# Method to diagnose parasitic drain by fuse voltage drop

Stephen Technician Tennessee Posted   Latest
Tech Tip
Electrical
Parasitic Battery Drain

METHOD TO DIAGNOSE PARASITIC DRAIN BY FUSE VOLTAGE DROP

Measuring the voltage drop across fuse terminals is useful to identify circuits with a current draw. The usual procedure is apply DVOM probes to the openings in the top of the fuse(s). Either by using two hands, one probe in each, or for the more gifted, one hand by holding both probes kind of like holding chopsticks. To make the procedure less tedious and free up a hand; I suggested to clamp an extended positive meter lead to the positive post of the battery using a fused(just in case) wire. Then probe the fuses with the negative meter lead. Probing one side of each fuse at a time. Diagram A.

John Chaplin of Metric Motorworks improved the method by using any battery terminal in the fuse block to connect the positive meter lead to, instead of running an extension lead. If a spare battery terminal is not available then a fuse tapper or fuse buddy loop could be put in place of any “hot” fuse for connecting the positive meter lead. Diagram B.

Steve …

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Dmitriy Analyst
Ontario
Dmitriy

So to get the voltage drop across the fuse you would have to subtract the measurements? I’m concerned about the precision of doing that as the measurements are taken at different times, and the measurements potentially include voltage drops from upstream fuses…

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Stephen Technician
Tennessee
Stephen

This test method is for identifying parasitic drain in a circuit via voltage drop(current draw) of a fuse. Any voltage drop indicates current flow when there should be none. The meter will indicate 0mv, where there is no draw. Where is the current flowing is the question, the amount is for later consideration. If there is no current flow thru the fuse it will show 0mv on both sides of the fuse…

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Dmitriy Analyst
Ontario
Dmitriy

If there is no current flow thru the fuse it will show 0mv on both sides of the fuse. For your method, not sure it is always so. For example, with R3 representing the fuse, and R1 representing an upstream fuse, R2 being a load on the other branch, and R4 simulating an open, what do you think about this circuit? diag​.​net/file/f6u146sot… ​

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Stephen Technician
Tennessee
Stephen

Nice circuit. The circuit would read the same, as you have calculated, regardless of using my method or the legacy(?)2 probe across the fuse method . The is no voltage drop across the fuse, so there is no current flow in that fuse circuit. That is the whole of the purpose of the test, done by either method, to identify if there is current flow in that fuse’s circuit.

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Dmitriy Analyst
Ontario
Dmitriy

Your method would give 0.6V on both sides, but only if the load is constant. If it is instead some module trying to do something repeatedly, your measurements will be all over the place… That could be quite confusing…

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Glenn Owner/Technician
Texas
Glenn

Hi Dmitriy, I would not say one fuse means one fault. It is seldom that can happen from what I have noticed. In the example about a ground relay fuse I mentioned, I did find 2 small relays in the circuit that were compromised because a previous tech tried to pry them out with a pocket screwdriver. Both had a hole in the housing and apparently one had been shorted by the attempt. Who says a tech…

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Kevin Owner/Technician
Arizona
Kevin

In the previous thread, there was mention of how calculate the actual mA value from the VD reading. I don't get very concerned about the mA value. I'm really looking to ID what circuits are active. Depending on the location of a fuse box, I may prefer to use a TI tool so as not to disturb the vehicle. leaving the windows down allows access to the dash panel and whatever is behind it. Yes, you do…

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Marty Technician
California
Marty

Hope this helps.

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Frank Manager
Tennessee
Frank

Amp hound. Works great

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Glenn Owner/Technician
Texas
Glenn

Hi Stephen, I often hear complaints about parasitic draw tests taking too long, but what I see all too often is people still using outdated and wrong ways to test. Most common, is they do not wait for all systems to power down as prescribed by the service information. Their sense of urgency outweighs using strategic thought. Second, is they don't prep the car by setting all latches and switches…

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Frank Manager
Tennessee
Frank

HA-Thought it was just me. Test light on each fuse terminal, tests good. Voltmeter-14 volts on one side-6 volts on the other. Cracked fuse. Need to use DVOM nearly always.

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Paul Mechanic
Illinois
Paul

I have been a fan of the voltage drop method for locating which circuit(s) may be the culprits and have had great success over the years, but was seemingly burned this week and still a little dumbfounded as to what was going on. 2003 E320 (211) came in with a 2.3 amp parasitic. I'm not a Euro guy, yet these vehicles seem to find me, but I advise customers/shops I'll look at it for a bit, but…

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Glenn Owner/Technician
Texas
Glenn

Hi Paul, The difference is: You took the initiative to research and give it a second look. Regardless of the name badge on a vehicle or machine… An electrical problem is still an electrical problem. I learned years ago not to be brand conscious, that way I could concentrate on a diagnostic strategy. In more recent years, internet has been a blessing in many respects because no one has all the…

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Paul Mechanic
Illinois
Paul

I do agree with not being brand conscious discriminatory initially- regardless of issue, after all, the basics are all still in play. They breathe the same air, burn the same fuel and have two obey certain laws of thermal dynamics to move it down the road. It's when specialized tooling or oe scanners/software come into play that I have start drawing lines. I have a shelf full of specialized…

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Bob Technician
Massachusetts
Bob

I have heard that floating voltage referred to as “ghost voltage”. It is a very good indicator of whether you have a connection or not. The other thing when doing this kind of testing is to use the milliamp range on you multimeter. You're checking for very low voltage readings and it's much easier to see the ghost voltage.

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Paul Mechanic
Illinois
Paul

Agreed, all my meters are Auto setting and start at mV- I don't leave a fuse until I get a value that stabilizes regardless of measurement. Still baffled as to why I read 0mV unless what Glenn mentioned about bad fuses, but seat would not function without that fuse inserted and didn't think about swapping positions with another at the time.

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Joe Owner
California
Joe

Here is some stuff that I accumulated over my years as a Technical Writer/ Instructional Designer-Developer for OEM's Try the Chesney Test. It really does work for quick identification of whether you have a problem or not.

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Interesting
Geoff Diagnostician
Hawaii
Geoff

Wow. Wonder why this has not been shared (widely) before. Seems like it should have come-up in every parasitic draw conversation.

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George Curriculum Developer
New York
George

Interesting. I wrote a class for CTI in 2007 covering this very topic.

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Justin Technician
California
Justin

Recently found a draw on a Jeep lift gate module and pulling the power fuses did not remove the draw . Had to unplug module . The module was at fault and the cause . It had incorrect liftgate pawl switch data yet pawl switch was ok . Did not have time to figure out why module had to be disconnected. It was waking bus up which would cause a 3 amp drain . Also the BCM now lists wake up reasons…

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