Power source Only hot when engine runs

mb911 · mb911's Garage

I am looking for a power source that is only live with thengine running. any ideas?

74 911 s engine

Ben

ruf-porsche · ruf-porsche's Garage

Output from the alternator? You may have to install a diode along the way.

mb911 · mb911's Garage

Thats what I was thinking. I am not sure which wire I think the blue one? Or simply come off the voltage regulator?

Thanks
Ben

fred cook · fred cook's Garage

What if you pulled a lead off a non-switched power circuit and ran it thru a vacuum operated switch with vacuum from the airbox? That way, the circuit would only be hot when the engine is running and you won't have to rig something off the back of the alternator.

Good luck with your project!

Fred Cook,

'80 911SC coupe

mb911 · mb911's Garage

I like that idea do you know were I could find such a switch?

Ben

77 911 3.0
75 914-6 2.7

Alan Cottrill · Alan Cottrill's Garage

where do you need the power?

what are you going to power?

fred cook · fred cook's Garage

You might try your local NAPA store for the vacuum switch. There was a company selling a system that would cause the ac clutch to cut out temporarily when at max throttle. If there is a switch that opens when put on vacuum, then there should be one that closes on vacuum. Depending on what you are powering, you also mighty want to tie the switch to a relay and pull the main circuit from the battery.

Fred Cook

'80 911SC coupe

dad911 · 07-13-2003, 11:13 AM

how about an oil pressure idiot light sender?

mb911 · mb911's Garage

I just need a switching power source fuel a fuel pump relay.

Ben

dad911 · 07-13-2003, 02:16 PM

So why not on with ignition circuit like most cars? Manifold vac or oil pressure will not turn relay on to start car. Oil pressure might build while cranking, but I wouln't want to have to run the starter that long to start every time.

mb911 · mb911's Garage

I have carbs on this car and the fuel in the bowls will start the car. I am conserned that if in a crash I could have a problem. Safety I spose. This is why I have been considering the voltage regulator for a switching source but have not recieved any conformation that this may work. I tested the regulator in my 911 without it running but key on and no power but not convinced yet I need a little first hand knowledge

Ben

77 911 3.0
75 914-6 2.7

dad911 · 07-13-2003, 06:48 PM

There is an 'inertia/impact' type switch available for that purpose. That would also stop the engine.

Why would the engine stop in a crash on its own??

mb911 · mb911's Garage

I have heard of those switches but I am running with an idea and I am kind of stuburn.

I just don't want gas pouring every where If I were knocked out or something

Ben

77 911 3.0
75 914-6 2.7

RoninLB · 07-13-2003, 08:17 PM

Hey Ben.
I just did a fast eyeball thru the Summit cat.. didn't find anything.. but maybe a phone call to them.. 1-800-230-3030............Ron

emcon5 · emcon5's Garage

Quote:

Originally posted by dad911
There is an 'inertia/impact' type switch available for that purpose. That would also stop the engine.

Why would the engine stop in a crash on its own??

You can score an inertia fuel cutoff switch from most junkyards. Mid-late 80's Fords came with one. On my 86 Mustang it was located right next to the latch for the hatchback, against the rear sheetmetal. It has been a long time, but I am pretty sure it was a white plastic box with a black reset button on it.

Tom

mb911 · mb911's Garage

Is the regulator hot only with the engine running?

Ben

dad911 · 07-15-2003, 10:01 AM

Quote:

Originally posted by mb911

I just don't want gas pouring every where If I were knocked out or something

If you are knocked out, what makes you think engine is going to stop running?

scottb · 07-15-2003, 10:09 AM

How about an oil pressure switch like those used on aircraft? They only operate when there's oil pressure (i.e., when the engine is running). This is what they look like:

The only "catch" is that you'll have to find an adapter that will allow you to install it in the system.

304065 · 07-15-2003, 10:12 AM

Ben,

The voltage regulator continuously measures the alternator output and adjusts the strength of the alternator field to keep the voltage within limits. If you tap into that circuit with anything else, you will change the voltage that the regulator sees, causing it to drive up the alternator's output, potentially overcharging your battery and cooking the diodes in the alternator. That's in the black wire circuit called DF. The blue wire circuit is hot with the ignition on and is the same as a switched 12V. The brown wire is a ground and doesn't do anything. And the red wire is connected back to the battery through the starter, it's hot all the time.

I admire your persistence but I really think if you tap into that circuit you will have trouble.

1980SC · 12:11 PM

Quote:

Originally posted by dad911
If you are knocked out, what makes you think engine is going to stop running?

Because your foot comes off the clutch. When I crashed my car backwards into a wall at a racetrack, people tell me it did some leerching before it finally stalled. The car was still in 3rd or 4th gear, and trying to run, but I was sleeping...

Without the switch, the fuel pump would continue to pump gas since the ignition is still on, but the engine is stopped. Could be a real bad situation.

Rob
1980sc