BenzWorld Junior Member
Date registered: Jul 2009
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Further to my original post, the general answer for using an ammmeter is that you have to.
The problem is that you can't capture all the (potential) parasitic current drain with just an ohmmeter. You need to test the circuit under power (12V) in order to capture its on-power leakage.
It was my thinking originally that an electrical curcuit that was malfunctioning would in the first instance show a lower resistance than normal (whatever normal might be). In addtion, with a dead battery you aren't in a position to conduct amperage tests -- you need a good battery to do that in the first place. And a test of resistance is less likely to get you into trouble than a series amperage test. So for field work an ohmmeter seemed to be a reasonable shortcut prior to putting in a new battery, calling AAA or taking the car to the repair shop. Debuggng a circuit that presents only a 100-ohm resistance can get you a long way to finding the source of your problem -- no auto mechanic, repair shop or ammeter required. Identifed and corrected, in the field, you just jump-start your car/pop the clutch and you're on your way.
But as the poster above correcly implies, a circuit powered up at 12V is dynamic, not static, and is liable to leakages as a result of faulty relays or control circuitry, faults that do not occur when there is no power in the circuit. You need an ammeter for this.
Measuring just the static resistance will not show the circuit resistance under 12-volt power. Under conditions of 12V power, even a properly-functioning, low-leakage electrical system will show current drain -- running the clock, for instance. But even that is not steady-state, it seems. I tested my 300SE and found that its parasitic current drain was about 25 milliamps -- but that it pulsed every second, like a heartbeat. You need an analog multimeter to see this effect clearly. This heartbeat was surprising, and would not have been detected with a properly-functioning non-invasive ohmmeter, nor with a digital multimeter. Prior to this test with the ammeter, I had measured the resistance at 600 ohms, thereby arriving at an estimated 20 ma parasitic drainage current -- not far off from the actual.
Even this low current load will cause 0.025 x 24 x 30 = 18 / 50 = 36% load on a 50 amp-hour capacity battery over the course of a month. Not recommended, of course.
In addition, my tests this week with four multimeters showed that with the possible exception of one of them, they were all terrible at meauring resistance. It's worth finding several multimeters, testing them against one another, and returning the ones that are clearly out of spec. In addition, as good as some of them are, they are not all idiot-proof. I fried one in the course of these tests.
-- Roy Zider