Replaced the alternator 4 times in 2 years

Nah..that's way too high a setting Steve...you're expecting to see a MUCH lower figure, ALTHOUGH he could set it to 10 Amps initially, then if he sees no reading he should select progressively lower settings till he gets a reading near half scale...You should (normally) see something like...oh, I forget now but like maybe 10 to 15 mA?...lessee now ...100 mA is 1/10 of an Amp, so 10 mA is 10% of that or 1/100 of an Amp...that sounds reasonable to me...isn't it?...

In any case a typical car battery is rated at about 50 amp hours, meaning if the car battery is dieing overnight the short has got to be pulling some serious current, 10's of milllamps isn't going to kill it. Even 100mA at 24 hours is only going to suck 2.4 amp hours out of the battery, still not enough to kill it. So the short is probably in the amps area, or at least several hundred mA. I would think if the battery has 30% of its life in it it will still start the car (wild guess).

Car headlights draw about 10 amps a pair, for instance, on low beam. The best sears diehards are rated well over 100 amp hours and will still start your car if you leave your lights on overnight in the winter (remember those old sears commercials with a car frozen in a block of ice with the lights on). Anyway, an exposed wire shorting to the chassis pulling a few amps could be the cause, or maybe a shorted power window motor, or shorted door lock solenoid, if its a relay embedded deep inside your dash, you done for! (unless you go to the disconnect your battery every night routine).

While I can agree with most of what you say there's a few zingers in there...like leaving the headlights on overnight...I realize that the figures stack up that way but just try it yourself (make sure that you have a friend primed to come over to boost you first though)

What I was referring to was a bad battery which can't handle even the slight current that modern cars draw when shut down. And I've had new batteries like this...

Also what you say about digital VOM's could well be true, not so with analogue units though...I hate digital VOMS, never use them...

Well I didn't say I had a Diehard, so I wouldn't try it, I just looked up mine, its 48 amp hour (Walmart $39 cheapo), so lights at 10 amps hour means I got probably 3 hours, give or take. I probably couldn't make it parked at a baseball game with the lights on, let alone overnight.

Oh, ok, I didn't think about that.

Really? They are so nice, big digital numbers to read, easy on my old eyes.

Well, it's like aircraft instruments, the analogue types are much better for 'transmitting' data to the human brain than digital types because a glance will impart an approximate value almost instantly just by the needle position where a digital readout needs some slight mental gymnastics to impart the data.

Some instruments include a digital readout right on the face of the analogue gauge for extreme accuracy when required.

In any case, I've become used to the analogue type and find it easier to use...I agree with you though that the digital likely will measure low values better (when the range is selected much higher)

I don't do much electronic repair now (unlike in the past) my poor old eyes aren't what they once were... :)

Bluewagon Again!

I bought a digital voltmeter with a DC milliamps current function , Sears Craftsman Digital Multimeter (82082) I have questions about how to use it to solve my electrical problem. This link shows the meter and it provides a zoom-in for dial details:

Unfortunately this meter comes with minimum directions as to how to use it.

Your directions seem clear as to what to do but I don't want to blow the meter trying to get it right through trial and error.

Here are some of the meter options:

VÜ has 5 settings - 200m, 2000m, 20, 200, 1000 Comes with 2 leads - red and black Provides 3 input plugins for the leads, V(omega )mA, COM, 10A Max

Here's what I need to do:

1. take a battery lead off (negative lead suggested)
2. set the DC ammeter to a fairly low scale, maybe 0 to 300 mA Question - I don't see a 300mA on the Sears meter - Is the VDC 200m the closest to it?(9 o'clock on the dial)

Answers are greatly appreciated Bluewagon

Bruce L. Bergman wrote:

So far, so good.

The Greek Omega symbol is Ohms, as in resistance.

See the big dashed line around that group of settings? They are all "DC Amps" scales, just different degrees of them. Ranging from "a whole lotta amps" to "itty bitty tiny slices of amps." ^_^

Always start the test at the highest setting - 10 Amps. If you get any significant readings there, you've got a big draw. Like a glovebox light or trunk light that does not turn off when you close the door - the switch is bad.

If everything is working correctly, you should be able to get down to the 20m setting before you see some current draw. This should show somewhere under 5 milliamps draw for the basic car circuits - the computer memory and the clock. Though I wouldn't freak at 10 milliamps, if the draw with everything off is at or over 20 milliamps you may well have a problem.

Forget about the microampere settings with the Greek "mu" symbol, that's too small to be of concern looking for a battery drain in a car. If you were looking at battery drain in something running off D batteries, then it would be useful.

If you want to read the 10 Amps scale, you go between the '10A' and 'Common' jacks to use the DC shunt built into the meter. The meter is really reading across a calibrated resistance inside the meter - maybe .01 ohms, but enough to read the voltage drop across.

Once you have eliminated big problems, you use the 200ma or 20ma scales, and for that you use the 'V/O/MA' and 'Common' jacks.

There's one trick you should use - put a series resistance in line with one of the meter probes before you hook it up, so if you overload the meter by tuning on the headlights on the 10A scale, or opening the door on the 20ma scale, you don't pop the protective fuse inside the meter. Black lead from meter to one side of the lamp, other side of the lamp to the chassis ground.

If the meter didn't have the fuses inside, you'd blow out the meter electronics instead, which would be bad. If you do pop the fuse, and it's easy to do if you forget and try reading voltage with the leads in the 10A position (I've even done it a few times) use the exact replacement fuse specified. "Close enough" doesn't count on this one.

For the 10A scale, use a high-beam headlight bulb - a 55 watt lamp will let through about 5 amps and protect the meter on the 10A scale.

For the 200ma scale, a backup light bulb like an #1156 lets through roughly an amp at full brightness, and 20ma scale would be a pilot light bulb like a #57 You can get small lamp sockets with leads at the auto parts store for the #1156 and #57, and crimp connectors or a headlight socket to plug on to the headlight bulb.

Light bulbs with tungsten filaments have a very handy quality that we are taking advantage of here - their 'cold' resistance is very low, almost zero effectively, and as you apply full operating voltage and heat them up to incandescent the resistance goes up to regulate the current. Meaning that they won't affect the circuit resistance very much if you don't run enough power through to make them light up.

In the old days, you could ruin an analog meter movement by running it with the polarity backwards... But with a digital meter it shouldn't really matter - the black lead to the Common jack and the battery post, and the red lead to one of the other two jacks and the car chassis should read right. If the display shows a big minus sign at the left, it's telling you the connections are backwards.

Where the polarity display on the meter would matter is if you were monitoring the alternator, and you need to know if the net current flow was going into (+) or out of (-) the battery at that moment.

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You don't need those exact lamp numbers, anything you have on hand that has the right voltage and wattage will do. You just need to look up the ones available on the spec sheets in an auto lamp catalog. (This is the thick book the parts man uses that has the bulb specifications, not necessarily the one that tells what lamp goes in each car. Though I've seen both sections printed together in one big book.)

The book will either list the current draw of the lamp, or you can get a very close idea with the wattage rating of the lamp and a bit of math - 55 Watts divided by 13.8 Volts = 3.9855... Okay, 4 Amps. With that lamp connected in series with the meter and the chassis ground even if someone tries the starter only 4 Amps are going to go through

- the meter will be fine.

Or the reverse Volts X Amps = Watts. Google "Ohm's Law"

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