AUTOMOTIVE TECHNICIAN TRAINING CENTER
355 65th Street.
Brooklyn, New York 11220... Between 3rd & 4th Ave
Bob Starr 718-492-0985, B.Starr@NYATTP.com
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Vehicle Electrical Troubleshooting Tip
January 24, 2011
Greetings to all who have recently signed up for our newsletter!
Two weeks ago I was teaching an electrical troubleshooting
workshop in my home town of Brooklyn, NY. It was nice to visit the old
neighborhood again and meet so many dedicated technicians who gave up
their weekend to attend in very cold Hew York weather. My thanks goes
to Bob Starr who hosted the training event at his Brooklyn training
center on 65th Street. (I grew up on 53rd Street.) And, boy-oh-boy was
the Brooklyn pizza for lunch superb to say the least.
During the training session I mentioned, as I always do, the
importance of grounding the DMM COM (black) test lead to -BATT
(battery negative post). I'm used to seeing the quizzical stares
because most technicians have carelessly assumed any piece of metal on
a vehicle is a good ground for the DMM. I then go on to demonstrate
how this misconception can cause incorrect DMM readings. It could
result in a good DMM reading in a circuit with a problem or it could
result in a bad DMM reading in a circuit with no problem. Either way
the technician has committed a serious error in his troubleshooting
procedure that will lead him to the wrong conclusion.
I learned this the hard way in 1968 from my earliest days
doing electrical work on automobiles for profit and having problems
due to my incorrect troubleshooting procedure of grounding my
Voltmeter on any available metal. I learned a lot the hard way very
fast. What a difference it made when I learned my lesson and began
using the battery negative post to ground my DMM. Here's why!
A DMM will only display the voltage difference between its
probe tips. It doesn't matter how much voltage you are measuring. All
the DMM can do is display the voltage difference between its two probe
tips. If you measure a point in a circuit with the Red test lead where
14V is present, and ground the Black test lead to a point in the
circuit you think is a good ground (voltage of 0.00V) but actually is
at 4V, the DMM display will indicate 10V (14V - 4V = 10V). See the
problem? You will think the voltage is 10V where you have placed the
Red test lead to test the voltage in the circuit but it is actually
14V.
Solution!
Ground the DMM Black test lead at the battery negative post
which is always considered to be 0.00V. Now test the circuit voltage
at the same point with the Red test lead.. Grounding at -BATT, the
battery negative terminal, clearly shows a correct reading of the 14V
present at that point in the circuit (14V - 0.00V = 14V) A correct
reading of the voltage present at any point in a circuit being tested
can only be indicated on the DMM when it is grounded at -BATT. So
simple, yet so often neglected.
To ensure a good connection to the battery negative post, it
may be necessary to drive a thin carpet tack between the battery
negative terminal and the battery cable end. A thin carpet tack will
not disturb the integrity of the connection between the negative
battery post and negative cable terminal end. Leave the carpet tack
sticking up about a ¼ inch above the battery post for a handy
connection point for the alligator clip at the end of the DMM COM test
lead. This ensures a good electrical connection to the negative
battery post. If the battery has side terminals connect the Black test
lead to the bolt head holding the negative battery cable to the
negative terminal.
Vince Fischelli, Instructor
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