Parasitic Draw trick

[ourkid2000]

Posted this in the wrong forum…..it’s better suited for here.

I learned this trick from one of the best diagnostic technicians going. Make sure you have a good multimeter that can measure mV’s. Enjoy!

http://www.vestest.com/HowtodoParasitic … sfuses.pdf

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[twiggy]

This is a nice trick but I’m not too happy with the resolution you would get in a milliampere range current draw. But it did give me an idea that I could take a blown fuse, place maybe a 10 ohm resistor accros it and use that for a makeshift ammeter that I could measure each branch at a time. The higher resistance would give me much more resolution at lower currents.

[spelunkerd]

I like the idea of a fixed resistor, as above. If you choose a one ohm resistor, then current is exactly the voltage drop across that resistor. However if you do that you need to remember that the resistor should be capable of significant power consumption (10 watt, 1 ohm resistors are in stock at most Radioshack stores). If the current is high I would turn it off quickly to avoid overheating wires/electronics down the line. An inline reversible circuit breaker would add a safety factor.

[Tony01013]

We just went over this in class. He taught it a little differently. You have to wait until all the modules are in “sleep mode” like it says, but before you do this disable the driver door switch so it doesn’t turn on. Then wire you DVOM in series with the battery. That is, take the positive cable off and connect one end of the meter to the cable and the other to the positive terminal of the battery. Set the meter to milliamps. When the car is in sleep mode, the draw should be less than 50 milliamps. If not, go to the fuse box and start pulling fuses. When you pull them don’t put them back in, it may wake up the modules. Just make a note of where they go or lay them out in order. Keep and I on the meter and when the amps drop below the limit you know what circuit is causing the excessive draw.

[spelunkerd]

I’ve come back to this post a couple of times, since it sounds like a real time saver if it works. And I’ve seen the technique referenced a few times on the ‘net by experienced technicians, so they are telling me that it does work. Of course the assumption is that each fuse will have internal resistance that varies depending on the class of fuse, the amperage rating, and probably the manufacturer. Presumably also the current going through that fuse causes a change in resistance as it heats up, ultimately cascading to infinite resistance as the fuse blows. All you really need is a clue regarding which circuit is involved, and then you can isolate that circuit and test it in line or with a resistor to get more accurate current data. So, it doesn’t have to be completely accurate.

Doing the initial screen this way saves the tedious job of sequentially removing fuses, especially considering that you should not reinsert fuses until finished, to avoid the confusion caused by rebooting microprocessors. I’m going to try this the next time I have an opportunity.

@ourkid2000, thanks for posting the data chart.

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