Well I think its waste although clearly others think its great. Whatever works for you.
I think far more useful is to colorize the circuit of interest on the factory diagrams using tracing paper. Then redraw it as a functional diagram. This does more than add color, it helps us learn the circuit. (See June 1968
Master Tech Conference)
With the digital shop manuals, I either print and draw on the paper or screen shop and use MS Paint and IRFAN to colorize and move items around.
Make a few more measurements and you'll know if there is a significant voltage drop in the system before the fuse box.
Only one branch circuit goes to the headlight switch before going back to the fuse box.
That means if you measure anywhere else in the fusebox, the voltage drop has nothing to do with the headlight switch.
This is no knock against adding headlight relay system. A great idea IMO, but not the issue. If the reading is the branch to the instrument lights, its the rheostat and everything is fine. If the reading is anywhere else there is resistance in the main circuits.
Dana is being modest. He's taken apart, IIRC has even fixed, a headlight a headlight switch. I wouldn't replace unless its broken. The failures often are at the connections. But again we're getting off track.
When the engine is off, voltage on the hot buss should be the same as the battery voltage under the same condition (lights on).
When the engine is running, the voltage at the fuse box should be the same as the alternator. Normally 14 to 14.5 V.
View attachment 1716218173
Measure voltage at the battery.
Then turn the headlights on.
Notice how far the ammeter swings over to discharge.
That's the amount of electrons flowings from the battery to the lights. It will be about 10 to 12 amps.
Now measure voltage at the battery again. It may be a little lower now.
Measure voltage at the alternator battery connections (big stud).
Measure voltage at the fuse box on each side of the fuse.
Do the same with the engine running. But let the battery recharge first. You'll know its rechargeed when the ammeter needle moves to center.
With the info from both sets of measurements we can tell if there is resistance, and which circuits its in.
Remember this. When there is no current flow, there is no resistance causing voltage to drop.
Think about it. Took me a while for that to sink in, but its really a key to understanding what's going on.
When we turn on the lights (or anything) the voltage should be the same as the power source right up to the item we turned on. Some voltage drop is inevitable, but it should be small. 1/2 Volt might be acceptable. 2 Volts is not.
