Ralph77

Pretty much what the title says.

Been pouring over fuse box diagrams on the internet and can't find what I am looking for.

Going to try and put a auto dimming mirror from a WJ in my XJ.
Out of the 5 wires only care about 2. Power and ground.
The write up I am using says to wire the power to a ignition on only circuit. Which makes sense.
What he does not mention is anything about using an inline fuse.

So I am wondering what amp fuse is used for the auto dimming mirror?

Thinking not much.
All the Junction Box and PDC diagrams I look at don't seem to list it.

Noah911

I believe a 10 AMP fuse would work. The auto dimming rearview mirror recieves 12 VOLT power from a fused ignition switch circuit. The fused ignition-(run)/start circuits are of the 10 AMP variety. I am not sure which one of the pins off C1, C2, C3, or at the BCM.. for where the wiring runs on a factory installed auto dimming rearview mirror unit from a WJ to look and see?



This diagram has the exact circuits labeled on it though...

The dark blue/white wire and the black wire are the two most important wires of the unit. The dark blue/white wire should get +12V with the ignition on only (not a constant +12V). The black wire is for a ground. Running the violet/black wire to a +12V reverse lamp feed will automatically turn the mirror dimming feature off when you put your Jeep in reverse. The remaining two wires they run to an optional driver side auto dimming side mirror available on the WJ.

Ralph77

I have seen that diagram in the write up I am using to do this.
All I am using is the wires in cavities 1 and 2.
Dark blue/white and the black one as you pointed out.
Also aware it needs to be to a ignition on only source.
I just figure you WJ guys have a fuse that is labeled for the mirror somewhere.
Junction Box, PDC, ect.
I just figured I would use the same amp fuse inline when I wire this up.
Just not seeing it on any fuse box diagrams for WJ's that I am looking at.
One guy who did this on another forum said you just spliced into one of the wires in his overhead console.
Ignition on only one and with no fuse. Also the write up does not mention a inline fuse either.

BlueRidgeMark

It probably does not mention an inline fuse because it assumes you are working with a fused circuit. That's a poor assumption.

I suggest you start small and work up. Use a 5 and see if it holds. As long as it does, you are good. If it blows, go to a 10.

I also suggest you use a relay. Find an 'ignition on' source and use that to trigger the relay, but source the power from a constant on circuit that has enough beef to be adding a load to it. Generally speaking, battery power circuits are rated higher than ignition on circuits.

Just because some clown tapped into an existing circuit and wrote it up does not mean it's a good idea.

Ralph77

Somebody on another forum posted schematics to a WJ and appears to be a 10 or 15 amp. Most likely the 10 amp with other stuff on the same circuit.
Not a dedicated line.
Also somebody on another did this and said they just tapped into one of the wires in their overhead console. No mention that they used a fuse.
I have an open fuse slot that in the Junction Box that I believe is only hot with the ignition on.
So I was thinking I would do what you suggested if I do use a fuse. Start with a 5 and go from there.

BlueRidgeMark

Do you know the gauge of the wire that feeds that slot? Do you know it's rated to carry that current?

Ralph77

Appears to be 20 gauge according to the schematic.

This question I don't understand.

BlueRidgeMark

You have a device that requires a certain amount of power to operate. Power is voltage X current. We know the voltage (12v), so we now need to know the current required. That's why we are trying to figure out the fuse size. If the mirror draws 14 amps, a 10 amp fuse isn't going to work. If it draws 6 amps, a 10 amp fuse is just fine.

Like fuses, a given length of wire can carry only a certain amount of current. Exceed that, and things start heating up, eventually melting and possibly catching fire. You must match the wire size to the current it's going to carry. In your case, you think the mirror may be in the neighborhood of a 10 amp load. That means you need wiring that can handle that 10 amp load. I can tell you without looking it up in a chart that a 20gauge wire is not going to tolerate a 10 amp load. It's going to overheat. Maybe not today, and maybe not tomorrow, but soon, and for the rest of your life....

Oh, wait. How did Bogart get into this???

Anyway,, not a good plan. You can use that circuit to trip a relay, as mentioned above, but don't plug that mirror into a slot that is fed by a 20 gauge wire.

Relays are the smart way to add loads. Unless you really know what you're doing, don't just go randomly plugging things in or tapping into existing wires. That's asking for trouble.

