is it safe additional accessory diagram

Ok heres the setup.

I've built a carpc system for my Chrysler 300M. I also built a simple relay box for the accessories/items that require remote turn on.

My question is this. I'd like to modify my setup to include a toggle switch, the switch would force the relay to stay on regardless of the car being on.

heres a diagram


Is it safe to add a switch from the 12v constant Source to relay point 86? My main concern is what would happen if I forgot to turn the switch off but turned the car on. Since they're technically both 12v+ connections it shouldnt really matter. But im wondering if I should put a diode inline somewhere. Or If I should go with a more complicated dual relay setup for this.

My Second question would be should I fuse any other lines. Like the main ignition line(which is technically already fused in the car and nothing else is connected to it as the factory system is gone)


In reality the barrier strip is actually a fused barrier strip, and all the accessories have STSP toggle switches to allow them to be turned on or off manually.

You should diode isolate the ignition feed.  If the car is off and you turn the switch on you will turn the ignition on.  Depending on what type of 'igniton' feed you have that could be several different things.  However, since we're just talking about a single relay here put a 1A diode inline with the igniton wire going to the coil.

As far as fusing the ignition wire it depends.  If you short out that ignition wire and it pops the OEM fuse will the car still be operable?  Where is the fuse that would pop and how easy is it to replace it?  In my car, a lot of my fuses are behind my battery and a real pain to get to, so I will add extra fuses just in case (so I don't have to pull my battery to change a fuse).  If the OEM fuse blows and it leaves the car undrivable then I would install a second, smaller fuse inline.  Here you could get away with a 3A automotive fuse or even a 1A glass fuse.

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Kevin Pierson

KPierson wrote:

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You should diode isolate the ignition feed.  If the car is off and you turn the switch on you will turn the ignition on.  Depending on what type of 'igniton' feed you have that could be several different things.  However, since we're just talking about a single relay here put a 1A diode inline with the igniton wire going to the coil.

As far as fusing the ignition wire it depends.  If you short out that ignition wire and it pops the OEM fuse will the car still be operable?  Where is the fuse that would pop and how easy is it to replace it?  In my car, a lot of my fuses are behind my battery and a real pain to get to, so I will add extra fuses just in case (so I don't have to pull my battery to change a fuse).  If the OEM fuse blows and it leaves the car undrivable then I would install a second, smaller fuse inline.  Here you could get away with a 3A automotive fuse or even a 1A glass fuse.

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Any reason not to use a larger than 1a diode?(I have larger diodes on hand)

If the fuse pops the wire is still operation, this fuse is in the cabin by the driver side door, you remove a panel to see a small fuse panel, nothing else ties into this fuse (I have the electronics schematics of my car along with the FSM) The car is fine if you blow it, this wire is just there to turn on the factory radio and factory amps. It's a 10A mini Atc.

A bigger one will be fine.  The relay will only draw about 250mA, so that is all you need, but bigger is better in this case.

Sounds like the current system is fused well enough.

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Kevin Pierson

new pictures(the 30k filesize limit here is terrible so low compression quality)

Anyways, I've decided to go with a dual relay setup for several reasons

-I need to be able to have both a system killswitch and a swith that allows me to force the system to stay on even if ignition is off.
-I have like 90 relays in my toolchest just waiting for usage :)

Look it over and see if everythings ok. My main concern now is if the diodes are in the right orientation/placement.

both setups do the same thing, the second one just seem to be safer and proper to me.

It's more complicated but its also nice practice for when I begin installing an alarm in my car.

tetracleric wrote:

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new pictures(the 30k filesize limit here is terrible so low compression quality)

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For images like this, you would do better saving it as a gif file rather than a jpg. It would look better and probably be even smaller.

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the12volt • Support the12volt.com

the12volt wrote:

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tetracleric wrote:

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new pictures(the 30k filesize limit here is terrible so low compression quality)

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For images like this, you would do better saving it as a gif file rather than a jpg. It would look better and probably be even smaller.

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tried it, didnt want to compress properly. Somethings up with my copy of ifranview, need to reinstall it because all gifs come out around 300kb regardless of compression settings.

Could do it in photoshop or another program but i'm lazy.

First off, I don't see a need for the second relay.  You could do the same thing by wiring the kill switch in between pin 87 and pin 86 of the first relay.

Second, you put the diode in the wrong place.  The diode needs to be in series with the ignition feed, not across the coil.

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Kevin Pierson

Theres another reason for going with a dual relay setup. The single relay method means I have to run long wires from the trunk to the front of my car for the switches. The dual relay method allows me to keep shorter runs. As the only wire going to the back is the first blue wire. It's just easier than trying to fish more wire under my carpet.


as for the diode on the coil of the relay. that was reccomended by various places to prevent some kindof backfire voltage. Just dont know if I have the orientation of it right.

