Author Topic: Digital Multimeter (DMM) - Voltage Drops  (Read 3471 times)

Offline K9DJT

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Digital Multimeter (DMM) - Voltage Drops
« on: May 12, 2014, 02:13:30 PM »
There are a lot of different ways to use a DMM (Digital Multimeter) to diagnose problems.  One of the more common usages is to find a missing voltage while tracing a circuit.  Most technicians will do the tracing by placing the negative (black) lead from the DMM to a chassis, if you’re measuring DCV, or to one side of an ACV power source, and then moving the positive (red) probe from point to point back from the load toward the source. The assumption is a known power being applied to an appliance but the appliance (load) is not functioning, i.e., dead.  What I would like to do is present a little different method of finding an open circuit, or a highly resistive one, by measuring the voltage drop “across” simple components such switches, connectors, lugs, terminals and wires.
What I mean is to test simple devices which “appear” to be OK when looking at it.  The car battery terminal in the picture is a perfect example.  There is no visible corrosion, it’s not lose, but it isn’t necessarily a good connection.  Especially under load, i.e., when attempting to start the vehicle and drawing a large current.  I helped my neighbor one day, who is a very good mechanic but was unable to figure out why the starter solenoid would chatter on his brothers car when he turned the key.  The battery was relatively new, the dome light and radio worked, all the connections looked good.  He was thinking it was the starter or solenoid when I asked if he measured for a voltage drop across the various connections.  He said, “no”.  So I made several  measurements similar to the one shown in the photo as he turned the key to start.  Guess what…there was a voltage of 0.4 Volts DC!  Look at the picture and ask yourself why would there be ANY voltage between those two points???  It should be ZERO!  If you leave the black probe where it is (chassis-frame) and put the red probe on the positive terminal of the battery, chances are you would think the voltage drop was due to a poor battery which wasn’t supporting the starter current .  Now the scenario even gets better.  Let’s say your conclusion is a bad battery, and you replace it.  You remove both battery connectors…clean both of them…clean the battery terminals on the new battery, and reconnect.  Wow…you fixed it.  It starts!  The downside is that it wasn’t the battery…IT WAS A DIRTY CONNECTOR, which happened to become clean when replacing the battery.  I’ve seen the same thing at the terminal of a starter.  Probe to center post and probe on the lug showed a voltage!  Why?  Again, it was a dirty and a highly resistive connection.  This happened on my sons jeep.  The garage he took it to wanted to replace the starter. I told him what I had found and the mechanics response was that he never seen anything like that…replacing the starter always fixes it.  Well, I am sure it does.  Again, remove the starter, clean the lugs and reconnect.  Yup…that fixes it.  But again, poor connection, not a bad starter.  Returning to the photo again, if both probes are placed on the center posts of the battery you wouldn’t have see the voltage drop at all.

OK.  The message I am trying to get across is to make voltage measurements across devices which you would not expect to be a problem.  Center post to lug, lug to lug of a wire, ground post of a device to ground,  the female part of a connector to the male part of the connector and even across a closed switched.  These are all places where there should be absolutely NO VOLTAGE INDICATED!  If there is ANY voltage indicated, it means the connection is highly resistive or it is open.  Could you find a bad connection using an ohmmeter?  Yes, but there are times when a fault will only appear under load…when a current is being drawn.