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How Can I Test My Ammeter?
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Mello



Joined: 24 Aug 2009
Posts: 34
Location: Lower Normandy, France

PostPosted: Wed Oct 28, 2009 12:45 am    Post subject: How Can I Test My Ammeter? Reply with quote

I have a 6v +ve earth '56 Ford Pop, with an ammeter that reads that it is discharging at around -9 amps with the ignition off. When the ignition is on and the engine is running there's barely any movement, but there is a little (especially if I tap it gently!).

My question is, apart from swapping the ammeter with a known good one, what else can I do to test its functionality? I have a multimeter, but I'm no electrician...

Advice welcomed Embarassed
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Phil - Nottingham



Joined: 01 Jan 2008
Posts: 1252
Location: Nottingham

PostPosted: Wed Oct 28, 2009 9:40 am    Post subject: Reply with quote

It seems that the dynamo is charging then - any evidence of it going flat. Does the ammeter flick to show a discahrge when lights are swtiched on engine running.

It could well be that the ammeter needle needs resetting/recalibrating (often by just carefully bending or repositioning needle depending on type if it does not show zero when engine off and battery discnnected
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Nic Jarman



Joined: 05 Oct 2008
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Location: Stoke by Clare, Suffolk

PostPosted: Wed Oct 28, 2009 10:28 am    Post subject: Reply with quote

You cannot measure current directly. The ammeter works by a very low resistance shunt and a simple coil/magnet deflected needle from the small voltage drop across the shunt. So if you have a good multimeter you can measure the small resistance across the shunt and reconnect it to measure the voltage drop across it. Then use Ohms law to work out if it is working.
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Peter_L



Joined: 10 Apr 2008
Posts: 2680
Location: New Brunswick. Canada.

PostPosted: Wed Oct 28, 2009 4:35 pm    Post subject: Reply with quote

Hi Mello.

Maybe this will help.




The current measured by the Ammeter is equal to the wattage of the bulb divided by the supply voltage.

So if your bulb was (for example 24 Watts and the Supply Voltage was 12 Volts) there would be a current of 2 amps flowing through the ammeter.


24/12 = 2 amps or 24 W / 6 volts = 4 amps

21W bulbs are maybe more common. 21/12 = 1.75 Amps

Now all of this is going to be a bit of a rough calculation thing, because your supply voltage will be not constant, unless you have a constant voltage power supply (it's a techie thing usually)

You say you have a multimeter ? The small ones tend to measure only small current, but if it will go to 2 amps, then it would serve as a bit of a test. Place it where it says AMMETER on the sketch. If it has a 10A facility then so much the better. You can up the load (that is wattage) and the current flow will increase. Then you have more to play with by comparing your meter and the car's meter.

Now you know what the multimeter says is flowing, so insert the car's ammeter and see what kind of a reading you get.

Remember that as soon as you connect the battery, the bulb will start to pull power from it and the voltage will drop. Putting a battery charger onto the battery will help out. Use your meter to measure the voltage across the battery or battery charger combined. Use this voltage in your calculations. You may find that with the charger on, it could be close to 13.5.

Use your multimeter to obtain the voltage across the battery terminals and use that figure for your calculation.

Of course you can't be sure how accurate your multimeter is,, so it is catch 22 a bit.


So at 12 volts if you put in 2 by 21 watt bulbs (in parallel) then the total wattage goes to 42.

42/12 = 3.5 amps another 21 bulb would lift the load another 1.75 amps.

Many ammeters, like you have on the car, have some form of adjustment on the back.

Now you need to juggle.

Test the ammeter with the connections + from battery to + on meter and - battery to - meter and note the ammeter needle deflection.

Reverse the connections on either the battery or the ammeter and you should have the same deflection the opposite way.

Of course, at no current flow, the needle should read zero.

It may involve a bit of needle moving and a bit of adjustment (if there is one).

I know its all a bit rough and not very scientific, but you never know.

If all of this fails. the you will need a large hammer.... Wink

Goodluck.
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Mello



Joined: 24 Aug 2009
Posts: 34
Location: Lower Normandy, France

PostPosted: Fri Oct 30, 2009 11:29 am    Post subject: Reply with quote

Wow- great post eter from Winnipeg - thanks!

I'll br trying this out and will update this once I have Smile
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ukdave2002



Joined: 23 Nov 2007
Posts: 4100
Location: South Cheshire

PostPosted: Fri Oct 30, 2009 12:13 pm    Post subject: Reply with quote

or you could get yourself a cheap (less than a fiver these days!) digital multimeter (switched to the "Current" setting) and put it in series with the existing ammeter (just disconnect one of the wires from the existing ammeter and connect one wire from the digital ammeter to the wire you have removed and the other digital ammeter cable to the existing ammeter on the terminal where you have just removed the wire.

Both ammeters should read the same.

Fave
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Peter_L



Joined: 10 Apr 2008
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Location: New Brunswick. Canada.

PostPosted: Fri Oct 30, 2009 3:38 pm    Post subject: Reply with quote

Good point UK Dave... I skipped that bit as Mello did say he had a multimeter and would likely need it to monitor the voltage as he made the calculations.

I have a high end digital multimeter, that goes back to my working days and will measure upto 20amps. The cheap ones tend to be a bit limited when it comes to amperes.

Of course we can get into shunts and the like, but sometimes simple is best.
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Nic Jarman



Joined: 05 Oct 2008
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Location: Stoke by Clare, Suffolk

PostPosted: Fri Oct 30, 2009 5:41 pm    Post subject: Reply with quote

I re read you post and it is clear that your ammeter is not working properly. A quick and very easy test to see if your system is charging is to park your car a few feet from a wall (in the dark), turn the headlamps on and give the engine a rev. If the lights brighten then your system is charging.
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Peter_L



Joined: 10 Apr 2008
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Location: New Brunswick. Canada.

PostPosted: Fri Oct 30, 2009 5:51 pm    Post subject: Reply with quote

Nic Jarman wrote:
I re read you post and it is clear that your ammeter is not working properly. A quick and very easy test to see if your system is charging is to park your car a few feet from a wall (in the dark), turn the headlamps on and give the engine a rev. If the lights brighten then your system is charging.


or.. clip your trusty little meter to the battery, (Voltage Scale) and rev the engine. If you do this with Lights on and Lights off you may even get some clues as to how good or otherwise the charging is.

Voltage across the battery should rise when the engine is reved. If it only rises when the lights are off, then there are problems with the charging system. Likewise, if it doesn't rise at all, then there is no charging going on.
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peter scott



Joined: 18 Dec 2007
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Location: Edinburgh

PostPosted: Wed Nov 11, 2009 3:03 pm    Post subject: Reply with quote

If you disconnect the battery then no current should be flowing anywhere.
If the ammeter still shows -9 amps then you need to bend the needle to the 0 amp position.

Peter
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Jim.Walker



Joined: 27 Dec 2008
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Location: Chesterfield

PostPosted: Sat Nov 14, 2009 2:54 pm    Post subject: Reply with quote

Why not simply do what peterwpg suggests but add a reliable test ammeter into the circuit in series? Both ammeter will read the same if both are good. You NEED the bulb or similar. If you connect straight across a battery without a suitable load (bulb) neither ammeter will ever work again (unless fused).
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Nic Jarman



Joined: 05 Oct 2008
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Location: Stoke by Clare, Suffolk

PostPosted: Sat Nov 14, 2009 3:45 pm    Post subject: Reply with quote

You might just find some bulbs at the corners of the car, ie turn the side lights on.
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