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MG MGF Technical - A little electrical help please.

Hi all,

96 VVC

After experiencing problems with battery drain, I thought I'd "have a dabble" so nipped down to Robert Dyas and got myself a multimeter.

DC 2.5-500V, 500microA-250mA
AC 10-500V
Ohms 10-1k
500mA fuse

Now, you probably guessed what's happened already, but...

I thought I'd test the draw on the battery when idle so set it to 250mA range and flick....dead. Blown fuse.

OK so the draw's over 500mA then, lol.

So, I'll replace the fuse but, and here's the question...

Can I modify the multimeter to accomodate say a 2A range by putting a resistor in series with the multimeter or replace one of the resistors on the PCB so that when selecting the 250mA range, the effective range is 2.5A?

Or is the multimeter I bought just not up to the job.

Or, as V=IR and I have V=12 (say) as the Resistance range is 10-1k Ohms won't that at least give me a 1.25A-12.5mA range using a calculator?

Here's hoping,

Matt Wood

Yes you can put a shunt in line and then measure the volts drop accross the shunt or you can come down to the Essex Roadsters meet next Thursday at the Dick Turpin 7:30pm onwards and I'll bring my meter. (Recruitment over)
Richard Petrocochino

No to elaborate a bit. If you want to measure upto 2 amps, ideally you fit a very low resistance shunt that is matched to your meter so that the ratio of current flow is high in the shunt and low in the meter. Alternatively you fit a resistor in line of about 1 ohm and measure the volts drop accross it.
Richard Petrocochino

Thanks Rich,

So by putting a 1 ohm resistor in line how would I calculate the new current range, if it was previously 0-250mA, in order to get a reading that would mean something.

Matt Wood

The problem with....

Ammeters have to pass all current thru themselves, for this they have to use as small a resistance as possible or the voltage would suffer, If you are putting the ammerter across the terminals of the battery (as your calculations suggest) then it will go pop, the battery can happily push out 100's of amps. To use an ammeter properly you have to connect it in series with the battery, but BEWARE! when you first connect the battery all the systems in the car start up, so for a short while there is quite a current draw - to mesure accuratly you need to put one lead on the battery post and one on the clamp, _then_ seperate the battery post and clamp, be careful, any slight slip and you'll have to start over in 15 minutes.
Will Munns

>>So by putting a 1 ohm resistor in line how would I calculate the new current range, if it was previously 0-250mA, in order to get a reading that would mean something.<<
You would need to know the internal resistance of the meter. If you use a 1 ohm resistor, then :-
1) use a shunt across the resistor, as Will suggests. This will need to be heavy duty, capable of handling tens of amps. Once everything is stable (could be several minutes), remove the shunt.
2) Now measure the DC Voltage across the 1 ohm resistor (this should be at least one watt - and will need to be MUCH higher wattage if you don't use a shunt. The current through the resistor will be equal to the voltage drop ( V/I = R (= 1)). So start with the meter on a high reading (like 20V DC), and reduce to get the reading, which will hopefully be less than 0.1V (or 100mA)
(The correct figures were posted recently)

Aha, ok thanks vm guys. It looks as if I may be barking up the wrong tree then.

What I wanted, was to compare my car with Will's readings given here:-

System at 12.5 volts. Car left overnight.
With alarm off and car unlocked drain was 0.032A
With alarm on drain was 0.067A

The MEMS relay holds in after switch off for approximately 9 mins (560 secs) but if the car is stone cold it switches off almost immediately.

So after switching off a warmed engine the drain was 0.13A for about 9 mins and then fell to 0.067A.
...end quote.

Is the way I am approaching this the right way, in order to get these readings?

Thanks again.
Matt Wood

These reading sound good, you might have a battery fault - how old is the battery?
Will Munns


Sorry, when I said "Will's readings" I meant "Bruce Caldwell's readings".

These are the readings I would like to take to compare to his. But I don't think I( am going about it in the right way now from what I have read so far..

Matt Wood

-remove last message - d'oh of course they sound right!

If you have a beefy ammeter then ou can just use that - BUT do not turn the dial whilst connected

Will Munns

where abouts in Essex are you? If your around Chelmsford I could have a look for you over the w'end, if closer to the 127 come down to the Dick Turpin and I could have a look for you there.

>So by putting a 1 ohm resistor in line how would I calculate the new current range, if it was previously 0-250mA, in order to get a reading that would mean something.<

When I said about putting the resistor in series and measuring the volts drop, I meant that you would use the meter accross the resistor in 'volts mode' and then the display would represent 1 volt = 1 amp


Richard Petrocochino

Will :o)

Rich, oic thx. I'll be in sunny Suffolk this w'end so giving the car a good run. I'm in Brentwood so not far at all. I'm now off to check what the colour band coding is for a 1 ohm resistor, then into my box of "stuff that'll come in handy one day".

Matt Wood

>I'm now off to check what the colour band coding is for a 1 ohm resistor.


gold/silver/salmon band = start/tolarence (ignore)

any number of black stripes followed by brown black stop.

Remember standard resistors are 1/4 watt (so 1/4 amp for 1 ohm)

You are unlightly to find a 1 ohm resistor in your box.
Will Munns

black, brown, blue and green, grey to answer the obvious
Will Munns

I have just had drain problems and thats despite having a battery conditioner connected while the MG is garaged.
Seem to have cured it by removing the relay panel behind the mems and uncovering it and spraying liberally with WD40 and working the relays with my finger - they are not enclosed.
It wasn't the battery as I argued a replacement out of halfords as my 3 year+ battery was under 4 year warrenty.
Dealer was useless said "they all do that".
Once again saved by the BBS !
Jon Baker

WD40 in hand.

Matt Wood

This thread was discussed between 27/11/2003 and 02/12/2003

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