The orientation is correct, but like I said, they arn't needed.  The backflow of voltage is caused by the magnetic field of the coil collapsing.  This back voltage can damage sensitive components, but won't affect your battery in any way.

The two relays won't hurt anything, I just see them as beeing unneeded (KISS).  You are already running a decent sized power wire to the back of the car, so adding one or two 20 guage wires isn't going to be a big hassle.  What is the idea behind have an 'on/off' switch and a 'kill' switch?  They are both doing the exact same thing.  Are you just installing them in different locations for convenience?

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Kevin Pierson

KPierson wrote:

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The orientation is correct, but like I said, they arn't needed.  The backflow of voltage is caused by the magnetic field of the coil collapsing.  This back voltage can damage sensitive components, but won't affect your battery in any way.

The two relays won't hurt anything, I just see them as beeing unneeded (KISS).  You are already running a decent sized power wire to the back of the car, so adding one or two 20 guage wires isn't going to be a big hassle.  What is the idea behind have an 'on/off' switch and a 'kill' switch?  They are both doing the exact same thing.  Are you just installing them in different locations for convenience?

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Nope, one forces the ignition line going to the back to stay active even if I turn the car off.

The other forcefully turns off the line going to the back. this switch will be hidden. It's more of a "mechanics" switch where if my cars getting service done I flip this switch so they cant access my carpc/system. It will probably be a key switch inside the glovebox or center console armrest.

Why would it be any different with the number of wires running to the trunk if you use the single relay setup? A simple isolation diode will do the same thing as the extra relay.
Wherever you were planning on putting that relay, you just connect a diode, it can be in the exactly the same location, and doesn't even need the extra ground connection needed for the relay.

Possibly the confusion here is that you're thinking the extra "second" relay we're referring to is the one in the trunk. What we're suggesting here is to get rid of the "first" relay.

If you just want to use up the extra relays you have laying around, then that's another matter-

dualsport wrote:

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Why would it be any different with the number of wires running to the trunk if you use the single relay setup? A simple isolation diode will do the same thing as the extra relay.
Wherever you were planning on putting that relay, you just connect a diode, it can be in the exactly the same location, and doesn't even need the extra ground connection needed for the relay.

Possibly the confusion here is that you're thinking the extra "second" relay we're referring to is the one in the trunk. What we're suggesting here is to get rid of the "first" relay.

If you just want to use up the extra relays you have laying around, then that's another matter-

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The problem is the switches have to be in the cabin. I already have relay 2 setup in the trunk, its not going anywhere.

In order to allow the killswitch and the bypass switch to work from the drivers seat i'd have to run 4 wires from that rear relay to the cabin, thats more wirework that I just dont want to do.

I know you're talking about getting rid of the first relay. But that relay allows me to have shorter wire runs, less work(I dont have any wire fishers, so pulling wire under carpet with the seats still installed is a pain.

Theres already an ignition wire that runs into the back of the car from the dash. On top of that I can use the power and ground off the old unused radio harness. If I do the dual relay setup then my switches from from the dash to where I want them. Saves me cable and time. In the end its actually less work to go with the dual relays than the single.

Also like I said I do have like 90 relays laying around, so it would e nice to use them.

What are the four wires you're saying need to be run from the rear relay to the cabin? I only see the same one wire, exactly the same as if you were to use the additional first relay. I can't see the problem here-

Maybe stretching out the wires in the drawing makes it clearer.



Does that help?

yes that clears it up, for some reason i couldnt get my head around the connection properly.

Wired it into my car today. Couldnt find my butane soldering iron so I used a crimp connection. when it comes time to redo it(After i finish my dash fabrication) I'll make the connections more solid.

Good deal-

You might consider adding a 1A fuse at the connection between the ignition switch line and your switch line, just to protect the ignition circuit if the long switch wire happens to short to ground somewhere along the run. The diode probably would pop first, but it's safer using a fuse to be sure.

That actually might be a good excuse for using your extra relay, to convert your ignition signal to a ground switched signal, so you can control your relay in the trunk with a ground (-) signal instead of a 12V (+) signal. If the long wire shorted out, worst thing that would happen is the trunk relay would turn on. No worries about fuses or wire meltdown. The extra relay in the front would only be energized when the ignition is on, so you don't have any extra standby current drain to worry about.
If you want to do this:
-Replace your 12V+ Constant Fused input with a ground
-Substitute for the diode: a relay that turns on with the ignition and switches a ground connection output.
-Connect term. 85 on the trunk relay to term 87 (+12V constant) instead of ground
-Remove the diode from the trunk relay (or at least reverse the polarity if you still want to use it)