A friend invited me over for a new engine start on a customer 67 Mustang. He had also replaced the entire front harness (the one that includes headlights, horn, voltage regulator, etc) and sourced a new alternator harness to go along with it so he wanted to do a smoke test too.
Both the front harness and alternator harness were correct for a factory tach car.
The car originally came with a tach so this was to replace old, unsafe wiring.
Prior to attaching the battery I performed a voltage test between the negative battery post and the battery cable and saw voltage. This mean something was on. I don't like sparking near batteries and not knowing how much amperage we were drawing I hooked a small alligator clip to the battery and another to the negative cable (away from the battery). I've seen battery explosions ...
A momentary touch with a small spark revealed the mechanical voltage regulator making a relay clunk.
That shouldn't happen with the key off.
I ran a voltage check to the "I" lead on regulator plug and got nothing with the key on. The A terminal had no voltage (which it should -- it goes to the solenoid battery side).
I asked him to remove the alternator to check on which color wire went to which alternator connection.
It didn't take long to determine that the voltage regulator plug has been wired exactly backwards. Since the plug is phased so you can't install it backwards this had to be a SD factory error.
The wires can be removed as the plug has a slot to release each of the tangs.
You don't have to consult a wiring diagram as the error (if you bought one) is easy to spot. The terminal are, in order "I ASF". The spaces between "I" and "A" make it impossible to plug in the harness incorrectly. But if the large yellow wire is not next to the space in the plug you have an issue.
Pic of the incorrect wire routing.
