Voltage Loss or Voltage Drop, That Is The Question?

We hear the terms “Voltage Loss” and “Voltage Drop” test frequently in the Automotive industry.  But most refer to it as what’s is really a Voltage “Drop” test.  This is incorrect, these are two separate test with two separate purposes – however, many-times the same failure result can be determined with both test.  So what is the difference between the two test, and why do we perform them?

Well, it’s actually what it states:

A Voltage Loss test is testing for a voltage “loss” across one or more connections or components.  This test is performed on “one” side of the circuit at a time – meaning to test the Positive side of the circuit, and / or then test the negative side of the circuit. 

A Voltage Drop test is to test for a voltage “drop” across one or more connections and components.  This test is performed across both sides of the circuit and requires a component – meaning to test from the positive side of the circuit to the negative side of the circuit.  This requires some sort of resistance through a load component like a light bulb or resistor, etc…

The below circuit examples shows a voltage loss test.  This test is typically not performed across a component – although you can in some cases.  It is typically used to test circuit hard wire connections – like a starter cable, or for a lose ground with lights flickering or dim on one side.  A “General” rule is 0.5 volt allowance across each connection.  If you are testing across several connections, it would be 0.5 Volts “per connection.”  Three connections would be 0.5 Volts X (3) Connections = 1.5 Volts loss allowance.  Now this is a general rule, realistically it would be typically in the 0.35 to 0.37 volt range depending on wire length, gauge, number of connections, quality of connection, number of components and the resistance or voltage loss value of each component (any resistance equates to a volt loss value).  I will not be going into formulas, however below is a link to a great site that explains Ohms law and formulas.

See This Site on Ohms Law

Volt Loss Examples (Click Examples to View Full Size):

voltagelossexmp1.jpg  Example 1 checks volt loss from the battery terminal to the fuse power input.  With the positive volt meter lead on the actual battery post, you will test for a loss of the terminal, wire, and connection to the fuse.

voltagelossexmp2.jpg  Example 2 checks volt loss across the power input and output of the fuse.  A poor connection to the fuse or open fuse would be the result.

voltagelossexmp3.jpg  Example 3 checks volt loss from the fuse power output to the positive lead of the light bulb.  The result of this test would indicate a bad wire or terminal /connection for the fuse / bulb.

voltagelossexmp4.jpg  Example 4 checks volt loss on the ground side of the circuit after the components (the light bulb in this case).  The fail result of this test would indicate a bad battery terminal, wire, terminal/connection to the light bulb.

Keep in mind that all of the above tests are assuming that battery voltage is being applied to the circuit in the first place.  If there in no (“0 Volts”)battery voltage (12 volts normal in this case) test results of a voltage loss test will test OK on all connections.  So be sure to check basics first…

The next four examples are demonstrating a Voltage Drop test.  The same rule applies, but instead of a 0.5 volt loss per connection (general rule again) you are allowed a 0.5 volt drop per connection.  So if your battery voltage is 12 volts, your allowable voltage would be 11.5 volts – 11 volts for 1 connection would indicate a problem.  Also, you are checking the circuit under load from the connection back to the battery, not across the component as it may appear in the examples (the light bulb in this case).

Voltage Drop Test Examples (Click Examples to View Full Size):

voltagedropexmp1.jpg  Example 1 checks volt drop across the two battery terminals, if the test leads are actually on the terminals.  If the test leads are on the actual batter posts, then all you are checking is initial battery voltage.  The failed result (possible 11 volts or less for two connections based on the general rule), if leads are on the battery terminals, would be bad, corroded or poor connection of the battery terminals – assuming battery voltage is 12 volts to start with.

voltagedropexmp2.jpg  Example 2 checks volt drop from the battery terminal to the input lead of the fuse.  The failed result (possible 10.5 or less voltage for 3 connections), would be a battery terminal, wire or fuse connection (if checking on the fuse itself). 

voltagedropexmp3.jpg  Example 3 checks volt drop from the battery terminal to the output lead of the fuse.  The failed result (possible 10 volts or less for 4 connections), would be a fuse, terminal, wire or battery terminal.

voltagedropexmp4.jpg  Example 4 checks volts drop from the battery terminal to the power lead of the component (the light bulb in this case).  The failed result (possible 9.5 volts or less for 5 connections), would be a light bulb connection, fuse, terminal (s), wires (2 possible) or a battery terminal.  Note:  if you were to now put the positive volt lead on the ground side of the bulb, you would be performing a voltage “loss” test and not a voltage “drop” test.

When doing a voltage drop test, the assumption is that the component (light bulb in this case) is good and drawing a load on the system (load meaning an “Amperage” draw).  You would not normally test for a drop across a component unless you knew the actual volt drop value of the component – referring to ohms law again and determining the formula for the draw and volt drop.  If there is no load on the circuit when doing the volt drop test, result will show system or battery voltage – test OK.  Check the resistance of the component, the bulb, or incorporating the use of an amp meter when performing this test would verify appropriate system load.

On a realistic note on the above tests, drop and loss tests, typically a good system under normal load, will only show a max of 1 volt loss across all connections – unless you have a lot of connections and components on one circuit, or have really lengthy wiring, or a really heavy load like a starter circuit drawing 200 and 300 amps with sub-standard gauge wire.  If I see more than a 1 volt loss or drop, I’m going to investigate the circuit further to see if there might be a problem.  Many time this results in a loose bolt, slightly corroded or contaminated terminal, etc…

You see many technicians waisting unnecessary and valuable time testing specific components in a circuit.  By performing the voltage loss and drop test, you can accurately diagnose a circuit at easy access locations to determine if further specific test are needed based on your result.  Thus, hours saved with unnecessary component testing by removing hard to get panels and parts.

– Jack Miller (Click to View Business Card)


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