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MG TD TF 1500 - Test Amp Meter

This is probably a fundimental question for most of you but I would like to ask: Is there a way to test an Amp meter? I have re-assembled the Amp meter and want to test it before I close it for good by bending the back tabs.

Thank you in advance,
Patrick
D P Earles

Get another meter, preferably a good test meter. Hook them up in series with a battery and some different loads - one light two lights, etc. Batt hot>>meter>>meter>>> load>>ground (other post of batt) Both meters should read the same. Don't overload the test meter, typically a multimeter - it will blow the fuse. Lacking a test meter, wire it the same way with just the ammeter meter; the loads go in parallel to each other. A standard 60W hibeam should give c5Amps on a 12V batt, two give 10A, etc. It's not a lab instrument!

FRM
FR Millmore

FRM
Remember, you are addressing to a true novice on this end. The Ammeter is sitting on the my home office desk. By "test meter" do you mean a little hand held multi-meter? Also, I have a headlight (that I think is good) that has three connectors, I'm assuming that one connector is ground, one if for high beam and one is for low beam. Good assumption?
Thank you again,
Patrick
D P Earles

An aside.... be really careful bending those little tabs back...be gentle... Mine were so thin one of them broke off and another is ready to...had to use a lttle 'tad' of epoxy to hold it together....
(and where i was able to clean up the other gauges, the ammeter had a paper dial with stains and didn't clean up...!
gblawson(gordon)

Gordon,
The little tabs were the reason I wanted to test the meter before I bent them back. I figure they only have so many "bends" in them before they break. As for the cleaning up of the guage and its face, the amp meter is the instrument of which I am least proud. The face didn't clean up as well as the others, but it still looks fine for what my uses will be in the distant future. Now if it will only work.
Patrick
D P Earles

Patrick,

You have it essentially correct. By standard 60W headlamp, that is the rating on main beam (not dipped). a Sylvania Halogen will typically be 60W. The three connectors on the back of the head lamp are Ground, Main, Dipped. A process of elimination will reveal which is which.

I would skip the known meter test and simply run it in series with the headlamp on high beam. If the meter is zero with no load, and 5 amps with the high beam, it works good enough for your car.

As Fletcher says... it ain't a laboratory instrument.

warmly,
dave
Dave Braun

Patrick-
" By "test meter" do you mean a little hand held multi-meter?"
Could be, they usually have a 10A limit, and if you apply a 10A load it may blow the meter fuse.
If you have no other device, like another ammeter, it might be easier to use a known load. Watts = VxA. V for a good battery, charged but not being charged, is around 12.5V, nominally and for this test practically taken as 12.0V.

"Also, I have a headlight (that I think is good) that has three connectors, I'm assuming that one connector is ground, one if for high beam and one is for low beam. Good assumption?"
Yes. You can tell which is what by looking at a car plug for the lamp; in Lucas, Black is ground, blUe/White is high beam, blUe/Red is low beam. Or if you hook two wires to any pair of terminals: if it's bright, one of the two is grd, if dim, you have the two "hot" terminals. Apply some logic and you can sort them. Wattage of the lamp should be on it. Modern are 60W high, 55 low; older ones may be less.
For practical purposes on this test,using a 60/55W lamp, you can just figure 5A per filament. So, if you hook it up Batt>>HB terminal>>ground terminal>>meter>>batt grd post, the meter should say 5A. If you hook it up Batt>>HB+LB terminals>>ground terminal>>meter>>batt grd post it should say 10A. This puts the two filaments in parallel.

FRM
FR Millmore

Patrick,
IMHO: Strongly advise installing a battery cut off switch when you hook this back up to the car. 12 volts from your battery can easly act as a small welding rig if you touch your wrench to the wrong place. Don't forget the "correct" way to install your meter will depend if your car is positive ground (as orginial) or negative ground if it has been changed over. If you meter "reads backwards" after you install it make sure you disconect the batery before you change the leads....otherwise we'll be calling you sparkey!
Cheers,
David
David Sheward

Thank you everyone. I ran the test tonight and got OK results. The meter moved a little maybe 3 to 4 amps in measurement. The unkown veriable in the equation was the Wattage of the headlamp bulb. Watts are not indicated on the bulb. I used the one that came off of the TD. The bulb is a modern bulb circa 1969 (30 years ago). Either way, for now, I plan to close up the back end of the meter and mark it complete so I can move on to the next part.

But before moving on, what is the purpose of the center screw on the back of the Amp meter? see attached picture. Is this an adjustment screw?

Thank you again,

Patrick


D P Earles

Picture of Completed Amp Meter font view.


D P Earles

Patrick-
Such screws are generally zero adjustments.
Quite likely your old bulb is lesser wattage, hence less amps. 48W = 4A, close enough. You can probably find a number on the bulb, but at the time 40/50W bulbs were common, and lesser not hard to find. Not certain, but I think originals were something useless, like 35/40W. Even with the OE generator, the car will handle 60/55, if you keep the engine speed up - and you can actually see!

FRM
FR Millmore

Thank you Mr. Millmore, I was guessing the same on the screw in the back of the meter. To the best of my abilitiy, I can't seem to find the Watt indicated anywhere on the bulb, but I am assuming just what you state, that the bulb was a lower Watt bulb.

Thanks again to all for the help,
Patrick
D P Earles

This thread was discussed between 30/10/2009 and 04/11/2009

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