How to determine switched or unswitched fuses in footwell fuse box

[Bossdog]

Without sketching out a circuit, I'm not convinced that your added device is truly fused when you run the add-a-fuse wire forward instead of in the rearward direction. I recall testing for voltages with/without fuses in place, to verify that the load is at the right-hand side of the fusebox as you face it.

IOW, if the main fuse blew, then your added device will still be powered even if its add-a-fuse blew.

I always run the wire facing the rear, and sometimes it's tricky to get clearance for it.

if the hot side is the right side as you look at the box, I think it’s correct as pictured.
the power has to cross over the fuse to exit the wire to the left.
It surly is important to get this part correct or you can burn your car down.

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[GTP]

if the hot side is the right side as you look at the box, I think it’s correct as pictured.
the power has to cross over the fuse to exit the wire to the left.
It surly is important to get this part correct or you can burn your car down.

I was saying that my memory tells me the hot side is forward and the load side is rearward. Opposite of what you posted.

 

[Bossdog]

I was saying that my memory tells me the hot side is forward and the load side is rearward. Opposite of what you posted.

I guess I don’t understand “hot side” and “load side” To me, I would used those terms interchangeably, but I know almost nothing about electrical circuits.
So I guess I’m getting ready to learn something.
when I ground my black lead on my multi meter and put the red lead on the rearward side with no fuse, I’d expect to read 12 volts. When I move the red lead to the forward position of the same fuse opening, I’d expect to get nothing.
I appreciate you taking time to school me on this. As I said, it’s important to get this correct.

 

[GTP]

I guess I don’t understand “hot side” and “load side” To me, I would used those terms interchangeably, but I know almost nothing about electrical circuits.
So I guess I’m getting ready to learn something.
when I ground my black lead on my multi meter and put the red lead on the rearward side with no fuse, I’d expect to read 12 volts. When I move the red lead to the forward position of the same fuse opening, I’d expect to get nothing.
I appreciate you taking time to school me on this. As I said, it’s important to get this correct.

If it behaves as you say above, then the load is forward and the hot is rearward. I just am remembering that it was opposite for me.

The issue is if you get it wrong, then you haven't protected your device. It will be connected to +12V whether the fuse is good or blown.

 

[NGOT8R]

Just to clarify, I went out and double checked this on my car today. I used a gutted 10A test fuse in the add-a-fuse accessory port and a good 10A fuse on the vehicle’s electrical circuit side of the add-a-fuse. Note: I could also see the indicator light on my accessory in my glovebox (which was off). I then installed a good 10A fuse in the accessory side of the add-a-fuse and the indicator light on my accessory was on, thus powering up the unit. Lastly, I removed the 10A fuse from the vehicle’s electrical circuit side of the add-a-fuse and left the good 10A fuse in the accessory port of the add-a-fuse and the light on my accessory remained out. This tells me that the orientation of my add-a-fuse is correct.

 

[GTP]

I went online and found a good resource that explains the situation really well.

TLDR - While both the main device and added device will work with either direction of the add-a-fuse lead wire, there is a good reason to identify the source and load ends of the fusebox, and route the lead wire properly.

1.
This image is the correct configuration and is 1 of 9 images that steps you through all the various combinations. The yellow X's indicate live circuits. Note that the only connection between the main (green) and added (blue) fuses is at the end of the add-a-fuse block, i.e. opposite of the lead wire end.

2.
This next image is when you route the load wire in the "wrong" direction. Note everything still works provided the main fuse is intact. But to get to the new device, the current must go through the main (green) fuse first. Which means if the main fuse blows, both that device and your new device will not operate. This is also a good reason for the main fuse to be at least as high of current rating as the new device fuse. This is because the main current must handle the current load of both the main device and the new device. The yellow X's indicate live circuits.

3.
Finally, this image is the same case of "wrong" direction AND the main (green) fuse has blown. You can see that the current path for both devices has been broken and so neither device will operate. The gray X's indicate dead circuits.

