[Healeys] voltage drop was intermittent OD part 1

Richard Ewald richard.ewald at gmail.com
Thu Aug 19 10:06:55 MDT 2010


On Wed, Aug 18, 2010 at 1:42 PM, Richard Ewald <richard.ewald at gmail.com>
wrote:
  If you can't find any instructions or they don't make sense, I will
be happy to post how to do this test tonight.
Rick

I had a couple of people write and ask me to post the procedure to the
list, so I will.
Before I get into the specifics of the OD circuit, I am going to
explain how an ohmmeter works, the problem with using an ohmmeter to
find resistance, why this is so, a little on ohms law, the advantage
of testing the circuit dynamically, how do the test, how to interpret
the results and finally how to use the test to isolate problems.
(Sorry if this seems long and ambling, but you can take the instructor
out of the classroom, but you can't take the classroom out of the
instructor, so grab a beverage and let's go)

How an ohmmeter works:
an ohmmeter puts a very small voltage (about .6-.75 Volts at 0.no
amps) out on one lead, and measures what comes back on the other lead.
 The circuitry then calculates the resistance based on the return
voltage.  This is fine for large resistances, but does not work very
well for small resistances in automotive applications.

The problem with an ohmmeter to find a resistance problem:
Consider your basic LBC starter motor circuit + cable--> Battery --> -
cable (long bastard under the car)--> solenoid -->cable -->starter -->
engine ground strap (assuming a + ground car)
One day after a spirited drive the car won't start.
You use an ohmmeter to check the continuity on the battery cables,
both show good.
So
You replace the battery.  No joy.
You replace the starter.  No joy.
You replace the solenoid.  No joy.
You replace the ground cable, and ground strap even though they test
good.  No joy.
You have now replaced every part in the starting circuit except for
that long bastard cable under the car.  So you climb under the car to
get it out so that you can get one made and you find that on your last
drive you hit a rock that somehow hit the cable and severed 99 out of
the 100 strands of copper inside the negative cable.  There is only
one single solitary strand of wire connecting the two parts of the
cable.
"But I tested it, and it tested good" you cry.  Yup you did, and yes
it did.  The problem is the ohmmeter is a static test at 0.no amps.
As long as that one last strand is intact, the cable shows good with
an ohmmeter.  The problem is the starter motor requires 100 amps or
more to start your LBC.  There is no way in hell you can pump 100 A
though that one strand.  What you need is a dynamic test that tests
the circuit under actual operating condition.  (more follows)


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