'90Cherokee

It does choke the system in a way, but not like you'd think of as comparing choking off a water hose and increasing the pressure. If you have high resistance that particular 'choke' point will get hot, but the rest of the circuit will have less power going through it therefore will run cooler. If that choke point is at the switch it'll get hotter, if it's anywhere else the switch will run cooler since it's flowing less power.

In my first post I specified 'light circuit' because with electric motors increasing the resistance in the circuit can actually cause the motor to draw more current and add load to the whole circuit and everything gets hotter.

cruiser54

You are over analyzing this to the nth degree.

If a harness isn't added, the headlight switch fails when the internal circuit breaker pops. Only happens at night though.

A harness is $20 and takes 30 minutes to install.

How much is a switch and the time to install? More $ and longer time.

But driving home with your flashers on a dark night is a good adrenaline rush...................


I really don't care if you don't install one, but I'm not gonna sit idly by while you may be influencing others to shy away from a proactive procedure to insure theirs and others' safety at night.

Biggsly

Thank you. I will probably order one.

'90Cherokee

I was in no way influencing him or anyone away from doing the upgrade, I was merely passing on a small electrical lesson.

I'll probably be doing both of my XJs.

extrashaky

That's where I got mine as well.

No, you're thinking of Ohm's Law. That gives you the capacity of the circuit. If you increase resistance, you don't decrease the load. You decrease the capacity of the circuit to carry the load.

To find the load, you use

Amps = Watts/Volts

For example, a 35W bulb in a 12 volt system pulls 2.9 amps, whereas a 55W bulb draws 4.6 amps.

If you try to pull 2.9 amps through a circuit whose resistance has increased over time so that the capacity of the circuit can no longer handle it, the light will become dim from voltage drop, and the circuit will generate waste heat wherever the resistance is greatest. If you try to brighten up your lights in that circuit by swapping in a 55W light pulling 4.6 amps, you'll increase the waste heat and the likelihood that the circuit will fail.

Fuses take advantage of this phenomenon. A fuse will have a section of wire or filament with greater resistance than the rest of the circuit it's trying to protect. If current draw exceeds the capacity of that filament, the heat given off will melt it, breaking the circuit.

You're risking melting your headlight switch instead.

cruiser54

Yep. All theory and electrical mumbo jumbo beside, these are the facts.

Proven time and time again in the real world.

toddreg

This has turned into a rather informing thread. Which has prompted me to now raise the question of what gauge wire you need to carry the needed voltage for E-spec 55/65 H4's? Does it matter?

Secondly one of the considered harness's has a separate wire to the battery for each headlight, the other has one. I guess the power is split in one of the connectors.

While I am at it, on the Putco harness is the fusible link the sections of wire that go from the first connector to the battery? If I want to put a fuse in where is the best place?

cruiser54

Do a supplemental harness and you're fine. I have 90/100s in my white Comanche.

http://www.ebay.com/itm/CERAMIC-H4-H...-/330997592807

Tbone289

EDIT: see comment in my next post

It's not the voltage that matters, it's the current. 16ga is sufficient for 65/55. You can "get by" with 18ga on a short run, but you will see some voltage drop due to resistance.

Current:
55w/12V x2 = 9.17 amps
65w/12V x2 = 10.83 amps
80w/12V x2 = 13.33 amps
100w/12V x2 = 16.67 amps

Wire gauge chart: Ignore the wattage shown below, and use current from above

Tbone289

Correction: The amperes I've listed above are double what is required for each headlight, so an 18ga run from the relays will be sufficient to each headlight.

The charts above will show you what wire gauge and fuse sizes you need for the power wires to your relays if you replace the fusible links with fuses.

cruiser54

Yep. It's a short run when you add the harness, compared to what it was from the factory.

Biggsly

Okay.
So out of these two, which is the best, or does it matter?

https://www.ebay.com/itm/CERAMIC-H4-...807?rmvSB=true

https://www.ebay.com/itm/Jeep-Cherok...8AAOSwCcZZ5rIr

Tbone289

They're basically the same. Between the two, I would buy from the seller who has 100% feedback. He's a Cherokee guy anyway.

jhc7399 = jhc6635?

Biggsly

Good point.
I just ordered it.
Thanks guys.

Ricksxj

I too just ordered one of these harnesses.

I'm wondering if there is anything special to installing the harness.
Do they come with instructions?
Do I even need instructions?