why is sensing voltage key on only?

The sensing line to the voltage reg turns out to be key on power only.
Is this necessary? That is, why is it not hooked up all the time?
Or is that how you kill the car?

That is correct.

This is how it is wired and suppose to be. It only needs to sense voltage when the alternator is spinning not when engine is not running.
No this is not how it kills the engine.

I would think it was part of it, since it taps off the line prior to the ballast resistor. So if it still has power there, the coil would have power too.

But regardless, is there a problem in wiring it hot all the time if I seperate it from the wiring to the ballast?

I understand it doesn't need it if the engine isn't turning, just wondering if it causes a problem, like points burning out when you leave the key on, not that this burns out points if you leave it hooked up.
Do voltage regs or the alt have a problem with power applied to the field all the time if they aren't doing anything?

Why are you wanting to change something that works like it was designed to?

rewiring the car for efi, and was trying to clean up the harnese a bit.
They need keyon power and it looks like I have to connect 3 wires together for it, 1 of them happens to be sensing voltage.
So I thought if I moved that, then all I need is power from start and run for the computer.
Not a big deal, was just wondering if it may cause a problem.

Of COURSE it is a problem. The sensing lead also supplies power through the regulator to the alternator field, so if you wired it "hot", then anytime the sense lead is "hot" then the alternator would be drawing field current--that much harder on the field/ brushes, and certainly would discharge the battery with the vehicle shut off.

The IMPORTANT thing about wiring (or rewiring) this connection, is that you want it to be AS CLOSE to 'true' battery voltage as possible--which means use large enough wire, and make sure ANY connections in the path are good and tight, and of course make SURE the ever present firewall connector is in good shape.

Thanks. Hadn't thought of that.
Gives me an interesting idea though.
Might put that sensing voltage on a relay based on the key to bypass the bulkhead connections entierly.

Man, I'm having to refresh myself on what I did for my EFI install. I needed a constant voltage to the coil from the key in run to start and back, with the balast resistor removed from the system. I also needed to have a voltage sense wire for the new Maggie alternator that got installed at the same time.

I think you may want to check your wiring diagram for a weld junction with 4 or 5 wires all solderd together at one point. That's where I ended up putting my voltage sense and also the feed for the coil. I'm not sure if the early mechanical VR wiring is the same, but maybe it is. It SHOULD be hot for key key on/run and start, but there's a chance it's dead to put all the voltage towards the coil for a hotter start with the original ignition system. I just can't remember off the top of my head if I had to change that aspect of my wiring. Somewhere I've got my revised wiring diagrams I did for my project, but the office got converted to a bedroom for Kiddo #2 a few months back and I haven't seen it since...

Clair