 

[Bossdog]

@GTP , thanks so much for your patient and knowledgeable input. I'm headed out today to try and wrap up this diff wiring project

 

[Bossdog]

@GTP , you are correct, the hot side is forward. When I measure the forward leg to ground, I get 12 volts. When I measure the rearward to ground, I get nothing.
The add a fuse I have has a continuous path on the leg opposite the wire exit. That would indicate I need to place the "add-a-fuse" upside down to have the fuse on the path/flow of the energy.

I'm glad I persisted and double checked.

 

[Bossdog]

@NGOT8R , you may want to double check your connection, as far as I can tell, and I'm a electrical "Novice", I need to install my "add-a-fuse" with the fuses facing down to keep the fuse in the flow of the electrical current.
I could be completely wrong but that is what the info I'm getting from my Multi meter is telling me. I had my neighbor who manages the service department for A marine engine building company double check and help with my sanity check and he got the same.

 

[GTP]

@GTP , you are correct, the hot side is forward. When I measure the forward leg to ground, I get 12 volts. When I measure the rearward to ground, I get nothing.

This is consistent with what I recall determining a few years back.

The add a fuse I have has a continuous path on the leg opposite the wire exit.

Correct. Of the two fuse rows, they connect at the block end, but do not at the wire end.

That would indicate I need to place the "add-a-fuse" upside down to have the fuse on the path/flow of the energy.

Incorrect. The add-a-fuse lead wire must point towards the load side. Which means it must exit out the rearward side of the fusebox.

 

[Bossdog]

With the add-a-fuse plugged into #35, fuses pointing down, wire exit pointing rearward. the add-a-fuse does not seat fully, only about 3/4 deep into the socket due to the exit wire coming in contact with the housing of the fuse box. It seems fairly solid but I'd prefer to have a full fuse seating for the Diff cooler.
(the 3/4 seat of the add-a-fuse will be fine for the oil temp sensor)

On to unused Fuse # 22, it's a triple.
My assumption given that the Micro 2 fuse is the same size as 2/3 of the triple, we can use 2/3 of this fuse socket with a Micro 2 fuse and leave 1/3 empty? is this correct?

Checking what is hot, the middle of the triple seems to be the hot.
So, my conclusions so far are, I would install the "add-a-fuse" fuse in #22, as follows:
- using the forward 2 openings of the triple fuse, the add-a-fuse would be installed fuses up, wire exit toward the front of the vehicle.
- using the two rearward fuse openings, you would install with fuses pointing down, wire exit pointing to the rear of the vehicle.

It looks like #11 is a triple with the two rearward openings being used

Sure would be nice if we had an automotive fuse box designer in the crowd, especially from Ford!

 

[Bossdog]

Correct. Of the two fuse rows, they connect at the block end, but do not at the wire end.

Incorrect. The add-a-fuse lead wire must point towards the load side. Which means it must exit out the rearward side of the fuse box.

I think we are on the same page. To have my add-a-fuse exit wire point to the rear of the car when I plug it into the fuse box, the two add-a-fuse openings are pointing down

 

[GTP]

@Bossdog interesting extra info.

And yeah, I get it that it is tough to get those blocks to fit into the OEM slots. Tight fit there. Plus it's tricky to route the exit wire sometimes.

I do not know about the 3-prong fuses. If the center leg was hot and each side a load, then yes, you could route the exit wire towards the front.

 

[Bossdog]

It looks like the other half of fuse #22 triple is fuse # 21, which is being used so if I'm going to use unused 22 (triple) I'll need to get a Micro 3 add-a-fuse. UGH! this is way more complicated than I expected.

 

[GTP]

It looks like the other half of fuse #22 triple is fuse # 21, which is being used so if I'm going to use unused 22 (triple) I'll need to get a Micro 3 add-a-fuse. UGH! this is way more complicated than I expected.

You are trying to power your diff pump, correct? Anything else you need to power?
My car has a dashcam and diff pump that are both on switched circuits via add-a-fuse wires. So I have 3 add-a-fuse wires in there.

I am redoing my dashcam soon, and can then post some pics, but that doesn't really help you today.

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