Adding a circuit that is hot during run only

I want to figure out a way to make a circuit that is only hot when the engine is running. Like, as it is, there is one side of the fuse panel that is hot and one side that is switched. I want one that is only hot after the engine is running. I'm building a car right now with some heavy draw items like a compressor for air suspension, a couple amps for a medium duty stereo, DRLs, and they normally would come on with the switch. How does one go about adding a run-only circuit so those items wait until the start is done and the alternator is making power?

i use a "fuse-tapper" or some name like that, it takes the spot of a mini fuse, and has two fuses mounted to it, one for the original circuit, one for a new circuit, and it has a wire coming out of it. it only holds upto ten amps, so i still tap the battery and use a relay. i picked it up at autozone or advance auto or someplace like that.

tell me if you have any trouble finding this thing

Thanks but I'm talking about a completely different thing. The fuse panel only has two choices; either hot while the switch is on, or hot all the time. I want an entirely different thing. I want an electrical source that is only hot when the engine is actually running. I'm not talking about adding a new circuit to run fog lights, I'm talking about a whole new set of juice.

Two possibles come to mind. 1) If you never leave your key in the 'run' position with the engine not-running, then you could find a tap that is 'run' only and trigger a relay off of that, or 2) you could build/buy (if available) a circuit that would trigger a relay based on voltage...say over 13.5 volts would turn it on.

ah i see, you could tap into the charge indictor light, but be carefull if the light is controlled by the PCM

Good suggestions. I'm somewhat knowlegable with electronics, but I'm far from an engineer. I know diodes only let juice go one way, but I don't know anything about their operating parameters. What I was thinking was a Diode between the alt and battery, then connect a relay to the alt. That way, when the alt is making power, the relay is active, but when the alt stops, no juice flows back to it. The problem is; its a 140-amp alt. How does one design a diode that is capable of that type of amperage?

Here's an idea, run your circuitry off of a key-hot source, then run it through a switch before it hits the rest of the circuit. Or switch to a relay to the circuit...
You'll get the idea.

naw curtis, good idea, but a diode on the alternator pos cable isn't practical. i was thinking about it today, i would find the wire to charge/battery light, put a LED inline(minimum increase in impedance). then line it up to a photodiode, seal the two up so no stray light gets in, then wire it to a N.C. relay that is hot with key on. that way the light goes off, with the key on, the relay is closed. light off key off - relay not powered. light on key on - relay open.

i think thats right..... i gotta go to bed...... :zzz:

Excellent idea! Now the problem is... The car is gutted of the entire interior and wiring. The old system used something in the external voltage regulator, but that's been bypassed since 1991. How does one make a charge light :D ? Once I get that charge light circuit figured out, I can do it.

Vacuum controled switch?? Get one from Greybar.

Oooh. Now we have our thinking caps on! I also thought about a tiny microphone epoxied to the block, but I think the heat and vibration would get to it in short order.

Old time bicycle generator running like hell off of the drive belt :slap:

Vacuum controled switch?? Get one from Greybar.

never knew that existed, i like that idea

Relay possible?
are you running an aftermarektt ECM? A lot of aftermarket ones have one or two extra 5V in and out for controlling things you set up. These can usually be set up in analogue or digital.
Curtis, I'm gonna head down to the race shop and ask our Electrical Engineer for a sure fire way to do this, just gimme one day =)

Cool. Tell him that its a serious old school car, so I have no ECM. Its a caddy 500 and the only "electronics" are the electric choke and the HEI ignition. :) Its old school

Thanks

I couldn't get in touch with him and yesterday at the Auto-X I had a sudden thought on this. The ignition has several positions:
Off
ACC
Run
Start
You're pretty knowledgable and know what these do. Would it be possible to find the trailing hot on the ACC and or Run position and run this to a relay? Just a thought

thats a idea, latching relay off the Start position.

or another idea i had, same as the alternator light idea i had before, but use the oil pressure sensor.

Oooh. this gets better every time!!

I still say vacuum switch. www.graybar.com list 13 made by Square D alone. You can get normally open or closed. And the amout of vacuum required is adjustable. This way if the engine stalls for any reason it will detect this. These switches used to be common on electronic water injectors. This was the most sure way to know the engine is turning. YOu did not want the motor pumped full of water just because the key was "on".

I'm sold on it. Vacuum switch it is, with a relay or solenoid controlling 50 amps worth of circuit taken directly from the battery by some 4 ga. wire. Daddy like.

Curtis, the vacuum switch will work, but what happens when you floor it?

I visited the graybar site and there are so many available thresholds for vacuum switchingt that I can choose one on a 0-30mm range or even one on a very small millibar range. Even at WOT there is still vacuum; as much as 3 inches of it, so I'll just have to get a switch that will stay closed above those pressures.

is that relative or absolute? you wouldn't want going thru the hills to be a problem.

