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Discussion Starter #1
I have a MK III, and the American Autowire carb harness from FFR. After reading all the posts about poor headlight performance, I've decided to relay the headlights. I'm going to use some aftermarket H4 headlights. Question is, how many relays for the headlights? One to power both lights, or one for each side? And does both the high and low beam circuits require a seperate relay for a total of four realys for the headlights? I'm also going to use an "old School" foot operated dimmer swith. I also plan on replacing all those "made in Taiwan" wire terminals/sockets that FFR sent me.

Thanks in advance, John
 

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FFCobra Master Craftsman
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Summit sells a Painless headlight harness for what you need. They sell a weatherproof version of it and that's what I bought.
 

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John,

There are massive number of posts on this, but...

1 - 40 amp relay will handle any headlight that exists. Feed it with 1 - 14 gauge wire for the high beams and 1 - 14 gauge wire for the low beams. Use 1 relay for the low beams and 1 for the high beams... So a total of 2 relays to power the entire headlight system... With this setup you could power a HID system... :eek:

Chris
 

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Discussion Starter #4
Originally posted by Chris L.:
John,

There are massive number of posts on this, but...

1 - 40 amp relay will handle any headlight that exists. Feed it with 1 - 14 gauge wire for the high beams and 1 - 14 gauge wire for the low beams. Use 1 relay for the low beams and 1 for the high beams... So a total of 2 relays to power the entire headlight system... With this setup you could power a HID system...



Chris
Thanks Chris, I spent all morning reading through the hundreds of posts concerning the headlight circuit, but I couldn't find anywhere as to exactly how many relays to use. My knowledge of electrical circuits is just enough to make me dangerous. :D
 

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Just to add a bit to this discussion.Keep the relays out in the engine bay and make all hot wires as short as possible.Control the lights by the ground side of the relays.that way the longest wire(running thru the firewall to the switch) is a ground. So if it gets pinched all that happens is the headlights come on or you switch beams.Actually this applies to pretty much all circuits also.Good luck
 

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John,

Here is a little help I gave another member this past week...

Kouros,

Here are the most basic rules of wiring a car:

1 – the frame is used as the common ground for every electrical load in the car
2 – primary grounds should be made by SANDING off the power coat or paint and drilling through the square-tube frame and bolting the ground cables (there are at least 2) to the frame. Put dielectric grease over the connections after completed
3 - every load in the car shares 6 common attributes:

a – it must have a (+) power source
b – it must have a (-) ground connection
c – there must be some way to turn it on and off (99.9% of the time). This can be with a switch or a relay controlled by a switch
d – the wire for any given circuit must be capable of carrying the required current without excessive heating (no fires)
e – every circuit, or group of circuits, must be protected by a fusing element sized correctly for the load using the 125% rule

If you wire each circuit, one at a time, and use (a-e) you won’t have any problems.

Remember… 95% of all circuit failures are caused by a failure in an electromechanical connection (loose wire).

EVERY circuit is just:

(+)--(fuse)--(switch)---(load)----(-)
This is called “high side” switched

(+)--(fuse)-- (load)---(switch)---(-)
This is called “low side” switched

(+)--(fuse)--(switch)---(FAN)----(-)
(+)--(fuse)--(switch)---(HEADLIGHT)----(-)
(+)--(fuse)--(key switch)---(IGNITION)----(-)
(+)--(fuse)--(key switch-momentary)---(STARTER)----(-)
(+)--(fuse)--(switch-momentary)---(BRAKE LIGHTS)----(-)
(+)--(fuse)--(switch)---(RADIO)----(-)
(+)--(fuse)--(switch)---(HEATER)----(-)
(+)--(fuse)--(shared switch)---(RIGHT TURN SIGNAL)----(-)
(+)--(fuse)--(shared switch)---(LEFT TURN SIGNAL)----(-)

A RELAY CIRCUIT:

(+)--(fuse)--(relay contacts)---(FAN)----(-)
(+)--(fuse)—(switch)---(relay coil)------(-)


Painless does not comply with all the rules, especially (d)… Save a ton of money and make your own harness… ITS SIMPLE TO DO….

Chris
 

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John,

I used 1 relay that is activated by the headlight switch. It feeds another relay that without power feeds the low beams. With power (bright switch) feeds the high beams.

One of many ways to wire it up. Very simple.

Ron
 

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John, I have the carb wiring kit from FFR, also. I believe that the "horn relay" is actually the headlamp relay and that should be enough depending on the current rating of the relay contacts. Actually the purpose of the relay is to allow a switch with a low current rating to switch high currents (or voltages). The H6024 lamps are 35W/65W (low beam / high beam) and so the current ratings are about 2.7A and 5.1A. The total current at 100W with both beams on would be about 7.8A. The total current is about 15.5A for both sides with both beams on. This current shouldn't be a problem for a single relay or even a beefier switch. The problem comes when you switch the lamp on: the initial current or inrush current can be up to 16 times the normal operating current. This is because the filament resistance increases with temperature: a cold filament has very low resistance. Luckily the inrush current lasts only a few milliseconds but the switch (or relay) needs to be able to handle it or the contacts will burn and eventually open.
Another issue is with wire size and lamp luminous intensity. The wire resistance (in ohms) increases with the length of the run and decreases with the increase in wire diameter. The resistance introduces a voltage drop, which increases with current, between the switch/relay and the lamp. There is also a drop between the lamp and ground. These voltage drops reduce the amount of voltage available to the lamp and thus reduces the light output. An example would be to have both beams switch and running off the same wire, as is the Autowire circuit. If he current is 15A and the wire is 14 gauge, then the voltage drop for 10 feet is 0.4 volts. This will beduce the lamp output by about 10%. It might we better to run a single larger wire or two separate 14 gauge wires to reduce the voltage drop to each side.
Short answer: size the relay (or switch) to the current load and check the wire gauge to minimize the voltage drop to lamps. Also, keep the ground wires as short as feasibly possible for the same reason.
 

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Thanks Tork. Finally, a guy who took the time to do what the rest of did by experimentation with a current meter! I was too lazy/busy to post my findings. I wired my low beams to the first pull on the light switch with the running lights and the high beams on the second pull. No relays, no issues. The switch contacts are only exposed to 1/2 the current they were designed for. I let you know in a year if the headlight switch gets fried but I am pretty sure it will live a long life.
 
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