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.
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