Electrical Headlight harness trouble

[1985fox]

Hello everyone, I am desperately in need of some help. I own a 1985 mustang gt 5.0 t5. I bought some led headlights

(https://www.hidnation.com/products/h4651-led-conversion-kit)

i also bought the h4656 2 set as well, I bought a headlight harness to go with it

(https://www.amazon.com/LIGHTING-Cer...pcontext&ref_=fplfs&psc=1&smid=A10QY6INFH3C39)

Now the problem i'm having is that my car needs 2 trigger signals, 1 for high beams one for low, the harness I bought is a gm style 4 beam that only takes one spliced signal, now I didn't know this until I came here last time but after I heard that, I went to look for a headlight harness for my car specifically and couldn't find one. The problem is that I have the ability to fix this harness I'm good enough at soldering but I don't have the know how for all this electrical stuff. How can I remedy this problem, is there a harness meant for idk a jeep or some other brand that would fix this or can I fix this harness? Now this is my plan, please correct me if I'm wrong could I buy one 2 prong male pigtail (for the 2 prong high beam plug) one 3 prong male pigtail (for the 3 prong low beam plug) and one 3 prong female for the 3 prong male connector on my harness. Now my thought is that I could match signals like Y solder the 2 prong ground to 3 prong female ground, Y solder 2 prong high beam wire to 3 prong female high beam wire and lastly, 3 prong male low beam wire to 3 prong female low beam wire. In my head this splices the 2 signals into one plug and all is well that ends well but I know how working on cars can be especially fords.

I truly am lost, any help matters. Please and thank you.

 

[AeroCoupe]

I mean it looks like the wiring harness will work. You just plug the male connector into one of the outer headlight sockets and then wire the rest of it up. The 3 wire plugs on the wiring harness would go to the outer lights and the two wire plugs would go to the inner lights.

I will say I think the wiring harness itself is garbage just looking at the relays as they do not look like they will survive being under the hood and in the elements.

 

[Noobz347]

Setup your new wiring harness by itself. What I mean is: Hook everything up according to the directions to the point where you can manually trigger each of the relays manually either by applying power or completing the ground (whichever way it is wired).

Once you get to this point (if this portion is clear) then you will use your trouble light to probe the OEM headlight harness wires coming out of the headlight and high beam switches. Once these are identified, you will use them to trigger you new relays for Power (from the headlight switch) and Power (from the high beam switch).

Hell, you can even tap the OEM wires at the headlight connectors themselves if it is convenient.

 

[AeroCoupe]

This is the wiring diagram I drew up when I put some H4 headlights in my 1983 Thunderbird in 2019. I built my own wiring harness and used Bosch 30A relays.

Mounted the relays on the back side of the front bumper:

Pictures of the headlights when I was done:

 

[1985fox]

I mean it looks like the wiring harness will work. You just plug the male connector into one of the outer headlight sockets and then wire the rest of it up. The 3 wire plugs on the wiring harness would go to the outer lights and the two wire plugs would go to the inner lights.

I will say I think the wiring harness itself is garbage just looking at the relays as they do not look like they will survive being under the hood and in the elements.

unfortunately it can differentiate the signals when turning on my high beams but not when turning them off. I am a little concerned about you saying the harness might be garbage. ive been somewhat aware of that but most people recommend making my own custom harness, and I don't know if I'm ready to tackle a project like that. I was hopeful I could do some minor modifications to the harness and be set but it also has been giving me backfeed issues to my turn signal indicators on the dash even after I cleaned up the grounds so I'm worried it might just be as you said that it's just junk.

 

[1985fox]

Setup your new wiring harness by itself. What I mean is: Hook everything up according to the directions to the point where you can manually trigger each of the relays manually either by applying power or completing the ground (whichever way it is wired).

Once you get to this point (if this portion is clear) then you will use your trouble light to probe the OEM headlight harness wires coming out of the headlight and high beam switches. Once these are identified, you will use them to trigger you new relays for Power (from the headlight switch) and Power (from the high beam switch).

Hell, you can even tap the OEM wires at the headlight connectors themselves if it is convenient.

what is the clean way to do this without cutting any of my oem harness?

 

[1985fox]

This is the wiring diagram I drew up when I put some H4 headlights in my 1983 Thunderbird in 2019. I built my own wiring harness and used Bosch 30A relays.

View attachment 740185

Mounted the relays on the back side of the front bumper:

Pictures of the headlights when I was done:

Looks really nice, how long did it take you to complete? also how did you randomly draw up a wiring diagram? this all seems quite foreign to me.

 

[AeroCoupe]

I can do simple wiring like this but cannot program to save my butt. I just think of it as completing a circuit like a light switch. I used to work at a car stereo shop back in the mid-90's so I am pretty familiar with these cars and how they are wired. So with that being said I looked back over my diagram and found a mistake that I needed to correct. Went back into my file folder for the car and found the original diagram and corrected it, scanned, and loaded it up in the original post. At the bottom I had from left to right the following:

"high beam 12V+ from switch (fuse as necessary)" "low beam 12V+ from switch (fuse as necessary)" "high beam 12V+ from switch (fuse as necessary)"

This should be low, high, low so that is corrected and I added a couple of call outs on the headlight pin outs at the top so hopefully they help with clarity.

So the first thing you need to know is how much power will the low beams pull and how much will the high beams pull. Simple math with Watt's Law which is I = P / V where I = amps, P = watts, and V = voltage. So a typical DOT legal front headlight is 55 W for the low beam light and 65 W for the high beam light. From there you simply divide the watts by the voltage and that gives you how much current each light will need. I use 12V as that will be the highest draw in most cases.

I = 55W / 12V = 4.6A and since you have two lights its 9.2A (at 14.5V this would be 7.6A)
I = 65W / 12V = 5.4A and since you will have four lights its 21.7A (at 14.5V this would be 17.9A)

From here you need to size power wire from the battery to the high beam relay and the low beam relay. Where I put mine was a little less than 10 feet away by the time it wound its way from the battery to the relays. I used an online wire sizing calculator like this one:

Wire Size Calculator

Looking back on this I could have used just two relays. One for low beams at 9.2A (call it 10A) and one for high beams at 21.7A (call it 25A). So for the high beam circuit you select 12V, 22A, and 10 feet and by looking at the chart 10 ga will work just fine as its a little less than 10 feet. For the low beam circuit you select 12V, 10A, and 10 feet and you get 14 ga will work just fine and again is a little less than 10 feet. You will need inline fuses and you can get them with 10 ga and 14 ga wire. I would use a 25A fuse on the high beam and a 15A on the low beam just to account for the inrush. You can try 20A and 10A and see if they hold up.

So that gets you the power over to the relays which I put on Pin 87. Then you can ground all of the relays to PIN 86. The 12V+ for the low beams and high beams from the original headlight harness will go on Pin 85 of the corresponding relay. Then the 12V+ output to the headlights comes off of Pin 30 of the corresponding relay.

From Pin 30 of the low beam relay you can run 18 ga wire to each low beam headlight. From Pin 30 of the high beam relay you can run 18 ga as well.

Hope this makes sense.