About Voltage Drop Testing

Voltage Drop Testing


                      Symptoms Of Unwanted Voltage Drops                       


      Dash gauges that tell lies

      False computer codes

      Intermittent loss or sluggish operation of an accessory

      Intermittent ignition misfire

      Lean injector pulse causing an intermittent lean misfire

      High computer sensor voltage values

      Low voltage power to the PCM forcing it into back-up mode.

      Sluggish starter or  generator operation

      Headlights, tail lights, dome light, etc. That flicker or are dim . 

      Other symptoms you may have encountered.


                                     Causes Of Unwanted Voltage Drops


                 Normal ageing (oxide formation /road salt) corrosion!!!

                 Electrolysis – use of dissimilar metals

                 Corroded wires someone else pierced long ago and never sealed

                Mis-matched connectors

                               loose connections on the feed side

                               loose connections on the ground side

                 Do it yourself “quick fix connectors”

                 Unsoldered wire connections.

                 Improperly crimped wire connections


                                              Voltage Drop Testing – Why?


Voltage drop testing can reveal the location of resistance that is sometimes impossible to find with an ohmmeter.  The control voltage used by an ohmmeter is usually three volts.  A three volt pressure is not high enough to bring some circuit resistance into play.  Some resistances in current paths only show up when circuit current is flowing.


An example of why an ohmmeter is not always the best tool to use to find circuit resistance can be found in a partially severed wire.  Consider a wire that has fourteen strands. If all but one of the strands was severed and you used an ohmmeter to check for excessive resistance in the current path that the wire is part of, the meter would show no resistance.  The ohmmeter is telling the truth about the one strand that did not get severed.  However, if this wire was in a current path to a high amp component, such as a rear window defogger, or a power window motor, the thirteen open strands would provide excessive resistance to the current the load normally sees.


A voltage drop test of this current path would reveal an excessive voltage drop due to the severed wires.  When you read an excessive voltage drop, you are reading excessive resistance. Resistance in any current path always causes voltage to drop. Trying to find that resistance with an ohmmeter is not always successful. Finding the resistance with a voltmeter is always successful.


                                             Voltage Drop Testing  – How?


Connect the voltmeter negative (-) to the battery negative terminal, and leave it there. If you are working some distance from the battery, use a length of gage wire between the battery negative and the voltmeter negative probe.


Use the voltmeter positive (+) probe to look for voltage drop.

Voltage drop testing is always performed with the circuit  operating. You cannot have a voltage drop without current flow!


Voltage drop testing always begins at the load and works back toward the battery on both the battery positive side and the ground side.  You will always find the unwanted voltage drop using this method.


Do a voltage feed and ground side voltage drop check of the chassis, block, and accessories as a first priority when trouble shooting electrical problems.