If there is no current flowing through the ballast resistor, voltage will be the same on both sides.
I do not know whether current wil lflow through the ballast during start, and if it does flow it may be intermittant as the coil resistance may be more lower than the combination of ballast (.7 ohms) and rotor winding resistance.
If there is any measureable resistance with an ohmeter, its probbaly too much. That's the reason for finding resistance using voltage loss with a known current. Also when using an ohmeter for small measureiment make sure you've zeroed the meter by touchiung the probes together and adjusting the meter or making note of the resistance.
review post #45.
I was avoiding the math at that time.
But this time lets include it.
We will measure voltage drop through any resistance between the battery positive and the main splice. We will measure voltage between the battery positive and the main splice by placing one probe on the battery terminal and the other on the alternator output terminal. No current flows through the R6 line as the engine is off. So the R6 wire acts as an extension of the probe.
Turning on the headlights will send roughly 12 amps from the battery to the main splice.
.15 V drop for 15 amps isn't bad.
Now lets do the math.
Energy level (Volts) = Current x Resistance.
0.15 Volts = 14 amps x R ohms
0.011 ohms = Resistance
(Example 2).
This time we're going to measure the resistance in the line between the alternator output and the main splice.
Engine running, battery fully charged, turn on headlights.
step 1. Start engine and allow battery to recharge. Ammeter shows zero. Measure voltage to ground from Alt output. It should be around 14 Volts.
step 2. Measure voltage difference between alt out and battery. Turn on headlights. Measurevoltage drop with lights on.
We're going to estimate the current knowing the only items drawing power are the lights, ignition, and alternator field.
At 14 Volts, the lights shoudl draw a little more current than at 12.5 V. 13 amps is reasonable estimate.
With points ignition at idle and low demand on the alternator, current for those may only be 4 amps.
Total of 17 amps flowing between the alternator output terminal and the mainsplice.
Voltage difference measures 0.9 V
0.9 Volts = 17 amps x R
0.053 ohms = Resistance.
What does 0.05 ohms in that line mean?
A couple things.
1) When the lights are on, there is so much resistance in the output line that the voltage regulator is seeing at less .9 Volts less than the alternator is putting out. If the regualtor is trying to keep the system running at 14.2 volts, then the alternator will have to be running at 15.1 Volts.
2) When more things are calling for power - things like wipers, brake lights, battery charging - then the current flow will be higher and the voltage loss even greater.
Also, voltage loss represents a loss of energy. Where did that energy go?
It goes to heat.
Once you know the resistance and condition of the two main power lines, then you can use the same method to check for resistance in the circuits going through the key switch.
On the other hand if you can measure resistance in any circuit or section of circuit with the ohmeter, then you know there is a problem. I dont see a need to do a voltage drop test to further prove anything.