Adding headlight relays

[KdkGrizz]

I am planning on installing relays for low and high beam headlights on my 82 Waggy. I had read a few articles about this. Only one thing confuses me a bit. The placement of the relays as it relates to the battery/power source. Most articles don’t talk about this at all. But a couple were adamant that the relays need to be as close as possible to the battery with short leads. Just wondering how critical this is. I mean you have to run wires from one side to the other to access the headlight harness. But a single 12gauge is a lot easier than 2 14 gauge loops.

Interested to hear opinions and experiences

TYIA

[tgreese]

You want a single large gauge wire to feed the relays (say 10 ga), and place the relays as close to the lights as possible. Protect the feed wire with a fuse or circuit breaker, as close to the battery as possible.

The main difference between big and small wire is the resistance per foot. Big wire has much lower resistance than small wire. Make the small wire runs as short as you can, and make the big wire runs as long as needed to make the small wire runs short.

The headlight wires go across the driver's side inner fender. That's where I put my relays. My feed wire comes from the battery side of the starter solenoid, immediately to a circuit breaker, then from the right inner fender, across the firewall, and up to the front of the left inner fender where the relays are.

[joe]

""relays need to be as close as possible to the battery with short leads"

I've never heard that theory before and always been under the impression you want the relays closer to the accessory. Don't know where over the years I got that impression but I've also never strictly adhered to it and generally mounted the relays where most convenient/easy access. On cars/trucks usually a fenderwell, On bikes, closer to the batt area under the seat to keep them semi-hiden/protected. For the trigger wire I've tapped off from any low current 12v source. For the high current feed wire I go from an aux fuse box. I agree with Tim that you do want the feed wire fuse close to the originating source otherwise it don't protect much.

[KdkGrizz]

Thanks

Great info from everyone. Thank you very much.

I like the idea of a separate trigger wire on a lighter circuit

[nograin]

[quote=KdkGrizz]I am planning on installing relays for low and high beam headlights on my 82 Waggy. I had read a few articles about this. Only one thing confuses me a bit. The placement of the relays as it relates to the battery/power source. Most articles don

[SJTD]

You generally have one wire to the relay then two wires from the relay; one to each light. So one would want to minimize the length of the single wire feeding the relay.

Unless you run a bigger wire to the relay or two of the same size as the wires to the lights. Then it wouldn't matter as far as voltage drop is concerned.

So using 10 ga to the relays like Tim suggested and putting them on the fender will be fine. You're getting the dash switch out of the picture and cutting out four of five feet of the original wire. Probably more than that on the high beams.

[tgreese]

...
When the engine is running, the alternator produces power at 14.2 Volts. The battery is only at 12.8 Volts. So it only provides power if the alternator can't keep up and system voltage drops. ...

Not correct. The potential across the battery when running is whatever the alternator is charging at, minus the voltage drop due to the wire resistance between the alternator stud and the battery. Measure it.

If the headlights are connected near the battery,
power is going to have to go from the alternator through the firewall,
to the main junction (a big splice) to the ammeter,
to the firewall,
and then through the fusible link.
For vehicles wired like ours, the shortest route is direct off the alternator. ...

Correct, but I would strongly advise you to bypass or remove the ammeter and connect directly from the alternator stud to the battery. Then the voltage drop from the alternator to the battery will be negligible, if it were not already. This is covered ad nauseum in the sticky thread about ammeter bypass.

[nograin]

That's a misconception. The battery potential that we see with the engine running is simply the system voltage. As soon as the alternator stops providing that 14.2 volts, the voltage drops to the battery's. There's a small, temporary, charge on the plate surfaces around 13 something volts, but there's no stored power to speak of on the surface charge.

The ammeter should show no current flowing to or from the battery once the battery is charged. In other words, normally no power should be flowing through it. Removing the ammeter does not make a route nearly as short as attaching to the alternator output. All removing the ammeter would do is reduce the number of connections by one.

The number of people who have got trouble from misinformation about the ammeter and its place in the circuitry makes it worth repeating.

