What causes system voltage to exceed 17 volts?

[Dangerous Doug]

To Bob's advise, I checked the ground for the voltage regulator and it ohmed-out fine (0 Ohms to block, firewall, and chassis ground). I also verified the wiring to the alternator, and the ERA VR wiring is an idiot-proof connector. Checked the Alt ground. All is well.

I read in a Mustang Monthly article that the resistance between the "I" and the "F" terminals of the voltage regulator should be less than 10 Ohms---and on both voltage regulators I have (one on the Cobra, the other one is/was my tester) my DMM read open circuit... Uh-oh!

Frankly, I am not sure if this test is valid (but I read it on the internet!). Anyone vouch for the I-F resistance test described above? 10 Ohms or less between I and F on the VR.

So, it's down to the VR and the Alt. I did get that open circuit reading between the I-F terminals on the VR---both the one currently in my Cobra AND my test one. So, if they were both fried by a faulty alternator (what if the Field signal is read incorrectly by the alternator in the first place?), then I should replace both at the same time.

Which also seems to be the common wisdom: if either the alternator or the voltage regulator fail, then replace both at the same time.

So, other than thinking this through and following the advise submitted here, and of course, procuring a new VR and Alt, I've totally goofed off today. As it should be, I took off with my 10-year old son today in my Jeep and explored the back roads up to Mt. Hamilton and all the way over to Patterson, and then looped around by the San Luis reservoir and back again. As much as I wanted to fix my Cobra, kick'n it with my kid is far more important.

So, in my garage sit a new alternator ($50) and a new voltage regulator (another $50) and I'll likely be out there at the crack of dawn tinkering away.

Stay tuned, and thanks for the help!
DD