Safety advice needed for use of multimeter in car

[Rollingstone06]

Noob here, and totally green at continuity measurement, etc. Sorry.

Anyways, I just bought a digi multimeter, and would appreciate safety guidance on its use.

1) If I were to test continuity, amps, whatever, with the multimeter, for instance under a car hood context, is it safe to start the car and then check measurements, or is there any safety prob here?

2) Or should only the battery be connected, and the ignition turned to the right, but no ignition?

3) Or should the ignition always be on "OFF" position?

4) Should I wear some sort of anti-static gloves/kit in some form? -- I seem to attract static, lol.

5) Any nOOb site/link you can recommend for me, where I can learn, in simple English, how to use a multimeter, and practice the skill with some exercises?


Excuse the qs. I don't wanna get any nasty shocks, electrical or static, by incorrectly using my new toy. All tips, tricks, and links will be much appreciated!

Cheers,
/RS

 

[Diyisfun]

It does depend on what you are trying to measure. Most test you will not need the engine running a few you may need the ignition on. I think you would be better asking the question when you have a particular thing you need to measure.

 

[Rollingstone06]

Thanks for the reply. Ok, I need to see if there's any continuity in my car's cruise control connector/wires/whatever -- I'll then know what to solder. (As an aside, I also want to see if there are any shorts on my modded Xbox before I hook it up.)

Advice, again, will be much appreciated!

/RS

 

[Yarn]

You can only measure the continuity between two points. Usually this is done to prove or disprove if the 2 points are closed circuit or open circuit.

Continuity of a cruise control connector does not make any sense.

Also do not confuse continuity with measuring voltage or current.

On a final point, some Multimeters will be damaged if you set them to measure continuity and then put them accross a voltage such as a car battery.

If you describe further what you want to solder, then you should get more advice on what to measure and how.

 

[Yarn]

Oh yeh, you should not get any shocks off a 12V car system IF THE ENGINE IS OFF. When an engine is running, there are very high voltages on the spark plug leads and possible the alternator.

Your biggest risk (seriously) is getting burnt by shorting something out with your meter.

 

[Rollingstone06]

Hi Yarn,

Thanks for the info. You asked for more specifics. I have a Benz chassis 126, and plan to fix its faulty cruise control system. First off, I want to know where the bug originates. I've been told to measure the resistances of all the five resistors on the code plug circuit, and that one resistor should be 1600 K Ohms, and the tolerance is +-1%.

Please let me know whether or not, for this test, the car should be turned on.

Cheers, and happy motoring...
RS

 

[Yarn]

Hi there,

I still do not understand what you are trying to do.

However, generally, to measure resistance you need to measure Ohms. (not quite continuity as continuity implies that resistance is 0 Ohms).

To experiment with measuring Ohms you can hold one probe in each hand and set the meter on to Mega Ohms (1,000,000 Ohms) depending on how sweaty you are and how tight you grip, you get a reading which in the region of .5 to 10 MOhms. (range is big because it really depends on the factors i mentioned above).

You do not need the circuit active to measure resistance. It is much better to have the stuff switched off.

However, unless you can actually see something is visible broken, I do think that you are qualified enough to fix a cruise control. I would leave well alone. Chances are that it will be a complex fault with a black box that is the cruise control circuit and you will have to buy a whole unit for lots of money.

 

[Yarn]

Unless a resistor has actually become broken away from the board, It will still be the original value. I have never known a resistor change value by more that 2% even on 20+ years old equipment.

If the circuit is designed by Benz then it wont need a sharp tollerance of 1% !!!! (that would be crazy for many reasons).

Go to a garage!!

 

[simon_d]

get a few batteries (AA, PP3 etc) and some resistors. good for practice and little chance of shock/accident

 

[oilman]

If you are such a novice with a meter, there is very little chance you will be able to diagnose the fault on a cruise control. I have done some failure mode analisys on luxury car electrics, and its surprising what can happen with only a few simple faults. Further, many control systems on cars work on a CAN bus. To diagnose faults on this you need the sort of machine autoelectrics places charge you £50 for a session. Pay the £50 its cheap.

 

[eggplant]

you cant measure the value of a resistor whilst its in a compete (i.e. not open) circuit as your meter will measure the total resistance of the circuit.