And if I can, how would I do it? Where should I turn the dial?



I need to test current to a 1984 W123 300D defroster relay... if you can tell me how to do that -- i.e., what leads I should touch to what -- that would also be helpful.

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1982 300SD -- 211k, Texas car, tranny issues ____ 1979 240D 4-speed 234k -- turbo and tuned IP, third world taxi hot rod

2 Samuel 12:13: "David said to Nathan, “I have sinned against the Lord.” And Nathan said to David, “The Lord also has put away your sin; you shall not die."

turn to ohms and read the scale while testing the circuit.

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Ohms aren't a measure of current. They're a measure of resistance. So, no, you cannot measure Ohms of current with that (or any other) meter.

MV

I am not trying to split hairs, but you are not measuring the current (Amps) to your relay. You are measuring the Voltage. With the negative lead on ground, and the meter set on the 50Volt DC Volt scale, put your positive lead on the point where you are looking for voltage. You then should read approximately 12 to 14Volts depending on whether the engine is running or not. If you have anything in that range, then you are getting voltage supplied to the point you are testing.

From what you have written, I would suggest that you google around and find a good explanation of Ohms Law. Learn that and you know the alphabet that electronics is based on. Basically Voltage is like measuring the pressure at a point. Ohms is measuring the resistance to flow. Amps is the volume of flow.

For simple testing of automotive switches and controls, you want to see voltage feeding whatever it is that you're testing.

Hope this helps.

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Annoy a Liberal, Read the Constitution

Upon further examination, that meter doesn't seem to be equipped to measure much in the way of current. Current is measured in amps, and that meter seems to only measure milliamps. 250 of them, which is pretty much one quarter of an amp. Most all the circuits you'd be measuring will flow more current than that.

MV

BAV,

I think the OP actually means he wants to read Voltage. I have seen this before with people who are not familiar with electronics use that terminology. It's okay though as long as he knows to simply look for voltage so that he can trace his circuit.

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

Annoy a Liberal, Read the Constitution

Could use a shunt circuit, right? (If it is indeed current that he wishes to measure).

Oops, I meant amps... I guess this won't be up to the task then, huh? I'll see if Autozone rents something heavier-duty.

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"Senior Luna, your sense of humor is still loco... but we love it, anyway." -rickymay ____ "Your sense of humor is still loco... " -MBeige ____ "Señor Luna, your sense of humor is quite järjetön" -Delibes

1982 300SD -- 211k, Texas car, tranny issues ____ 1979 240D 4-speed 234k -- turbo and tuned IP, third world taxi hot rod

2 Samuel 12:13: "David said to Nathan, “I have sinned against the Lord.” And Nathan said to David, “The Lord also has put away your sin; you shall not die."

What part of the circuit are you interested in measuring? 2 amps isn't much for a defroster.

MV

Jooseppi,

There should be no need to test current draw on an item like that. To measure current you will have to disconnect the circuit and put your meter in series. Determine what current you are looking for, so that you can ensure that you have a meter heavy enough to handle the current. With an analog meter you will also have to be DARN SURE that you get the polarity correct, or you can blow the meter.

A simple way to measure current is to get a high power, .1 Ohm resistor and put it in series with the circuit under test. Then measure the Voltage drop across the resistor and do the Ohms law math to determine the current.

Why are you wanting to measure current draw rather than just ensure that there is voltage to the component or point in the circuit under test?

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Annoy a Liberal, Read the Constitution

Not to put words in his mouth, but I recall from another thread he is trying to sell a rear window to someone (maybe funola?) and that someone asked to see how much current the window uses to assess its defroster health.

I replied and told him to measure the resistance of the heater lines connection at the back window and use ohms law to figure out the current usage, which, is most likely 10-15A? I can't remember what its fused to... 200 Watts = ~16 Amps. Are they fused to 25?

I know that MOST meters will not pass that much current...They are typically 10A max (unfused/fused). So if he were to go that path...it could/would likely burn up the leads and/or kill the meter.

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Well, it might very well be that as it heats up, the resistance changes resulting in different current draw. That might be why they are wanting to measure current draw. I've never messed with a rear window from an electrical standpoint, but I would would think that you could simply check continuity of each individual strand and make sure the connector is intact and you could call it good.

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The problem is the strands are usually covered in glue. Because they are also all connected together at both ends that makes testing an individual wire harder.

IMHO the best way to test is to fog the glass, then turn on the heater. Perhaps put it in a room that's cool with ac, then blow steam from a pot over it.

-J

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Yep. That's pretty much what I would do too. Is there not a MB doc on testing the rear heater? I would think there is?

I agree that it would change with respect to temperature, how much..not sure.

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82 300SD 145k
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89 560SEL 118k
90 300SE 262k RIP 5/25/2010
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I have never heard of rear window defrosters changing their resistance from new if intact. Continuity of all elements is more important.

I guess you could fog it and put twelve volts across it to see if all elements are still intact. I am making an assumption by design they recieve a full 12 volts. Otherwise feeder wires would have to have a larger gauge and that costs money that no manufacturer likes to spend.