Haynes Practical Electrical Systems

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Vanny
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Post by Vanny » Tue Jan 10, 2006 11:01 pm

plug the black lead into the 'COM' hole and the red one into the 'Vma' hole (ie NOT the 10ADC hole).

your only going to be using two sections on the car, DC volts (V ... and a bar over the dots) and the resistance (looks like an 'n' and is the greek symbol Omega (stands for Ohm's).

Always start on the highest setting and work down (for DC volts this will be 600 ish volts and for resistancearound 2000k), these are normally closest to the 'off' position.

Basic theory, DC means direct current, this means the voltage level shoudl remain constant unless a sytem part causes a change. Consider voltage as a water level (like 12v is 12 meters of water above the sea level), all electricity works as water and will slowly trickle from the highest level (think of a canl on a hill) to the lowest level(in the case of the car to a river 12 metres below the canal).
If there is nothing to restrict the water getting out of the canal down the hill and into the river then you'll rapidly find it all drains away so you need something in between which we know as things like interior lights, the radio, resistors, fans, wiper motors etc.
Also if you connect a pipe between the canal and the river the canal will instantly drain or bust the pipe (which is very bad!)

So with the the multimeter set to the highest volts setting, place the red wire on one battery terminal and the black lead on the other terminal. The multimeter wont register anything yet as the voltage range is too high!so decrease it to the range of around 20v. It should now give you a reading of around 12v and the difference in water level between the two terminals!

EASY!

PS it doesnt matter which way round you connect the red and black wire as it will still register, just with a minus before the value!

Go on then, have a play!

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Toddman
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meters

Post by Toddman » Tue Jan 10, 2006 11:13 pm

stinkwheel - Fluke 77 seem to remeber drooling over those about 15 years ago :D I expect there have been a few revisions since then.
I moved away from major electronics about 10 years ago spent the last 8 years of my career involved in inkject printers and cartridges but the old teachings served me well.
I think most Avos of th eold type just gather dust these days big old things and the input impedance was low so not an ideal bit of kit alongsid emodern meters.

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Gareth Wales
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Post by Gareth Wales » Wed Jan 11, 2006 8:25 am

OK then
My fog lights don't work - how do I used a multimeter to discover why?

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sleepy0905
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Post by sleepy0905 » Wed Jan 11, 2006 9:30 am

oooh I would still like an avo meter what beautys i used one when in the forces and they were indistructable and so accurate :roll: :oops:
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DavidRutherford
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Post by DavidRutherford » Wed Jan 11, 2006 9:58 am

Gareth Wales wrote:My fog lights don't work - how do I used a multimeter to discover why?
A multi-meter will only tell you what voltage is present at any point in the circuit. What you have to do is read and understand the wiring diagram, relate this to cables in the car, know that "this point should be at 12V" and then test to see if it is or not.

It will only give you answers to the questions you ask it. The trick is to ask it the right questions.
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sleepy0905
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Post by sleepy0905 » Wed Jan 11, 2006 10:09 am

Sort of OFF TOPIC

I know this is sort of off topic but while we are on the subject does anyone know of anyone who want to buy an oscilloscope in good working order :D
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Stinkwheel
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Post by Stinkwheel » Wed Jan 11, 2006 10:26 am

already got one, sorry.
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Post by M » Wed Jan 11, 2006 11:12 am

sleepy0905 wrote:I know this is sort of off topic but while we are on the subject does anyone know of anyone who want to buy an oscilloscope in good working order :D
Bloody hell - binned one last year :roll: mind you was very very old ex-BAC valve thingy.

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Post by Oscar » Wed Jan 11, 2006 1:41 pm

Gareth Wales wrote:OK then
My fog lights don't work - how do I used a multimeter to discover why?
Gareth

David's reply is quite correct -and the Haynes Pratical Manual will tell you how to ask the questions using the multimeter.

It really is worth a look, especially as I sense that you, like me, are a novice with electrics.

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Post by jeremy » Wed Jan 11, 2006 3:06 pm

Gareth

The basic electrical circuit for your foglights and just about everything else is battery + terminal
switch
bulb
battery - terminal.

However this is a car so return to battery - is via the bodywork
and there will be many wires and switches between the battery and the bulb.

To start - check bulb - resistance scale - lowest - smallest number - and probe on each bulb terminal. reading near 0 - bulb works otherwise it doesn't.

if it works - is there 12 (actually should be a bit more) present at one of the bulb terminals (Set to 20V, earth - probe, + probe on bulb supply) No reading - found correct bulb terminal - try other - still no reading - supply fault - not turned on - remember its probably on the dip beam circuit. Still nothing - try same test on switch - and so on working back to the battery. When you get a reading then investigate and go back the way you've just come.

If you have 12 volts - put bulb back and if it doesn't work check supply voltage again with bulb there - its probably dropped to 6 or something - bad connection - trace as above.

If supply voltage OK check earth - resistance setting - bodywork (bare metal) one terminal, bulb earth connection for other - should be reading close to '0' If not investigate earth. If you get a reading but it still doesn't work - try earthing to battery - if it lights now there's another earth fault.

All good fun - best to get the feel of the meter measuring the resistance of things like bolts, pencil lead and all sorts of things - then voltage of dry batteries (approx 1.5 volts for singel cell) and so on.

jeremy

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Post by DavidRutherford » Wed Jan 11, 2006 5:48 pm

jeremy wrote: The basic electrical circuit for your foglights and just about everything else is
battery + terminal
switch
bulb
battery - terminal.
Although in the case of a foglamp circuit there is also the fact that the feed comes from the headlamp switch (possibly from one side of a relay) thus meaning the foglamps can only come on when the headlamps are on. There may also be a separate relay for the foglamps (I don't know BX wiring as well as some cars) which would need to be checked. There's also a fuse in there somewhere too.

This is part of the reason why it's almost impossible to give step-by-step instructions for fault finding, and far better to understand the whole of the circuit concerned before diving in with the multimeter.

Another very useful bit of kit for testing car wiring is a logic probe. Not only will it tell you if you have +ve at a certain point, but it will also tell you if you have -ve, or just a floating cable. Makes finding dodgy earth's much much easier.
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FF

Post by Toddman » Wed Jan 11, 2006 6:25 pm

I agree with what you are saying David but whilst giving fault finding advice on a forum is tricky I think Jeremys post is ideal for someone who is noy heavily into electrical systems.
If you follow Jeremys steps and still have no luck finding th efault you can seek more advice but be sure that you have checked the basics and I agree it is most likely the bulb or a bad earth/connection.

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Gareth Wales
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Thanks

Post by Gareth Wales » Thu Jan 12, 2006 6:24 am

Thanks guys for a terrific typical response. The tenor of the information is just what I needed - so many undoubted experts have little idea how to communicate their expertise. I'll be out there as soon as it stops raining
Gareth