Fuse connection question. - VTXOA
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post #1 of 7 (permalink) Old 04-24-2017, 09:51 AM Thread Starter
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Fuse connection question.

I'm installing some Boss mc420b speakers. It asks for a switched 5 amp power source. Using the only 5 amp fuse spot it gets a constant power supply. Hooking it into the 10 amp spot (ignition/starter) it only works when I turn the engine on, which is what I want. Is it safe to plug the speakers, asking for 5 amp source, into the 10 amp spot or should I just go ahead and toggle it and use the 5 amp spot?

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post #2 of 7 (permalink) Old 04-27-2017, 02:24 PM
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The fuse is to protect the wiring. If a short occurred, wire rated for 5 amp may melt/burn instead of blowing the fuse.
Hijacking any 5 amp fused circuit was not what they meant.
On a 10 amp fused circuit you should add a 5 amp fuse inline to the amp.
The IGN/Starter fuse is for the starter relay and the Ignition Control Module.
If your add-on wiring shorts or the amp chip shorts out it could kill the engine.
Do not add anything to a critical circuit: headlight, brake, Ignition.
The meter/taillight fuse also feeds the optional fog light connector (2 x 25watt bulbs).
If you do not have Aux/foglights you could then share power from that circuit.
Best choice would be a new fuseblock fed by a fuse and relay.
The relay could be triggered from the meter/taillight fuse.

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post #3 of 7 (permalink) Old 04-27-2017, 05:27 PM
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I suggest installing a fuse block with common "hot"/ground, and then use a relay triggered for the switching so it only comes on when the ignition is turned on. I installed a bolt on my fuse block to use as the common ground, and used the other bolt that came on it as common "hot" to each of the fuses. This way, you can put in whatever size fuse you need for each circuit (up to 30A total), and still have them "switched" using relays, or you can have them constantly "hot", like for a cigarette lighter plug. You can also easily connect up other accessories to the fuse block, too.
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post #4 of 7 (permalink) Old 04-27-2017, 07:38 PM
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You "can" absolutely install your connection on any fuse point that is switched by ignition. You would want to tap in on the "hot" side of the fuse (the side that still has power with ign on when fuse is pulled)... and then be sure to use an inline fuse to your added wiring. Your fuse needs to be close to the power source, the fuse block. This is fine for small current draws, since the blade contacts are being shared. It is "not the best way" to do it, but it is somewhat adequately safe under light duty applications.

If the circuit you are on pulls 5 amps and has a 5 amp fuse, then adding an additional 5 amps (10 amps total on the output side of the fuse) would certainly blow it. Changing the fuse value to a higher value would not protect the circuit as designed. You need to tap the "hot" side of the fuse, if you go that route.

Try to avoid "glass" inline fuses.. the blade types are more reliable.

For higher current applications, or the more reliable way, it's better to go to the key, or a good connection point directly from the key.


Last edited by Mmusicman; 04-27-2017 at 07:43 PM.
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post #5 of 7 (permalink) Old 04-27-2017, 08:42 PM
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Good point Mmusicman.
My GPS is connected to the Tripmeter memory on the Hot side (Red)with a 2 amp fuse.
I carefully cut away 1/4" insulation, wrapped a few turns of new wire over the OEM and soldered.

Whatever you do, do not use a crimp-on wire splice.
It scores the strands and the wire usually breaks.

I believe that happens because the added wire is not taped anywhere and it vibrates.
High failure rate on 1800 TPS wire because that was supplied by Power Commander.
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Last edited by Hans&Feet; 04-27-2017 at 09:00 PM.
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post #6 of 7 (permalink) Old 04-28-2017, 12:21 AM
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As an added bit of protection when using a fuse block, use the Easy-ID fuses. When they blow, they have a 'light-up' feature to eliminate hunting which one is bad. LOVE 'em!

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post #7 of 7 (permalink) Old 06-26-2017, 09:37 PM
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You do not need a relay for something only using low amps. Relays got really popular because the power to the headlight goes through the start switch so that it dims when starting. A relay avoids that high use headlight wire from going through the start switch which has a tendency to fail. I agree with Mmusicman advise.

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