Jim Chambers

A while back there was a post describing a way to test for current draw by measuring voltage drop across individual fuses. Not sure who posted, maybe Alan. Can anyone point me to it or describe the procedure. I have searched with no success.

tveltman

I think it's simple. You just disconnect one fuse at a time, and measure the current across the fuse terminals. When you remove the fuse, you are breaking the hot connection. You then close the circuit through the multimeter and read the current drain. Once you isolate individual fuses, check the circuit diagrams and figure out what points could be failing, and test those individually. You should have the ignition off but the battery connected, obviously. Make sure doors/hatch/hood is closed fully while testing.

dr bob

Jim--

Each fuse is a resistor, a small heating element if you will. As more current passes through a fuse, it gets hotter. Too much and it melts inside.

Since the fuse is a resistor on series with the load, the voltage drop across the fuse will be directly proportional with the current passing through it. The fuses have tiny holes in the back that allow you to use a small-diameter probe to connect with a blade. Set your meter to read DC volts, actually millivolts for small drains, and put the meter probes in the two holes on the fuse in the circuit you want to test. If you see voltage across a fuse, it means current is passing through it, simple as that. Higher voltage across the fuse means more current passing.

Note that smaller-rated fuses have higher resistance, so a specific current flowing through a smaller fuse will have a larger voltage drop. If you know the actual resistance of the fuse, you could get a good idea of the actual current flow from the voltage across the fuse. This is simple "Ohm's Law" electrical math.


At some point I want to do a little picture tutorial on finding stray current drains in the 928. The fuses in the CE panel are certainly a great place to start, but... There are a few circuits that don't have fuses, a couple that have fuses elsewhere in the car, and of course there are the rogue "installation technicians" that tap in to wires randomly to find power for alarms, stereo gear, and old mobile phones. Even if that equipment is no longer in the car, the effects of a poor installation can linger on for decades. Fun stuff!

tveltman

Note that not all fuses have these convenient test ports, and also, some of those that do might as well not have them. Cheap chinese crap comes standard with some POs. Not saying that's necessarily your problem, but it could be.

EDIT: Oh yeah, if you use the technique I suggested (the direct way of getting Dr Bob's result), make sure you use a fused multimeter. My fluke 88 is fused to 400 mA. If you are drawing more current than that, I wouldn't expect the battery to last 10 days. It might be close, though. A constant drain of 400 mA over 240 hours means that the fully charged battery has to be 96 amp-hours. That isn't unreasonable, but it also assumes that your battery is fully charged at the beginning, and fully discharged at the end. The car won't start if it's producing under 10 V while cranking. A six cell lead acid battery has an open circuit potential of 10.5 V at full discharge. When you draw current on that battery, the voltage it provides will be less, especially if you are trying to extract a lot of current. I have seen batteries that read over 10 V (something like 11.5, if memory serves) that drop below 10 while cranking.

The moral of that story is used a fused multimeter, and as long as it's fused at around 400 mA, you can test the current directly without blowing the fuse, and even if you do, the multimeter is safe. Also, go buy a fluke 88 if you haven't already.

rbrtmchl

I think this is the thread you are looking for.

https://rennlist.com/forums/928-foru...ic-method.html

Jim Chambers

That's it. Thanks to all.