• The ammeter simply indicates whether the battery is charging or discharging.
• The fusible link protects against battery short to ground.

If the wires and connections are not asked to do more than that, and in decent condition, they'll be fine. An electric winch is an example of extra equipment with power demands that will draw on the battery. This is when some rewiring is a good idea.

[nograin]

Here. I know it can be hard to visualize. Here's a battery that didn't have enough power to start the car. It showed around 12 Volts with no load and less than 12 when attempting to start.

Voltage measured at the battery with a battery charger attached.

14.19 Volts in this case is driven from the charger. Since the battery was low, its sucking a lot of current, even at this modest voltage.
Over 25 amps, as seen below. This is the current that would be going through the jeep's ammeter if charging power was coming from the alternator at 14.2 volts.


Since batteries don't like high charging rates, especially AGMs, the charger setting was dialed back to lower current. Guess what the voltage did? It dropped too.


The battery didn't have 14 Volts in it. It was connected to a power supply that did. When that was reduced, the voltage at the positive terminal went down.

[KdkGrizz]

Well this is what happens when you get distracted

https://www.dropbox.com/s/5yp2fmemdxbdygz/Photo%20Oct%2011%2C%208%2023%2009%20AM.jpg?dl=0

Life lesson here folks no matter how much you tidy up your wiring and make it look all pretty. If you forget to secure it away from hot things like say exhaust manifolds your whole day can be ruined and possibly expensive...

Hopefully when the solenoid fried it did not back feed into my MSD box or my EFI ecu.

Hey but the new relays on the headlights worked great ��

[nograin]

That sucks.
I don't know what happend to my post from last night. (#5). Tim saw it and quoted from it, but now its destroyed.


The point of it was that power comes from the alternator, and on all SJ's through 1985, the alternator power goes to the main junction first.

The shortest route for relay power is to attach to the alternator output stud.


This also keeps the circuit from the main junction to the solenoid for just starting and recharging.


'85 Charge - Light Circuit

[tgreese]

Here. I know it can be hard to visualize. Here's a battery that didn't have enough power to start the car. It showed around 12 Volts with no load and less than 12 when attempting to start.
...

The voltage at the battery depends on the resistance between the battery and the alternator stud, and the amount of current the lights require, and the set point voltage of the regulator. Ohm's and Kirchoff's laws still apply. The voltage at the battery will always approach the voltage at the alternator stud, exactly less the resistance drop between the battery and the alternator stud times the current drawn by the ignition and accessories.

Stay away from those hot manifolds!

[nograin]

I basicaly agree Tim.

Its just that in a stock system, there is no current flow between the main junction and the battery once the battery is charged. So the voltage at those two points will be the same. It doesn't matter if the heater fan, lights and wipers are on, as long as the alternator can produce enough current to meet those needs.



'86 and up is different in that the main junction function was mostly relocated to distribution at the solenoid.

On these, attaching relays at the solenoid rather than the alternator stud means the power comes through a fusible link.

But on an '85 and down, attaching to the solenoid means current for the headlights must run through the firewall to the main junction (same as stock) then through the ammeter, back through the firewall and then the fusible link. Yes it avoids the headlight switches, which is good, but adds a constant load on the charge wiring, and that's not so good.

[nograin]

1986 and later variants sent current for the headlights through two fusible links (the green only 18 gage) then to the stalk based headlight hi/low and then back out to the headlights. It's a long route with lots of connections!

[nograin]

Hopefully when the solenoid fried it did not back feed into my MSD box or my EFI ecu.

When running EFI, electric fuel pump, etc, then the considerations mentioned about the headlight relay source are amplified. This equipment runs 100% duty cycle during both run and start. As mentioned, in the earlier wiring, the charge circuit was really only for starting and charging. Yes it will support running on the battery if the alternator dies, but that's a rarity.



This is the type of situation where modifications to that early wiring strategy can reduce the current load through the firewall connections, fusible link, etc.