What Am I Doing Wrong? - Multimeters For Dummies!

[rawkyjnr]

Hi Everyone,

Just a quick bit of advice needed on how to use a multimeter, because what I believe is the way to use it isn't working!

Bit of background; I have tried to use a diagnostic kit on my 928 recently, only for it to not work at certain times points. The owner of the kit mentioned that grounds may be the problem. He is to be trusted here and he said he has come across this before on other cars.



This is the diagnostic plug in the car for the kit. He said that I should measure the voltage across the two diagnostic circuits (Pins 12 + 13) and the ground plug (10) individually. He also mentioned that I should use a small bulb between the pins and the ground individually to create a small current draw (I used a 50mA bulb). I completely understand that you have to measure voltage across a component so I got a small bulb and the associated wiring to bridge these pins.

Here came my confusion; I made the circuit, the bulb lights up, fine. I then got my multimeter out, put in in the 20V range, with the wires in the right sockets, etc with the expectation to see whether the circuit has 12V across it. Instead, the multimeter shorted the bulb when placed across it and blew the fuse in the car.

Have I got the multimeter in the wrong range or something?

Also, how would I check that the voltage to the ground in the plug was the same as when grounding it to the car body?

All Help Appreciated!

Edited by rawkyjnr on Saturday 17th April 22:41

[Vic Cooper]

I'd say You used the wrong socket on the multimeter. So instead of the voltmeter part You used the current meter (or how it's called) which is basically a shortcut.

Edited by Vic Cooper on Saturday 17th April 22:52

[rawkyjnr]

I definitely had the plugs in the right sockets (V and COM)

I made doubley sure of this because it just doesnt make sense to me! Maybe it's the multimeter?

IS there a difference between measuring voltage in an DC circuit compared to an AC circuit?

Edited by rawkyjnr on Saturday 17th April 22:59

[Vic Cooper]

It shouldn't be a problem measuring in the wrong voltage range and/or AC instead of DC. It shouldn't display anything just lines, but can't make a shortcut. Then the multimeter itself can be faulty, I guess.

[Ozzie Osmond]

As others have said, a correctly functioning voltmeter will be incapable of shortcircuiting anything.

The chances of a digital voltmeter being sufficiently faulty to shortcircuit anything are very small indeed. Most automotive fuses are quite "big" and the internals of the meter are likely to melt before the car fuse...

[leeeeshad]

Try it out on a battery you know works so you can work out where your going wrong.

[Vic Cooper]

Or one more thing. I happened to me once -the pins are pretty long as You clearly know- while I was holding the car's socket, the two pins created a shorcut by my mistake. Or the bulb's external wires with the pins, could cause a shortcut, a bad movement is enough.

No more ideas, tho.