw the bulkhead to the switch then back out to the headlights it just goes from the bat/alt. to the relay then right to the lights. Is that the short of it? Also if I run it off the starter relay(bat. is in trunk) then I'm still running the power threw the bulkhead when the car is running right? But if I run it from the alt. I still will be going threw the bulkhead at low idle or when the car is off right? Where do the two connect? Last question I don't want to throw off my amp gauge so is the alt. the only choice for power?

OK maybe one or two more questions. My Mallory ignition seems to have a built in relay so I think that is good to go and the only other high draw is my electric fuel pump which I can put a relay in the trunk for that. Would putting in a relay for the brake lights help them be a little bit brighter? I have some real nice reflective foil tape behind and around the the bulbs that really helped out but would like to go brighter without those LED conversions and the electric flashers. Last thing I'm staying with the original headlights.
 
Tail lights don't draw that much, so putting a relay in won't help much. I installed red LED tail lamp bulbs, and they are MUCH brighter (plus a deeper red, and lower current draw). Several people have noted my brighter lights, so I guess it works.

As for headlights: Yes, power comes directly from battery/Alt source though the relay to the headlamps. The relays are then energized by the wires that USE TO GO TO THE HEADLIGHTS.

Also, if you hook it to the starter relay then yes, it's still running through the bulkhead. However, by "bulkhead", people are implying the factory bulkhead connector that was notorious for burning up. The less high power circuits you can draw through that connector the better.

I actually installed a buss bar as a central point under the hood for power to go to. The battery (in the trunk) feeds the bar. The Alternator, ignition, starter all run off it. Makes for a clean install that keeps any single stud in your car from taking all the power.
 
Guys,

It's basic electronics and basic math!!! Wattage = Voltage (12v) x Current and Voltage = current x resistance (Ohms Law). Basically the relay takes the high current instead of it going through the firewall, connectors, headlight switch etc. The coil circuit for the relay needs milliamps of current so no big wires needed there. The load side of the relay is where all the current goes (and through the lamp to ground) and should be as close the the power source (battery) as possible. I seriously doubt you need 10 or even 12 guage wire; It all depends on the wattage of the lamps and how much current they draw.
 
Reducing the gauge / weight of wire in a car is the main reason relays were created. Reducing the bulk of switches and their contacts is a plus.
The wire count has gone up exponintelly. The weight of the wiring really hasn't changed much. Ton of bricks or ton of feathers, both weigh a ton.
When circuit configuration allows a relay can be wired hot into the coil and the signal / ground side is switched. A voltage drop occurs in the coil winding thus reducing the current seen at the switch contacts. Switch lives longer.
 
you can get the relays at napa. just ask for foglite relays. i think i paid around $12-15 each.
print off a copy of that diagram above and take with you to make sure you get the right ones. i wasnt able to return electrical parts even though the counterman gave me the wrong one the first time.
 
Instead of the starter relay (which would not be a totally bad place) I would go upstream to the alternator itself for my supply. Here's why: All alternator output is via a 10 AWG conductor which runs through the bulkhead connector through the ammeter, and then back over to the starter relay. If you feed directly off the alternator, you main headlight draw is removed from the charging circuit. As has been often mentioned, our bulkhead connectors and ammeters work better and last longer if you reduce the amount of current running through them.
 
C130 Chief said:
Instead of the starter relay (which would not be a totally bad place) I would go upstream to the alternator itself for my supply. Here's why: All alternator output is via a 10 AWG conductor which runs through the bulkhead connector through the ammeter, and then back over to the starter relay. If you feed directly off the alternator, you main headlight draw is removed from the charging circuit. As has been often mentioned, our bulkhead connectors and ammeters work better and last longer if you reduce the amount of current running through them.
Click to expand...
Thats true but then that headlight relay is hot at all times. Try this , Buy one more relay, let the blue wire at the altenater signal it or any switched 12v source. pull the heavy gauge wire and current from it to as many relays as the heart desires. Be hard to leave the lights on this way too :)
We could over engineer this thing (diagnostic LEDs)
My 67 has a couple wires beyond the altenater harness to the horn relay. I spose I would tape 2 wires in with it from my altenater to a good relay on that inner fender. From there ... who knows.
 
Just rewire the whole car!!:toothy10: That's basicly what I ended up doing, but I have zero power problems now.

- Battery in trunk grounded locally by 1/0 welding cable.
- 1/0 cable to buss bar mounted on inner fender.
- 8awg wire through fire wall (isolated by 60amp maxi fuse) feeds full time power circuits under dash from Buss bar
- 2nd 8awg wire (with another 60amp maxi use) feeds ignition circuits under dash from buss bar
- 4awg wire connects alternator charge circuit to battery buss bar.
- 16awg sense wire connects buss bar to voltage regulator for "true" battery voltage (through ign powered relay)
- Starter relay and starter main wire feed off buss bar

Amp gauge and original power wire/fusible link through firewall are abandoned.
 
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