Where to get Under hood switched power..?

edman1000 · Dec 23, 2013

Anybody know where I can easily pick up a 12v key switched power under the hood? I have to pick up a +12v for my plow relay. Right now I have it hook directly to my battery and because I'm an idiot, every once in a while I leave the plow switch in the down position which keeps the down solenoid on and drains my battery.

Truck is a '96 4cyl 5 speed

Earl43P · Dec 23, 2013

They make an add a fuse pigtail that will let you do that.

Pick a circuit that you know only comes on with the key, and pop it in the socket!

I'd add a link, but all the auto parts stores will have these, cheap, in the elect. section.

bobbywalter · Dec 23, 2013

the ecm relay or the back side of the diode for it in the pdb box.

i often hook the relay to the headlights so they have to be on for the plow to work.

edman1000 · Dec 25, 2013

hmmm, both good options. Not sure I'd like to have the headlights on but I guess tht'd work. The plow setup I got is a meyers E-47 style... How much amperage do you think that relay draws?

adsm08 · Dec 25, 2013

The fuel injectors also have a 12V key switched power source.

RonD · Dec 25, 2013

Yes, use one of those fuse taps they are easy to install and then remove without leaving spliced wires.
And run a wire, small gauge is fine, from the inside fuse panel to the relay's location near battery, where power for plow will come from.

Here is a typical fuse layout for Ranger
http://www.therangerstation.com/tech_library/EDiagrams/files/Diagram_Powerdistribution_1991_3of7.JPG

You want to use an active fuse when key is in RUN position, NOT START.

The point of using a Relay is that is doesn't draw many amps, 1 amp would be maximum.
The amp rating of a relay is it's ability to "pass amps", so a 10amp relay can safely pass 10amps to a load/device.
A 60 amp relay can safely pass 60 amps.
10amp or 60amp relay would use under 1 amp to close contacts to pass the amps, the power to activate the relay is not related to the power used by the load/device that is being controlled.

The Load does need a fuse, between battery and relay is best, and under the rated relay amps is best.
If relay is 40amp then use a 35amp fuse, but 40amp fuse would be OK, not 50amp though

There are at least 4 wires for any relay.
Two are smaller wires to close relay contacts, a power wire and a ground wire, under 1 amp draw, when power is activated(key on) then the electro-magnet is active and it pulls the metal contact closed, this connects the 2 Load contacts in the relay.
When power is turned off a spring pushes the load contacts apart, if you use an under-rated relay then the spring may not be able to open the contacts to power off the load/device.

The two wires for the Load are sized for the amps they will carry.
This is the "switch part", connected or disconnected.
This is where the Load rating of the relay needs to be correct.
The device(load) is already grounded so the contacts are just for passing 12v power.
12v in.....12v out when relay is closed

edman1000 · Dec 26, 2013

RonD - Excellent explanation of a relay!

I meant to ask what the enable side of the realy draws in amps. I know it's much lower than what the plow pump motor would draw.

4 wires on a relay.... Be careful. some starter relays are grounded through the mount.

My main point was to get under hood switch 12v signal to prevent going into the fuse block but no biggie, guess I'll just run a new wire on an add a fuse.

RonD · Dec 26, 2013

Oops, yes starter solenoids have chassis ground, so only 3 wire points.

In your application I would probably still have a switch in the cab to activate the relay, but from Key on power, that way the plow wire is only "live" when being used, and if you forget to turn it off it would go off with the key, so no dead battery.

If you use a fuse pigtail insert and use it on the load side of the fuse then no other fuse would need to be added to the relay activation circuit, as the existing fuse would protect against shorts.

As adms08 said all the fuel injectors have 12v when key is on, also the coil, and the MAF sensor have 12v from same source, EEC relay.

edman1000 · Dec 27, 2013

I'd be kind of nervous about tapping into the FI wiring... guess I'll just go into the cab...

Earl43P · Dec 27, 2013

The add a fuse does NOTHING to the circuit you tap into.
Doesn't add to its fuse's load, but simply uses the base of the fuse holder tab as its OWN source (called a bus) of 12v. The original circuit's fuse is "relocated" to the add a fuse, but remains electrically separate.

All you want it to do is power a relay coil, which is a really small draw regardless.

http://www.amazon.com/Littelfuse-FHA200BP-ATO-Add-A-Circuit-Kit/dp/B0002BGELQ

edman1000 · Dec 28, 2013

Yes, I do understand how the add a fuse works... It does not tax anything onto the cirucuit you put it on... (When installed correctly).

Thank you all very much for the info.

Again, main point of thread was to tap in underhood an prevent running a wire into the cab.

adsm08 · Dec 28, 2013

edman1000 said:
I'd be kind of nervous about tapping into the FI wiring... guess I'll just go into the cab...

Sorry, but you aren't going to find a lot of switched 12 volt wires under the hood that aren't feeding a critical component or aren't controlled by a switch in the cab somewhere, like the head lights.

6.2 · Dec 28, 2013

another option, which is what I do for my switches i have connected direct to battery, is a switch that lights up when its active. that way it grabs your attention letting you know its active and using power.

kryptonitecb · Dec 28, 2013

6.2 said:
another option, which is what I do for my switches i have connected direct to battery, is a switch that lights up when its active. that way it grabs your attention letting you know its active and using power.

Same here. It's a good idea to wire the switches to relays of you are running wires to the cab. All my switches have 5a fuses just in case cause burning down a cab when you're in it is never a good thing.

Sent from my SAMSUNG-SGH-I547 using Tapatalk

6.2 · Dec 28, 2013

Yeah always throw a fuse on the switch and a relay. Safety first.

Sent from my XT1058 using Tapatalk

Where to get Under hood switched power..?

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