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Using an add-a-fuse (or fuse tap) can be a quick and easy way to add an electrical circuit to your car, but there is a right way and a wrong way to use them.

This is the product in question:





The premise is, you remove an existing fuse, plug that removed fuse and an appropriate fuse for the new circuit into the device, and then plug the device into the slot from which you removed the fuse...and use the pigtail wire on the device to power your new circuit.

But to use it correctly, it has to be plugged in in the right orientation.



With reference to the above diagram:

The red line indicates that, with no fuses installed, terminal (or leg) A is connected to BOTH of the two slots above it, while terminal B is only connected to the slot immediately above it.

Now, in your fuse block, if you pull a fuse, one of the slots will have 12V+ (the hot side) and the other will not (the load side).

You have to plug terminal A of the add-a-fuse into the hot side (i.e. the add-a-fuse only works properly one way round), here's why:



Once you install fuses in the add-a-fuse, each pair of sockets is connected (the blue lines). Thus, if we apply 12V+ to terminal A, power flows 1) From terminal A, through the lower (fuse 1) fuse, and to terminal B...this completes the original circuit, independent of the added circuit. Power also flows from terminal A to the left-hand socket of the top (fuse 2) fuse, through fuse 2, and out the pigtail wire. This is the proper way, where both circuits are independent and the fuse sizes do not need to be matched. This is illustrated by the pink arrows.

However, if you plug the add-a-fuse in the other way around (the incorrect way), with terminal B being plugged into the hot side of the terminal in the fuse box, you get the following:



In this situation, 12V+ will flow from terminal B, through fuse 1, and out terminal A, completing the original circuit, and misleading you into thinking everything is OK. BUT...because there is no connection between terminal B and the right socket of fuse 2 (the top fuse), 12V+ will only flow out the pigtail wire THROUGH fuse 1 in series. Again, this is shown by the pink arrows. As such, it is quite possible that the combination of loads on fuse 1 will blow the fuse. Also, if you install the add-a-fuse this wrong way around in an open slot in your fuse box, you have to install BOTH fuses in the add-a-fuse even if you are only powering the one new circuit (this is a big clue you have it the wrong way around)

Bottom line - you have to plug the add-a-fuse in with leg or terminal A on the hot side in the fuse block slot.

By the way - in the 124's interior fuse block, the LOWER of the fuse slot pairs is the hot side, so add-a-fuses should be installed with the pigtail wire pointing up.
 

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I'm not sure how you worded that last sentence Bill, but the orientation depends on which row of fuses you are using. This diagram I made for my article, should clear it up.
71883
add-a-circuit22[1].png


(I only inserted the image once, so I don't know why your blogware displays it twice?)
 

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Billavista - many thanks for this one. After hard-wiring my radar detector and adding a lighted wind deflector, both using the add-a-fuse option, my radio, touch screen and nav went on the blitz. I tried several unsuccessful alternatives before remembering this post. After reading it in detail, I went out to take a look, and sure enough, I had both add-a-fuse circuits installed the wrong way. I flipped them around and I'm back in business. Thanks very much for your expertise and your willingness to share with the rest of the group. Much appreciated!
 
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