Checked the bulkhead connections, all are snapped in tight. Checked both + and - connections on the coil, both appear to be good. Checked connections under the dash, all seem to be secure. I had the voltmeter + wire attached to one of the ignition wire going into the ballast resistor. Since there’s two wires, top and bottom of the BR I’m not sure I have it connected correctly. The voltmeter still fluctuates like the turn signal is on.
Really, Andrew's methodology to troubleshooting was good advice.
The voltmeter fluctuating may or may not be a problem. If its stablizes when just above idle, certainly by 1250 rpm, then it could be normal.
But if there is a difference in the voltage between the alternator
output terminal and the regulator's input, that is a big clue there is resistance in the circuitry.
If it the difference increases with lights or other accessories on, then the resistance is in the shared line. The most likely locations of the resistance are in the connectors. A hand hand voltmeter will reveal where the resistance is causing voltage to drop
when current goes through them. (When no current is flowing, then the resistance is zero and there is no voltage drop.)
This same technique can also be used to identify resistance in the return (ground connections).
The fact that the plastic coveres are locked together is good, but does not guarentee the terminals inside are making good contact.
The terminals may have backed out when a connector was removed and reinstalled;
or the contact surfaces may have become oxidized, corroded, or burnt;
or the wire strands crimped to the terminal may have become broken, oxidized, corroded or worked loose from crimp.
Being a '74 truck, I would expect it to have been equiped with a dual ballast resistor. But regardless, the feed wires when running will be at, or close to the same voltage as the alternator output terminal (big stud). The coil connection while running will be something less, as will the ECU connection if its still a true 5 pin ECU.
Usually the 'run' or 'ignition 1' wires are dark blue, and dark blue with stripe.
Usually the 'start' or ignition 2 feed to the coil side of the resistor is brown.
But as the '79 truck diagram in the other thread shows, at some point the ignition wire color scheme was changed.,
