Starter Solenoid - Good 12v Source?

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skizot722

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I need a constant 12v source to power a relay harness and I don't want to run a cable all the way to the trunk to connect it to the battery. Is connecting a wire (with a ring terminal on the end of it) to the 12v post on the starter solenoid acceptable for this?
 
Why not? That's where the +12v battery cable wire connects to. If it's not a good idea, I'd like to know why.
 
Simple... it's called power surge.

Given that wire is only about 6 gauge... and when the starter is turning, it's typically going to be pulling almost the max amperage through that wire. What happens is the available voltage/amperage drops as the starter is operating. Once the starter stops operating, there is a VERY short and quick surge of power. Think of a water hose; when you quickly block it, there is a brief surge of pressure until the line fills up and equals out to the supply right? Same thing with electricity.

So, what you'll be looking at is... the supply of power to that relay will drop significantly when the starter is operating, upon return of the starter to a static state, there will be a quick surge. Then everything will be equaled out. Either way, it's a good way to ruin an electronic device.

Just run the wire back to the battery.
 
The other part of the problem is that the power is always there so 1) You could drain the battery if something is wrong with the equipment tied to it and 2) You could cause a fire if anything happens to that line and if it is not fused.

The camper shell company that installed my camper thought pulling 12v from the starter on my F250 truck was a good idea. A few years later, I went on a camping trip and someone left the camper lights on. When we got back after 5 days, the truck would 1) not start and 2) when I opened the hood there was a little smoke coming out from the wiring to the solenoid. The wire to the back of the truck was melted in several places and the light fixture was "bur5ned up". From what I could tell, I was mighty lucky I still had a truck after that.

So, if you do wire this up be sure to put in an appropriate sized fuse and heavy wiring.

The issue about power surge also makes sense depending on how sensitive your equipment is.

I would look for a power line that is controlled by the ignition switch or a line with a relay controlling it and fuses etc.

Good Luck,

Jim Henderson
 
NateRW21 - That really doesn't make any sense to me. It's still the same circuit. Running the wire all the way back to the battery does not isolate anything. And, since the wire powering the solenoid is *the* +12v wire coming off the battery, how does the surge not affect anything else? This wire goes to the fusebox as well, so how the surge you're talking about not blow anything else?
 
Jim Henderson said:
1) You could drain the battery if something is wrong with the equipment tied to it and 2) You could cause a fire if anything happens to that line and if it is not fused.
Click to expand...

1) The relays are triggered to open by the stock low and high beam harness, so they won't be open unless I leave my lights on. That would be a problem right now.

2) The line is fused with a 30 amp fuse.
 
There is some resistance in the cable from the battery to the starter. The voltage at the battery itself will be the most stable and least surge. The voltage that the starter end of the cable will drop when the starter is engaged, and may surge high due to inductive kickback when the starter is released. There are better places to tap +12V from in the front of the car. Get a wiring diagram. When you do tap, make sure you fuse the new wiring (at the tap) with a proper size fuse. Otherwise, it could be a good way to burn a car up.
 
joegr said:
There is some resistance in the cable from the battery to the starter. The voltage at the battery itself will be the most stable and least surge. The voltage that the starter end of the cable will drop when the starter is engaged, and may surge high due to inductive kickback when the starter is released. There are better places to tap +12V from in the front of the car. Get a wiring diagram. When you do tap, make sure you fuse the new wiring (at the tap) with a proper size fuse. Otherwise, it could be a good way to burn a car up.
Click to expand...


Thanks for explaining that one.
 
NateRW21 said:
Just run the wire back to the battery.
Click to expand...

So if I go that route, how do I get the cable through the firewall? It looks like the pass-through cover is designed just for the positive wire to pass through and no room for anything else.
 
I have not looked, so I can not swear to this, but the firewall should have a couple passthroughs which are like a rubber diaphragm; it'll be a tight fit, but if you tape the wire to say... a gun cleaning rod, or thin screwdriver, the rigidness of it will allow you to slide it in along whatever else is passing through.

