You said you have a Pertronix module.
That means you don't have 17-19 volts. Pertronix only guarantees them up to 16 volts and I suspect the reality is a little lower.
You could have a volt meter badly in need of calibration.
What you probably have, is a "floating ground". That means the battery neg post; the body ground; the frame ground; and/or the engine block may not all be sharing the same ground. Start engine, and while you're getting that 17 volts at the coil, put one volt meter lead on neg bat post(post, not cable clamp). With other lead, check voltage at engine block; frame; firewall; and radiator support. Also, read from post to the same bat clamp. Ideally you would read "0" volts at all these places. In reality, .2-.4 volts. Anything above .5 volts, FIX.
Do you really have a "one-wire" alternator? I suspect that you have a standard GM "three-wire", but that you have it wired incorrectly with only one wire. Above 1500 rpm when the alternator finally starts charging, the current flow in the charge wire, reverses and then you get the correct reading. Where do you have the ground lead for the volt meter connected?
These guys are very good at explaining why you should wire your alt with three wires.
http://www.madelectrical.com/electricaltech/onewire-threewire.shtml They have several good pages of info, especially this next one that you should look at.
http://www.madelectrical.com/electricaltech/remotevoltagesensing.shtml