Ralph77

OK. All that I understand.
Thing is according to the schematics there looks like a ton of other stuff on that circuit.
It is not on a dedicated circuit.
Cause I seriously doubt an auto dimming mirror draws much. But then again what do I know.
Somebody on another forum was nice enough to post schematics for the mirror, and related parts, in a WJ.
Maybe you can give them a look?

https://www.cherokeetalk.com/threads.../#post-1156785

Also am I wrong in the assumption that while bigger gauge wire might not be necessary it can't hurt?

Noah911

I understand and see why. I believe Mark was definitely correct in saying it was a poor assumption made on my part in referencing the 10 AMP fuse how I figured it.. to say the least.

By looking over the schematic; seeing as it is only a 20 gauge wire size being used. I believe it is safe to assume and say it could only be a 10 or 5 AMP fuse.

The maximum current of 20 wire gauge size (AWG) at 20*C is up to 11 AMPS. (18 gauge = 16 AMPS, and 22 gauge = 7 AMPS).

I believe since this is not a dedicated circuit for only just the mirror, it would be safe to assume using a 5 AMP fuse is the most logical and safe conclusion. I do not think using a 10 AMP fuse would be sufficient enough in being able to achieve the necessary adequate protection.

I think you could probably get away with using a 3 AMP fuse in this application. If so, the smaller fuse would provide you with an excellent level of protection. Using this as a starting point. You could probably safely take it up to a 5 AMP fuse if it is required.

If the wiring is dedicated and with a minimum size 20 gauge wire.. A 10 AMP fuse may be used safely too though, right?

BlueRidgeMark

All the more reason to not add more stuff to it. Look at it this way: A relay approach is safe. Another approach might be. Or might not.

I see a 10 amp fuse there, so you know for sure that's a safe size.


You ain't wrong. Bigger can't hurt.

Ralph77

So there is another thing I did not know, I will just add it to the list (LOL), that temperature affects the amps a wire can handle?
Assuming ambient temp and the higher the temp the less it can handle.
This is part of a bigger project.
We are talking adding extra factory switches for lights, an aux fuse box in the engine compartment with a circuit breaker, relays,
going to need 12V runs sources for the switches, power sources for the relays, getting all the factory switches to illuminate
when you turn on the headlights, adding an extra underhood light similar to what Comanche might have, might even try to put in a glove box light.
They stopped using them in '99. And probably a few other things I am forgetting.
Frankly I want to use 10 or 12 gauge everywhere.
Overkill but figure it won't hurt?
Will be soldering the 10 or 12 gauge wire though to pigtails that are probably 16 gauge at best.
All this to go with a 155 amp alternator I have and the 2 gauge battery cables I did last year.

Noah911

I am totally fascinated by electricity! It is a very unique and dynamic thing. I do not think it could ever be fully and truly understood. It sure is interesting.

Electricity is basically the movement of electrons from one atom to another. The materials used in making a conductor are choosen based on this fact. Good conductors are composed of materials in which their atoms contain the most free electrons. Some electrons are tightly bound to their atoms in such a way that they are unable to make the jump to other atoms. Only free electrons may make the jump for how it is needed in the production of electricity.

When a free electron bumps into and collides with the captive electrons (or other free electrons), this is something to cause resistance, friction, and heat... and makes for a loss of potential. Conductors have a positive temperature coefficient. Insulators have a negative temperature coefficient. Larger diameter conductors simply have more room for the electrons to be traveling around on them.

The addition of heat results in changing the baseline behavior of conductor atoms. The atoms vibrate more when heat is applied to them. Increasing the baseline vibration of these atoms causes more of the free electrons to end up having collisions.. which results in more friction to occur, and more heat production.. and then, more collisions etc...

Ralph77

Said it before and will say it again.
Of all the things I do badly or know little or nothing about auto electric pretty much tops the list.

dave1123

I was astounded that the XJ uses 20 ga wire in the tail/brake light circuit! Every GM vehicle I've ever worked on had 16 ga! If you loose a couple of strands in a 20 ga wire, your wire heats up! My toolbox is full of 14-16 ga splices, but I had to go buy 18-20 ga ones.

If you want to see how electricity works, try running a power saw on 200 ft of extension cord! The plugs melt and the saw runs like crap!