Automotive Repair: Locating Battery Drain Using Voltage Drop

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You can't measure voltage drop across a resistance unless current is flowing, right? Well, if something is draining the battery, there must be current flowing where there shouldn't be. SO...let's measure across easily accessible resistors...the fuses! And see where that current flow is.

Category:

Autos & Vehicles

Tags:

• voltage drop
• battery drain
• battery
• electric
• auto
• repair
• service
• technician
• mechanic
• motor age

[clampingdiode]

What about the square type fuses that don't have any exposed metal to test? A lot of vehicles out there that have these.

[MotorAgeMagazine]

@clampingdiode If you mean the spade-style (mini- or maxi-) they do have test points at the top of the fuse. Look for the two small exposed strips on either side of the fuse rating.

[clampingdiode]

@MotorAgeMagazine No that is not what I mean. This is what I mean:

acimg auctivacommerce com / imgdata /0/2/1/7/9/9/ webimg /3226922 jpg

They are in a lot of vehicles.

Guess that you have to use an inductive amp clamp or a resistor like this clown has in his video:

youtube com / watch ? v = B __ DqK90IIc&feature = related

Usually I cringe when I see this guys videos but this time he has a nice test. Same principle as you but it checks total draw. Note: had to break links to post.

[MotorAgeMagazine]

@clampingdiode Gotcha! I, too, use a low amp clamp attached to my scope to check draw. This also prevents opening the circuit. And the fuses you are talking about can be checked using this method by removing the upper clear cover. Chek out our other videos on testing for draw!

[MotorAgeMagazine]

My pleasure!

[MotorAgeMagazine]

@Pacer: When testing a circuit's voltage drop, looking for an unwanted cause of excessive resistance, I believe it is important to check the entire circuit path and not just the links within. 0.1 volt drop across any one connection is a rule of thumb for many, but what if there are 10 connections? It depends on the current load the circuit is carrying. A starter motor can withstand more drop than a computer sensor can. This test, though, is used to ID a circuit that is on when it shouldn't be.

[heystarfish100]

There is a set of test leads available to measure the voltage drop at the source and test the circuit all the way back to the battery without the usage of extensions on your DVOM leads. These leads replace the standard DVOM leads that come with your meter. The tool is reasonably priced and helps alleviate a lot of frustration. Electrical system diagnosis isn't very well taught and most techs I have worked with have struggled severely because they were afraid to learn before they were forced to.

[heystarfish100]

Although using a voltmeter to test all automotive circuits is the best idea, techs need to remember that using continuity and resistance measurements can lead you to a path to nowhere. Measuring voltage drop under a load is the most accurate versus a static voltage check. Following a manufacturer's diagnostic procedures can take you to replacing unnecessary parts for the purpose of testing. This is really difficult for some parts that require software downloads to operate.

[heystarfish100]

Electrical draw can be difficult to find. I start at the DLC and measure my voltages of the HI LOW Speed CAN networks first to find out which circuit may be at fault by not going to sleep after ignition key off. Once I identify the circuit remaining active I can check my wiring diagrams to identify modules on the circuit. Sometimes removing the CAN wires from the connector of one module and jumping them to another can eliminate a bunch of modules and wiring that may be shorted.

[Placer]

Right Pete. I was just making sure the idea of splices/connectors and such were true. Probably not too relevant to the video but it was something I needed to be sure of. Thanks