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From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: RV tech forum?
Date: Wed, 23 Mar 2005 15:52:23 -0500
Message-ID: <>

On Wed, 23 Mar 2005 13:42:15 -0700, cew <>

>cew wrote:
>> (Where/) Is there an RV tech forum?
>> Looking for an engine running 12v wire in front of the firewall of a
>> '97 GMC Sierra.
>> Thanks.
>I want this to control a solenoid in my camper charge wire. Close the
>solenoid when the engine is running, to connect the camper battery to
>the alternator and truck battery; open the solenoid all other times to
>isolate the camper battery. I'd rather do this based on engine state,
>rather than manually.
>Guess I should have been more specific -- and maybe I asked for the
>wrong thing.

Several ways to do that.

* Pick up switched 12 volts going to the ignition coil(s).  This works
but if you leave the ignition on it will run down the house battery
along with the cranking battery.

* Pick up the "generating" signal that is usually available from the
alternator. Excellent solution if your alternator has the signal.
Will require a low current buffer relay in most cases.

* (best way) Install a second oil pressure switch that closes on oil
pressure rise.  This will ensure that the house/cranking interconnect
only happens when the engine is actually running.

One other comment.  Attach the wire to your camper to the alternator
output terminal.  This terminal can be as much as a volt or more above
the battery voltage.  This helps compensate for voltage drop through
the wiring.


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: RV tech forum?
Date: Wed, 23 Mar 2005 21:37:45 -0500
Message-ID: <>

On Wed, 23 Mar 2005 15:01:25 -0700, cew <>

>Thanks. I am going to connect directly to the alternator -- about 10 -12
>feet of 6 gage wire. And I'd thought of the 'coil', which actually
>appears to be an amplifier of some kind, and is directly connected to
>the rotor by a high voltage line. I just verified that (what I take to
>be) the input has four light gage wires, two of which go hot when
>ignition switch is on.
>You'd suggest tapping one or both of the wires that go hot? (Why four
>wires in?)

My memory's not that good.  Does your engine have a distributor (HEI)
or a coil pack?  If it has a coil pack then don't mess with the little
wires.  Those are the individual "fire" wires that trigger the proper

Again, my memory's not all that hot, but I believe that the main power
wire to the coil pack stays hot all the time.  It'll be a large wire,
probably at least a #8 and probably a #6.

Is your fuse block under the hood?  If so then it will be easy to pick
up switched 12 volts from there.

On the alternator, the extra wire normally drives the "charge" idiot
light.  It does so by sinking the line to ground when the alternator
is not charging.  I have no idea how much current this line could
handle but since it normally drives only a couple watt indicator lamp,
I'd not push it.

Installing the extra oil switch is easy.  Pick up a female 1/4" brass
Tee and a 1" long 1/4" brass nipple along with some teflon tape at the
local hardware.  Get the close-on-rise pressure switch from NAPA or

Unscrew the old sensor and screw the nipple in its place.  Screw the
old sensor into one side of the brass Tee, the new switch into the
other and screw the whole assembly onto the nipple.  Most of these
pressure switches go to ground when the contact closes so you'd hook
one side of the solenoid coil to 12 volts and the other side to the

>My alternator does have an extra wire(s) attached. I'm hesitant to
>disconnect this while the engine is running. If this is the right thing,
>you're saying this is too weak a source to trip a solenoid?
>I think installing a second pressure switch is beyond my capability,
>although I like the 'actually running' result.

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