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Discussion Starter · #1 ·
Just wondering if anyone on here has installed a relay kit on your headlights. I ordered one from NPD and it really was a true plug and play. But now I have a problem with high beams, don’t have them. There is no way you could screw up the install, but if there is a way I’m sure I did it. New dimmer switch and still no high beams. Rechecked all wiring, nothin out of place. Replaced both relays and then had no lights at all. Put the original relays in and had low beams, no high beams. This is on a 66 coupe with the 200 6 cylinder. If you have any ideas, please let me know.
 

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First, isolate whether your issue is with the stock wiring or with your new relay wiring. Plug the headlights back in with the stock plug and check for both low beams and high beams. If good, you have a mistake only in your relay circuit. If you have an issue without the relays, then trouble shoot the stock wiring again.
 

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Yes I did. Bought one from Daniel Stern Lighting to get higher quality stuff. Also purchased his recommended bulbs. Works great. I think it is a lot better to NOT run power through the switch. Lights are bright!
 

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I built my own and it's working fine so far.

How many relays do you have? Can you post a pic or the relays? Or of the wiring instructions?

Rufus68 is right, you need to figure out if the problem is the car or the relay setup. How is the relay wiring tied into the car's wiring? If it's easy to get to you could just unplug the relay wiring from the car and see if you get power on the low and high beam wires there.

For example I unplugged the stock headlight pigtails and plugged my relays into the main harness where the passenger side pigtail used to be plugged in in front of the radiator by the frame there. So on mine I could disconnect the relays there and test and that would tell me if the headlight switch, wiring and the hi/lo switch are working. You should be able to do the same where you plugged yours in.

If that works then the issue is contained to your relay wiring and if you can share some pics and the diagrams for it we should be able to figure that out.
 

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68 Mustang Coupe
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What diagnosis have you done? Where does the power stop? It should be fairly simple to check.
 

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Advance notice-#hijack!

Guys,
Can you describe number of relays and how the high / low works.
I haven’t dug into it too much but I think the relay kits come with 2 relays.

How does that work? one for the pair of high beams and one for the pair of the low beams? Both sets triggered by old wires.


I’m very familiar with relays, have 9 on the car already.

Also, how is the horn wired with one?
Both horns on one relay, triggered by old wire?



Sent from my iPhone using Tapatalk
 

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One relay for low beams and one relay for high beams. I used the original low beam wire to trigger the low relay and the original high beam wire to trigger the high relay. One large fused wire from always on power(side of solenoid) to power on both relays. Two wires from each relay to the new headlamp connector. I'll see if I have a schematic at home when I get there.
 

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Also, how is the horn wired with one?
Both horns on one relay, triggered by old wire?

In stock form, the positive (hot) wire for the horn runs to the steering wheel area. This means that it is easy to cause a short if you have the horn button off and you accidentally short the hot horn wire. It would be better if the horn button in the steering wheel was completing the circuit by being the ground wire instead of the hot wire. With this said, when you add a relay for the horns, you can change the horn button to complete the ground wire instead of completing the hot wire. To do this, refer to your wiring diagram and identify the wire that is feeding +12v to the horn button under the dash. Disconnect this +12v and instead connect a ground wire to it which will feed a ground to the horn button. This means that the hot wire under the hood that connects to the horns is now a ground wire instead of a +12v wire. Use this ground to connect to Pin 85 (the relay ground) of your relay that will blow the horns. This means the horn button will actually trigger the relay. Take the +12v from the new horn relay pin 87 and connect it to both horns. Now when you press the horn button, you will complete the ground on the relay which will feed +12v to the horns. Now your horn +12v wiring is only under the hood between the battery and horns and your steering wheel is no longer hot for the horn. Win-win
 

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Discussion Starter · #12 ·
Update on my problem. It is not the relay wiring. Disconnected this morning and tried the stock wiring. Still no high beams. I have power down to the dimmer switch and going out of the switch. I have power to the low beam side coming from the main plug under the hood. I do not have power on the high beam wire. So it must be a break somewhere in the high beam wire from the main plug to the lights. This is a new wiring harness also. When I think about it, I may not have had high beams after the new wiring harness was installed. I will pull the wire out of the harness starting at the plug and working my way to the lights. If still no power, I guess under the dash I go. A wire may have come loose when I was wiring.
 

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In the case of the headlights it will give you brighter/more intense lighting by drawing power directly from the battery. As opposed to drawing it through the headlight switch.
 

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Relays are a way of isolating load from a switch or control. If you look at modern cars, many things are controlled by either the computer or by switches that are too delicate to handle a large flow of electricity. The trigger mechanism in a relay draws a small load when operating while the contacts inside the relay are capable of allowing much higher amps of electricity to pass through. So you will find lots of relays in a modern car. When our classic Mustangs were built, the switches were balanced with the loads they were controlling so the engineers allowed the switches to directly control the loads (ie, headlights, horns, wiper motors, heater blower motor, etc.) Now that our cars are so old, adding relays to the headlights both reduces the stress on the headlight switch and dimmer switch as well as enables us to run bulbs that draw more power through newer and larger wires to reduce voltage drop due to resistance in the old wires. In the case of the horn, the relay also allows us to switch the horn button from controlling the positive feed to controlling the ground thus removing the live wire from the steering wheel.
 

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The best example of a relay is the starter solenoid. You wouldn't want the full battery amperage going through your ignition switch, so instead the solenoid (which is what a relay is) handles the load and the ignition switch just triggers it.

Relays have gotten a lot less expensive so it's now easier to add them to your circuits.

I find drawing out the electrical schematic helps me understand how to wire a relay. Typically you have two sets of circuits, the low amp (switch) and the high amp, each with a power and a ground.
 

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Can those of you who have done this please post pictures of where you've put the relays.

Thanx,

Harry Z

No pic at the moment but I installed mine between the battery and the radiator support. Mounted as high as I could so I can still remove the battery when I need to. Can't really see them unless you are looking.
 

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