I was a bit confused, in your first post you wrote 3,7 volt.
38mA can't drain your battery very quickly, a good battery should last several weeks.

If it is really only 38mA and your battery drains it is probably at its end.

Rob

I wrote 037.8......the 3.7 ref was an "if" situation. Review my opening post.

The battery is fresh as were a few that came before it. Left connected it will drain in a handful of days...maybe a week. I've been living with a Quick Disconnect for a good long time because the local MB dealer and independent shops want nothing to do with it. Oddly enough, all of the wide-eyed 19 yr. olds that work in radio/alarm installation shops tell me they can remove the Clifford alarm without issue. This alarm was installed before any of them were born. I doubt they would understand it's construction and more importantly how it interfaces with MB electrical/electronic systems in a 23 yr. old car which is likely why the MB dealer and the indy shops turn and run.

There's a draw somewhere. I'm very suspicious of the alarm or after-market radio, but suspicion is bad science. I'll get to the bottom of this sooner or later.

Thanks for everyone's participation.

__________________
'91 300-SEL

I will throw in my .02 for what it is worth. I am an electronic technician and EE.

The value of the resistor shunt method is that it is something the average person can understand and do, usually without causing much damage, and it gives accurate results.

Yes, most meters can measure DC current through their probes, but it is my experience that almost no one can do this without blowing the fuse on the meter. The meter needs to have its probes connected correctly and then the meter probes need to be put IN SERIES with the circuit, which is different than how voltage is measured. Easy if you understand but most people not "skilled in the art" do not get this detail. So the result is they put the probe in the current measuring hole, then immediately measure a voltage and blow the fuse. Then they wonder why the meter doesn't work. Even an experienced person can forget where the probes are plugged in and do this. I work with a bunch of professional wiremen and half the meter fuses are blown all the time.

A DC clamp-on meter is a wonderful thing. As was said you have to be careful buying one since the vast majority of clamp-on's measure AC current only, including the ones on eBay advertised as "AC DC CURRENT METER". You have to look at the details. A good cheap one is the Mastech MS2108A which you can get for about $40. It will not read small currents as accurately as the resistor method but will measure them well enough. I used one this weekend to track down a short in some trailer wiring. I just sniffed the current along until it stopped. Like magic. Took 5 minutes and I didn't have to touch a wire.

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

I never blew a fuse of a multimeter.

I did blew up a complete multimeter once (a rather nice and brand new one) when I discovered that Olivetti used 220V AC fans in their power supply instead of 12V DC like the rest of the world. The display ended up upside down on the case. Replaced it with a Fluke, less features but nearly indestructible.

The clamp on meter I use is not very accurate with small currents.

Rob

You can't expect 19 year olds to understand eight tracks.
The electrics of a 21 year old car is simple enough to make removing a car alarm not that complicated.

If the battery is 55A and drains +/- 40A in week the power drain is (40/7)/24 = 0,23A/H.
If you measure 38mA (0,038A) and your battery is good it can only be an intermitted power drain. These are sometimes hard to find.

You can disable the radio and alarm to check if one of these causes the drain.
Other suspects are the light switches in the "always power on" circuits, like interior, doors, trunk and glove compartment.

If not, a multimeter with a MAX hold option can help to find the problem. If you know how much current is used during the drain it is easier to detect the component.

Rob

Ohm's Law is applied.
Let's back up a little.
The 1 OHM resistor will restrict the current that flows form the source back to the same source, your Battery. In the course it causes a Voltage drop across the resistor. That's what the DVM or whatever Volt meter indicates.
According to OHM's Law if a Voltage of 1V is applied across a 1 OHM resistor the resulting current is 1A
If there is only 38mV drop across one ohm resistor then 38mV/1Ohm=38mA

The 1 Ohm resistor makes it easy to calculate, however I don't like the Current Drain measurement using a 1OHM and measure the voltage drop across either.

Here is an example why not:
assuming there is a higher current flow than 38mA, lets say 3.8A
according to OHM's law the voltage drop would be 100 times greater at 3.8V.
We have only a 12 Battery so 12-3.8=8.2V is left for the rest of the car since we already dropped 3.8V across the resistor. Aside the fact that this is already over 14W of power, most of the gear will not work properly if at all.

As stated before, the 1OHM will probably work if the current flow is very little.
A 55AH battery can supply a current of 1Amp for about 55 Hours.
if the Current is double (2A) of course the battery is drained faster at about 27 hours.
and so on.

If measuring the idle-load (your car does nothing, only keeps a minimal equipment alive) battery current first of all one should give it an educated guess from last time the battery was drained and the time it took. If this would be around 1AMP or so, you should use a meter that is fused at 2Amp, many do this only the 10AMP is usually not fused. Set it up for current (DC) measurement, disconnect a battery terminal, which one doesn't matter, in the worst case your Meter shows a negative current. If the Positive is disconnect attach the positive lead to the positive battery post and the negative lead to the battery clamp. Now you can read the current directly off the meter.
The advantage, it is a current measurement where the voltage drop is very minimal and fairly stable with the consumer operational.
I believe Mercedes states that idle currents of ~200-350mA are within specifications. This would be about 14 days on a 66AH battery.
Don't quote me on the current, it may be much less.
So, your reading might be right it also could be wrong, current measurement as above.
If it is within this range and your battery is drained too quickly, then either your battery is too small in terms of capacity or it simply met the end of it's life.

Boy that was along one.....

Word of caution, **DO NOT START THE ENGINE** while the current meter is attached. A current surge would probably burn the fuse, to say the least.
The starter can draw 300Amps or more, plus your test leads will not cope with this type of current. e.g. your car would only make one click....

__________________
Mostly, I don't know notin, I just know where to look.
I am looking back, to over 30 years in Electronics Design.
Electrons don't care if they move in a car, computer or relay!

95 W124 E320 M104.992 - Because, I love to repair, naaaah!
Over 221,000 Miles
Cheers,
Norbert

8-Tracks were done in the U. S. by the late 70s; early 80s. My 23 yr. old 300-SEL came equipped with a CD player. Purchased used a handful of yrs. after it was released.

__________________
'91 300-SEL

Tenma is a decent house line of electronic equipment

Clamp Meters within Electrical Meters / Testers - MCM Electronics Category

I've had an alternator on a Ford that would occasionally drain the battery when the engine was off so don't rule out a failing diode.

There are actually youtube ect tutorials on meter use. Nothing wrong in not knowing but if you can read, you can do.

__________________
1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."

Yeah, but they usually have a range from 40A upwards!
Maybe good to test the current flow while starting the engine.

__________________
Mostly, I don't know notin, I just know where to look.
I am looking back, to over 30 years in Electronics Design.
Electrons don't care if they move in a car, computer or relay!

95 W124 E320 M104.992 - Because, I love to repair, naaaah!
Over 221,000 Miles
Cheers,
Norbert

Tenma Compact True RMS Clamp Meter | 72-7226 (727226) | Tenma

40A is the limit for that specific DC Amp range, it will work fine for finding low draws. We are less interested in a specific reading and more interested in if the draw goes away when we lift a fuse.