Remember to up-size your wire gauge. You'd be best off going to an electronics shop that carries high quality, high strand wire. Chances are that 10 gauge would be enough (how many amps are you going to be drawing on this?)

If you can't find one of those diaphragm thingies, you should be able to get a quality grommet from that same electronics place; opt for a diaphragm style because it will self seal, but if that isn't an option, size it close to the wire and hit it with a bit of RTV to seal it up.

If you for any reason for see yourself installing additional accessories up front, do yourself a favor and run a 6ga wire; it will supply you enough for additional accessories (electric fan conversion if you have an 02 or earlier). If you don't mind soldering and are good at it (key thing is being good at it)... I'd solder everything; makes a stronger, better connection. Hit any soldered connections with either liquid electrical tape then standard shrink wrap, or at the electronics shop (not talking :q:q:q:qty radio shack)... they have shrink-wrap that is glue lined; it's self sealing and water proof. Hands down the best way to go. I go through that stuff like toilet paper.
 
NateRW21 - Thanks for that info! I'm still just trying to feel out all my options. As you and others have already said, the starter solenoid +12v is a no-no. I was looking at some wiring diagrams for the car and noticed that there's a 4 gauge positive wire that supplies power to the Auxiliary Junction Box. Do you think it would be possible to tap in there (at the AJB) for my power? joegr mentioned that there are better places to tap into +12v in the engine compartment and I'm just trying to figure out what those are.

As for the amperage, it will be around 5.5 amps with the low beams on, and around 14 amps with both low and high beams on.
 
10ga would run 14A.

If it's 4ga, it seems though it would be safe. Your only issue is, tapping into 4ga is really a huge PITA. When I do it, I usually cut the wire, strip about 1" from both ends, grab a 2 or 4ga ring terminal (solder type); cut the terminal off, so you're left with a band, split open the band. I then use that band to clamp the twisted ends together (In-line). Now this is where it gets tricky... coat the union with flux... next I pull out the torch; usually a small butane torch, but if you've got a propane torch with more of a needle flame as opposed to the typical soldering wide flame... that will work quicker (I go straight to the propane torch these days for 6ga and higher butt splices; but I've got a pretty good hand). Trick is to rotate the sides you're applying the heat to and don't give it constant heat; if you do you'll either melt or burn the insulation. Once it's hot enough, using silver solder, fully solder the joint. With this size wire, it will take a substantial amount of solder. But once you've got it fully soldered, make sure there aren't any droplets hanging on. Let it cool, clean it up with some flux remover or contact cleaner. Then seal it up with the liquid electrical tape/shrink wrap or the glue lined shrink wrap. (you'll end up with one side having just your 4ga wire and the other, 4ga wire and your tap wire)

That, is in my opinion... the only way to tap wire, and the improvised clamp is pretty much mandatory with anything bigger than 6ga wire or it won't stay together and you'll end up with a hodgpodge of wire and solder that looks like a$$.

HUGE PITA... if you can find some kind of bus bar that the 4ga wire runs to, that is without a doubt the ideal solution.



AND as a bonus... another option that I use rarely... you can strip about 2" of insulation from the 4ga wire, about 2" from the tap wire... wrap the tap wire around the exposed 4ga. Solder together with silver solder. Coat exposed wire and 1/2" of insulation with liquid electrical tape, might have to do it twice. Then wrap with electrical tape once dry. If you can get the shrink-wrap electrical tape (yeah, they make it...)... that would work. But the stuff is very heat sensitive; too much and it melts, not enough and it doesn't do a damned thing... It's expensive too. This would work for your tap... but, the prior explained method does provide a better connection.
 
After looking a bit closer, the 4 gauge wire actually screws into the junction box on the right hand side of the box. If you find the Auxiliary Junction Box under the hood (up front on the passenger side, right behind the passenger side headlight), and you open cover, you can see this screw and the 4 gauge red wire. I think I should just be able to take my 10 gauge wire with ring terminal at the end and attach it right here - no splicing! Is this acceptable, or are there going to be any problems with tapping in here?
 
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