Rookie electrical question

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Wagon of Death

Aussie Barracuda Fisherman
Joined
Aug 30, 2009
Messages
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Location
Brisbane,Aust
Im fitting relays to my headlight circuit. Does it matter how far from the headlight the relay is? Want to be fitting them under dash and running cable forward...For the record Im probably going LED ...Plan B is something on the front of the rad support but hoping this is unnecessary..
Thoughts?
 
Rad support. Shortest distance to battery and lights. Trigger wires can be any length. There is someone here that makes up a kit, simple install.
 
Assuming this is going to be a street caer with an alternator.

Here's the thing to understand.
There are two power sources, right?
But the one can only supply power at 12.8 Volts in the best of situations.
The other can supply power at 14 volts give or take a little.
So between the two possibilites, electricity always flows from the higher voltage source, as long as it has enough power and the resistance is about equal.

The ammeter is in line with the battery.
It only shows current flowing in or out of the battery.
basic-charge-circuit-charging-animated-gif-gif-gif-gif.gif

View attachment 1715613174

After the battery gets recharged, the flow through that line is essentially zero.
upload_2019-9-27_20-48-1-png-png-png.png

View attachment 1715613175

How you design your trunk mounted battery system makes a big difference on determining where to tap into power for a headlight relay circuit.

In general, on a stock electrical system the best place (in terms of least resistance) to draw power for forward lights is at the alternator output.
 
I was going to run the battery cable to a pole under the dash, that goes through the firewall also...Power starter from engine bay side and alternator wire to this pole in the engine bay also ( Im looking at "hiding the wiring")...The bulkhead fitting will be under the dash and all the wiring for the lights will be run through conduit in the wheel arch and out to the front... I anticipate powering the dash, switches, relays etc from the pole under the dash...hope that makes sense...is there anything in there that sounds like it could cause major issue?
 
Moving the high current fire hazards to inside the cabin is opposite of how newer vehicles are wired. Relays keep large wires and 120 amps from their alternators outside the cabin. Everything inside is small wiring and fuse protected. You can walk around a yard like LKQ and find quite a few engine bay fires. The human beans didn't burn (pardon the pun).
My old Plymouth already has too much high current inside the cabin and not nearly enough fuses.
 
LEDs don’t pull much current, so I would suspect it doesn’t matter. If you use large enough wire and don’t use the bulkhead connectors, you can put them anywhere you want.
 
I was going to run the battery cable to a pole under the dash, that goes through the firewall also...Power starter from engine bay side and alternator wire to this pole in the engine bay also ( Im looking at "hiding the wiring")...The bulkhead fitting will be under the dash and all the wiring for the lights will be run through conduit in the wheel arch and out to the front... I anticipate powering the dash, switches, relays etc from the pole under the dash...hope that makes sense...is there anything in there that sounds like it could cause major issue?

Get rid of the bulkhead fitting if you can. At least bypass it for the charging system. And bypass it for the headlight circuits.

In other words put the relays as close to the lamps as you can with larger gauge wire.
 
Hooking up accessory items for power on an OEM style wired mopar at the battery is the WRONG way to do things most of the time.

You want power wire runs as short as possible. Source to the powered item.
 
67 Dart...this was the first thing I did....deleted the amp gauge too, Ill run a separate Volt gauge to keep an eye on things...And Cracked...definitely wont be pulling any accessory wires from Mother Mopar harness ...The battery cable does run iinside the cabin...I have a circuit breaker at the battery, and everything will be fused and relayed under the dash, dont want my Barracuda to be a roadside BBQ...thankyou gents
 
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What poster #12 said. LED lamps draw very little current [ compared to incandescent factory type lamps], so mount the relay where it is convenient.
 
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