They're all relative. They are pressure differential like the old diaphragm spring switches, just electronic. There are absolute pressure switches, but again, I don't think that even 15,000 feet would make the difference. Even if it only drew 1" at sea level, at 15,000 ft, that 1" should translate to .92". I'll just make sure to get one that switches low enough.

These work really well. We used to use them on "electronic" water injectors. We used water injectors to reduce detonation. This was so we could 87 octane in our high compression engines during daily driving. Drove thru the mountains of Tennessee enroute from Florida to Indiana without any problems. Good Luck.

The oil pressure switch sounds like a better choice. I'm still not real clear on why you're doing this. Would you care to elaborate?

The oil pressure switch sounds like a better choice. I'm still not real clear on why you're doing this. Would you care to elaborate?

Sure. First let me say that it doesn't need to be done at all, its just that I would like to incorporate it if possible. With newer cars, the ECM knows when the engine is running and when it isn't. There are outputs from the ECM for this. Most GM cars (like one I have) uses this circuit for the electric fans and a few other items. If you're sitting in traffic and the fans come on, then stall it with the clutch, the fans turn off. The idea is, why tax the battery if its not entirely necessary. (To that end there is a secondary thermal switch that will click on hot if the engine is too hot to have the fans shut off)

My 1966 Bonneville is a tow mule for me. It is currently gutted of its wiring, interior, engine, tranny, everything. A new wiring harness is going in and I'm adding power windows, locks, mirrors, DRLs, two amps for small speakers, A/C, and an air compressor for an air suspension. It already does plenty of work with trailer brakes, trailer lights, and other assorted goodies. The air compressor for the suspension is a biggie, as are the amps. The amps will take a total of about 30A and so will the compressor. Add to that a moderately high compression Caddy 500 engine to start and the battery being in the trunk, it sure would be nice to have at least the compressor and DRLs on this mystery circuit so its not competing with the starter and placing draw on the battery every time I turn the switch on.

To answer your next question (which I can just feel coming :), I know that GM switches don't send power to accessories during cranking, but I will be using a Bosch switch which does keep it hot during cranking. I also don't want the compressor to be running if I'm sitting in the car and just want to hear the stereo.

One more reason I'm leaning toward the vacuum switch is because I only have one port in the caddy 500 for oil pressure, and its occupied by the sender for the gauge. I guess I could tee off of it, but its on the back of the block just above the tranny belhousing and its a tight fit in that area. I have probably six unused vacuum ports so it just made more sense that way... unless you know of a different oil pressure port on the caddy ??? :)

The oil pressure switch sounds like a better choice. I'm still not real clear on why you're doing this. Would you care to elaborate?

I'm building a car right now with some heavy draw items like a compressor for air suspension, a couple amps for a medium duty stereo, DRLs, and they normally would come on with the switch.

hes adding some items that draw alot of current, enough that he is going to need a heavy-duty alternater, maybe even two. with the engine off, the current draw would be so great his battery would be dead in a matter of minutes. plus some things like the air compressor are basicly worthless when the engine is off. the main advanitige to the oil pressure switch i see is if he lost oil pressure it would take a large load off the engine, and it may be a little more reliable. but i'm sure curtis has his reasons.

Curtis, Yup, makes sense that your situation does require some unique attention. (Sierra - I read the post, and as Curtis saw coming, the ignition does not feed the accessories during cranking. He had not mentioned using a different switch.) Just thinking out-loud here, but perhaps dual batteries - or an isolated 2nd,

oops, mouse got crazy! anyway, ....may work for you. It seems unlikely that you would start the car with all your accessories going, the stereo cranked and the suspension running. I'm not familiar with the caddy engine, but may find a plugged oil passage near the filter. They have to drill a feed passage somewhere. It's your baby...I was just curious what the project was. One final thought is that you could run a normally-closed relay to feed your accessories, and us the trigger line to the starter to cause it to go open. Good luck on the car!

Thanks! I posted a message over at a caddy forum too, so maybe they'll have an idea of where to get the pressure.

Excellent idea on the isolated battery... much like a motohome is set up with a coach battery isolated from the van battery. The thinking caps are ON here.

A very big thanks to all who've chipped in on this!

I like the oil pressure switch idea. Run thru a relay it would probably work better than my old vacuum switch idea. We used them because the are also available with a pot so the output would vary with load. ..... Here comes the boss, gota go.

curtis, are you going to add a override switch to? like on/off/run

That would be a smart idea. That way I can bypass it entirely if I don't need it or force it to come on if necessary.

This project is really going to tax my electrical abilities. I have a 21-circuit aftermarket harness. I'm going to be adding 12v plugs in the rear seat area, and also making the interior lighting be hot, switched with the door, or off.

that doesn't sound to hard at all(at least for me) just post if you need a wiring diagram drawn up or such.