1991 MB 300-SEL

Working a battery drain issue. Found a maybe somewhat different way to testing for current drain in a car magazine article. Demonstrated on this youtube video as well:

Fixing Battery Drain In Your Car - YouTube

Using the infamous Craftsman multimeter that reads duty cycle. This meter has auto-ranging. Meter set to DCV. With the meter hooked up, I see the following reading:

037.8

If it read 3.78 I would think 3.78 volts. The 037.8 reading, especially the decimal place has me stumped.

I do realize that I can test for amp draw at each fuse with fuse out and a lead set to each end of the fuse holders with the meter on the appropriate amp scale. What I'm doing here is more of an academic exercise. The voltage reading has me stumped.

Thanks for your time.

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'91 300-SEL

If you are doing like the you-tube video and are using a 1 ohm 10 watt resistor then what you are seeing is 37.8 mili volts or .0378 volts. Using the formular I=E/R or amps = voltage/ohms. Then amps = .0378 or 37.8 mili-amps. That seems like a normal reading for a 420.
My 84 500SEL will go dead if not started for 3 or 4 weeks and it reads about .03 milamp if I remember correctly.

Paul

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84 500 SEL (307,xxx miles)

I'm on a slow connection so u tube won't work. Can you describe the test setup?

With auto ranging, 037.8 might be Milivolts = 0.0378 V . The best way to test for an amp draw is to use a clamp on amp meter. Clamp Meters within Electrical Meters / Testers - MCM Electronics Category I use an older Temna brand ( this is the house brand of MCM )

Make sure the meter can read DC , the less expensive ones don't read DC,figure on spending around $100 for something decent.

The big hitters for battery drains are trunk / glove box lights and shorted diodes in the alternator. Testing draw per circuit lets you find the specific circuit causing the draw.

Paul:

Thanks.

Yes - that would make sense. The battery is dropping of fairly quickly. How many days 'til it's dead? I can't say as I normally have a Quick Disconnect attached.

I see the Open Circuit Voltage dropping quickly if I leave it attached. This is not the case of a bad battery or alternator. I strongly suspect the after-market Clifford alarm, but the test I'm using would suggest that I do not have a serious drain?

Onward.

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'91 300-SEL

Pickup up a 1 ohm 10 watt resistor from a place like radio shack. Looks like this:


http://www.radioshack.com/product/index.jsp?productId=2062290


Connect some electrical line(I used 18 gauge wire) to each wire protruding from the ends of the resistor. Some people solder; I wrapped the wires together and then wrapped in electrical tape. Used about a 1 ft. length of wire on each side. youtube test leads are much longer than mine. I was concerned that excess length would increase resistance and skew readings. On the end of each wire, connect an alligator clip large enough to connect to batteries negative post. Walmart sells a set of these(red and black) for $2 and change.

Remove batteries negative cable. Connect one alligator clamp from your test lead to the negative cable and the other to the negative post. In my case, this sets off the alarm. I have the horns disconnected, but see the lights flashing, indicating this process has activated the alarm. The cars lights flashing stops after awhile. I wait 10-15 min for things to go to sleep, then connect my meter with small alligator clips on the meters probe sticks to the wires coming out of the resistor - one meter lead on one side of the resistor; the other meter lead on the other side of the resistor. Set meter to millivolts; then turn on and take reading.


Partial photo of test lead in this article:


http://www.motor.com/magazine/pdfs/042010_02.pdf

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'91 300-SEL

OK, you are making a shunt and measuring voltage drop across it. With such a low voltage drop across a 1 ohm resistor you will be seeing milivolts.

The setup will work, it is just a bit more cumbersome than an amp clamp as it requires breaking the wiring..

Using a resistor to determine the power drain is not a good idea.
If you measure 3,5 volt over the resistor it means you measure the power consumption at 9 volt not at 12 volt. Especially with electronics that is not good, it is possible the electronics won't work at 9 volt at all or al least not as they should.

An amp meter is much more accurate.

Rob

Rob and 97, I agree with both of you. A clamp on meter is much easier to use but not better in this case. In our shop, we had both a clamp on amp meter and an in series meter. Both had their place. We also had a little box that did exactly what the OP is doing. It also had its place. The two meters cost about $750 each. The little box (that I made) cost less than $5. For intermittent drains, the little box did just fine, and it only needed a DC digital meter which you can not get for less than $20. That would leave the expensive, easy to use meters for checking alternators, starters, etc. In fact, for small drains of 100 miliamps or less, the $25 setup was more accurate.
I have to disagree about the 9 volts damaging the electronics in an automotive application. If it were a problem, then any car/truck with a weak battery would be history the first time the key was turned to start. I agree that some things may not work correctly but that is not what we are working on in this case. The problem with using the 1 ohm resistor in this case is the resistor getting hot and burning your fingers. That is why I put mine in a box and added an inline fuse for the times it was being used when it shouldn't.

Paul

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84 500 SEL (307,xxx miles)

Most multimeters have a amp setting, even cheap ones.
The amp circuit is often not protected, so you will fry your meter if the amps are to high.

I never said that a low voltage damages electronics, only that i may give a different power consumption at a lower voltage. A car alarm may not work at all if the voltage is to low.
If you want to know the power drain at 12 volt you have to measure it at 12 volt, not at 9 volt.

Rob

How is it than I'm measuring with 9 volts vs. 12? Could someone throw up the math? Not doubting anything that's been said, just looking for clarification.

Thanks.

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'91 300-SEL

I stand corrected. Thanks. You are correct, you will get unreliable information with low voltage. But the question still remains, Is an expensive clamp on meter worth the cost when it may only be used 1 or 2 times over the life of a car when a $2 resistor and cheap digital meter will give the same information under the same conditions?
When I made that post I knew I would regret it.

Paul

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84 500 SEL (307,xxx miles)

Will pursue a traditional amp draw test which tests in series; then go back to the test I started out with - resistor in the middle of a lead and compare the two. Millivolts read using the resistor lead supposedly line up one for one with milliamps.

.040 mV equates to .040mA, or so it's been said by others with an electronics background on another forum.

Still curious about the math.....9 volts?

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'91 300-SEL

It is rather simple.
If you measure 3 volt over the resistor it means that the voltage drops 3 volt over the resistor, leaving 9 volt for the rest.
The same as putting two 6volt lamps in series on a 12 volt battery.

Rob

An expensive meter is only useful if you can use it professionally.

Even simple 20$ multimeters have an amp setting, so there is no need for resistors to check the power drain. Make sure you buy one with a 10amp setting and always start with the 10amp and go lower from that (otherwise you blow up the meter).

I have a couple of cheap meters that I keep in my car and motorbike, but I nearly always use my 30 year old Fluke because it is much easier to use.
For 200$ you can buy a very good meter that is practically indestructible and last a lifetime.

Rob

I appreciate the math, but...

Not measuring 3 volts. Measuring about 38mV. If 3 volts were measured, you have approx. a 3A draw.

12 - .038 = 11.962

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'91 300-SEL