Automotive Repair: Locating Battery Drain Using Voltage Drop

[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.

[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:

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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!

[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

[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.

[Placer]

yeah fuses/relays/connectors and splices add a bit of resistance. about .2 should be reasonable for each ,so I was taught. Pete correct me plz if I'm wrong

[MotorAgeMagazine]

@lainarico: The fuse is metal, but it is also a resistance. Fuses work thermally...when current in excess of the fuse rating is applied, the heat load melts the fuse link. Even minor current loads (in the hundredths of a millivolt) will heat this material, and cause its resistance to change. And that you can measure...through the voltage drop that occurs across any resistor.

Thanks for the comment!

[lainarico]

Is 29 mV voltage drop across the fuse, not fuse to ground? The voltage drop should be about 0 since the fuse is a piece of metal. A supply voltage so loaded should blow a fuse.

Battery is a stiff voltage source (internal R 100 times smaller than load), should supply a .1 ohm load at 12 V given an internal resistance of .001; more than 100 amps!

Don't understand how a chunk of metal has large voltage drop across it with the measurement done as indicated, assuming meter is good.

[MotorAgeMagazine]

@primitivepop: Sorry, couldn't write it all in one comment box, so start with the third one down and work your way up! ;>)

[primitivepop]

@MotorAgeMagazine Hey Thanks so much, I really do appreciate the help. I assumed "disabling" & disconnecting was the same as far as door switches go(common problem on these). Did this to the door switches that control dome lights, the auto door locks & electric seat belts. Intend to replace the belts with manual ones.I still think it is something in the 93 geo metro belt electronics causing this, but there is no schematic for them, not in a Haynes at least-not sure if they are fully disconnected

[MotorAgeMagazine]

@primitivepop: Do this one by one until the light goes off. Don't rely on "disabling" as you don't know whether or not that took it out of the loop. What if you closed the door switch mechanically, but not electrically? Identify the components that do work, not switch something else on or off.

Hope this helps, and if not seek a GOOD tech in your area to help you. Check out iATN.net for a "shopfinder" feature to get you started.

[MotorAgeMagazine]

@primitivepop: I watched your video and you have identified a drain related to the circuits supplied by the #3 fuse. Your test at the fuse box was not correct as far as the video test I posted. What you measured was a complete circuit with power on one side and ground on the other.

Leave your test light connected at the battery, and get a good schematic. First, is the fuse supposed to have power at all times? If so, identify all the components on that fuse and disconnect them at the component

[MotorAgeMagazine]

This test is more than enough to find the source of excessive parasitic drain, but it is recommended that you have a good, professional quality, meter.

[spelunkerd]

I've heard about this technique before. For it to work, there must be at least some resistance within the fuse, which I guess is appropriate since the fuse is designed to be a bottleneck that blows first. Do you find that the technique is sensitive enough when dealing with small current draw? A back of the envelope calculation makes it appear difficult to find anything less than about 200mA. Or am I missing something?

[MotorAgeMagazine]

Glad it was useful! Be sure to "favorite" it!

[StevenChandlerArtist]

Thanks this helped alot!

[johnre2000]

buuuuuu



[MotorAgeMagazine]

we apologize for the audio track. Turn your PC volume up as well as the volume control on the player. The use of standalone speakers is a plus.

[austn12345]

Can anyone hear this?