Tapping the main fuse box

LJDC

LJDC

TJ Enthusiast
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Feb 20, 2023
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I’ve done a fair amount of electrical work in my Jeeps, but I’ve never tapped a fuse. This is a basic question that will make you wonder if I’ve done anything at all.

In the main fuse box under the hood, every fuse currently gets power, with the ignition off. Is that right?

When I use a volt meter to test each fuse, every one registers current. I figured (for instance) the fuel-pump fuse would only get power when the engine is running. Am I just completely misunderstanding how this is supposed to work?

Inside the Jeep, the ignition wire that triggers the radio only comes on when the ignition is on. So that’s working as I expected.

In case any background is necessary, I’m using a relay to trigger power only when the ignition is on. But the relay is always on, because the signal from my fuse tap is always on.
 
Without looking, I can't think of any ignition only power sources under the hood.
If it's going to use a switch, get your relay trigger from inside the cabin.
 
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That’s where I am now—probably I’ll tap something in the fuse panel behind the glove box. But I was hoping for something that would trigger only when the engine is on (as opposed to ACC).

If I could tap a relay under the hood, that would probably do it. But I have no idea how to do that cleanly.
 
The trigger wire on the fuel pump relay would be activated when you turn the key to the start position if your dead set on this as opposed to ACC?
Either pin 1 or 2. You could pull the relay and test the sockets for power.
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Akitadog said:
The trigger wire on the fuel pump relay would be activated when you turn the key to the start position if your dead set on this as opposed to ACC?
Click to expand...

I need a trigger for things that should only run when the engine is running—compressor, fridge, etc.
 
LJDC said:
I need a trigger for things that should only run when the engine is running—compressor, fridge, etc.
Click to expand...
LJDC said:
That’s where I am now—probably I’ll tap something in the fuse panel behind the glove box. But I was hoping for something that would trigger only when the engine is on (as opposed to ACC).

If I could tap a relay under the hood, that would probably do it. But I have no idea how to do that cleanly.
Click to expand...

If you want only when the engine is running a few ways to do that. You could tie into the fuel pump trigger of the relay. I would recommend agains this as an issue in the circuit could shut the vehicle down. You may be able to tap the stator circuit off the alternator which supply power when running. It will be Ac voltage but should trigger a relay. You could also pick up a Chevy Vega oil pressure switch. This switch has two terminals which connect together when there is pressure. The third is for the light. Tee it in with the existing switch and just use the two terminals.
You could use a low voltage disconnect relay that would shut the power off before the battery was drained. This would be ideal with a second battery setup.
https://www.amazon.com/dp/B00GZOP53M/?tag=wranglerorg-20
 
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How about the AC compressor? That only runs when the engine runs…
 
LJDC said:
How about the AC compressor? That only runs when the engine runs…
Click to expand...

The fuel pump trigger would be a ground but you could tap into the wire going to the pump and that would be 12 volts. The a/c compressor will get power without the engine running it is just the belt drives it When the engine is running. I wired my compressor relay just to Acc circuit. This will be hot when the key is on but not when cranking. I just make sure I start the engine when I use the air compressor.
If I went to a fridge I may look into a second battery setup with the low voltage cutoff relay to separate the two batteries when voltage drops. I know two batteries is a squeeze in a tj. I was thinking of using a 12v battery from a jumper box as these are smaller.
 
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NskLJ said:
The fuel pump trigger would be a ground but you could tap into the wire going to the pump and that would be 12 volts. The a/c compressor will get power without the engine running it is just the belt drives it When the engine is running. I wired my compressor relay just to Acc circuit. This will be hot when the key is on but not when cranking. I just make sure I start the engine when I use the air compressor.
If I went to a fridge I may look into a second battery setup with the low voltage cutoff relay to separate the two batteries when voltage drops. I know two batteries is a squeeze in a tj. I was thinking of using a 12v battery from a jumper box as these are smaller.
Click to expand...

The only place left to put a battery is in the cargo area, and I’m not wild about the wiring. I’m also considering one of those separate battery systems, with a low voltage cutoff set very high, meaning it would cut off if left on ACC for a few minutes. Not ideal.
 
LJDC said:
The only place left to put a battery is in the cargo area, and I’m not wild about the wiring. I’m also considering one of those separate battery systems, with a low voltage cutoff set very high, meaning it would cut off if left on ACC for a few minutes. Not ideal.
Click to expand...

I think you best bet would be tap the fuel pump wire or us an op switch and a tee to trigger a relay.
https://www.amazon.com/dp/B00153AZWY/?tag=wranglerorg-20
 
I think I’m going to go searching for a place to tap the fuel-pump wire that seems unobtrusive, put a 3A fuse on it so it (hopefully) won’t cause any big draws, and use that to trigger the relay.
 
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