A bit about Ohm's law. E=IR or if your prefer V= A X R. Think of
Ohm's law as a teeter totter. For any given voltage if resistance
goes up, amperage goes down. If resistance goes down amperage goes
up. If the resistance remains constant and the current draw goes up,
the voltage drop goes up. So when subjected to a 100A draw the
resistance of the battery cable caused a huge voltage drop which
caused the car not to start. (I actually had this exact problem on a
car that got towed in once, there were 15 strands out of a 200 or so,
the car did not start.)
let's do a bit of math
Example 1 100A X .005 ohm resistance in cable = .5 volt drop in the
cable not bad.
Example 2 100A X .05 ohm resistance in cable = 5 volt drop, car
probably won't start.
A dynamic test
If there was only a way to test a circuit dynamically you cry. Ah
grasshopper there is. Rather than measure the resistance and try and
guess the effect on a circuit, how about we just measure the voltage
lost due to the resistance while the circuit is in operation. We will
then know instantly if there is a problem. Even better if we go and
fix something we can retest at will and see the effect our repairs.
How to do the test:
What you will need:
You basic DVOM (Digital Volt Ohm Meter) It can be a $20 special from
Radio Shack or Harbor Freight or a Fluke 88
Depending on the length of circuit some jumper wires with alligator clips
"I got that", you say, "how do I do the test?"
OK let's go back to our basic LBC starter circuit
+ cable--> Battery --> - cable (long bastard under the car)-->
solenoid -->cable -->starter --> engine ground strap (assuming a +
ground car)
We want to test the negative cable circuit Here is how:
1. Connect one lead from the DVOM to the negative battery post.
2. Connect the other DVOM lead (using jumper wires as needed) to the
battery cable lead on the starter.
3. Set the DVOM to read DC voltage
(now this next step is critical, write this down, highlight it if you
print these instructions but for God's sake don't forget this step)
4. Energize the circuit, in this case crank the car. The reading
will fluctuate a bit and after a few second settle down to one value.
This value is your reading. Write it down
Please note you are not disconnecting anything, you are adding the
voltmeter in parallel to the existing battery cable.
(more follows)
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