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Discussion Starter · #1 ·
I plan on installing a boost gauge, A/F, and oil pressure gauge in my center dash and I plan on running the wires down to the passenger side fuse box. It seems most people use add-a-circuits for 12V constant, dimmer, and switched but my question is can you run all 3 12V wires into one add-a-circuit?

If so, what amperage fuse should I use?

If not, what other fuses can I use for the constant, dimmer, and switched 12V?
 

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Re: add-a-curcuit question

You cannot run them all to the same circuit.

By definition switched != constant!

You'll need to tap the dimmer wire, most likely behind the HU. The constant can go to your add-a-circuit in the fuse box. There are several places you can go for the switched power (radio harness, cigarette lighter to name a couple).

I'm not sure what your gauges are rated for current draw. They should provide this info somewhere in your documentation. Just user a fuse slightly higher than the sum of the rated current for all gauges. If I had to guess, I'd think it's not much and even 5 amps would be enough.
 

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Discussion Starter · #3 ·
this was my understanding of the voltage sources:
12V constant - 12V draw even when car is off, keys are not in ignition
12V switched - 12V draw when car is on (acc or engine)
dimmer/illumination - used to dim gauge when headlights are turned on

so then for 3 gauges i would need 9 add-a-circuits?

i feel like many people have already wired 3 gauges simultaneously, there must be a simpler solution.
 

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couldnt you just hook them up to a switch or the cig lighter?
 

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You do not need 9 circuits.

One add-a circuit can supply 12v constant to all three devices.

One connection to the lighter/acc/whatever will provide 12v switched to all three. One connection to the dimmer can be connected to all three.

You need three separate connections.

You've got the concept right, you're just looking at it from the wrong direction. 12v constant will provide a constant source of voltage. 12v switched will only provide power when it is switched on (key in acc position). It doesn't matter when the device is trying to draw power, it is when the voltage is available. An add-a-circuit to the fuse box is a constant 12v source. If you hooked the wire intended to go to a switched source to a constant 12v, the gauges would work fine but you are likely to have problems with a run down battery because the device will stay on even when your ignition is switched off.

The dimmer signal is required so that when you turn on your headlights the gauges either light up or dim (depending on the configuration). If you hookedf the dimmer wire to dsomething else, your gauges would be too bright at night or not illumined.

Make sense?
 

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Discussion Starter · #6 ·
yea that makes perfect sense.

i'm sure the constant 12V is for memory like high/low warnings etc and won't use much current, but what fuse can i tap into for that? i've been told the door lock fuse is constant, which makes sense because even when the car is off the locks need power to unlock via the remote.
 
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