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MG MGA - Nervous Electrician

I have been reading Dominic's Horn thread with interest. I am fairly comfortable with basic electric circuits, having fully rewired the MGA without any issues and doing modifications such as the third brake light, fog light etc. However, I have never really got fully to grips with the electrical meter. Mine can supposedly test housing circuits and the like, but when I look at the small diameter of the test wires I have fears of melting them or of blowing up the meter.

Presumably I can attach the meter directly to both battery terminals provided I have the correct voltage range dialled in, but what if I have set up too low a voltage range, say 0 to 5 volts, will it damage the equipment? Also, from Dominic's thread that covers 'high resistance', I can understand checking the voltage drop, but is there another way by directly measuring resistance? If so, what do I need to set on the meter and what do the readings indicate.

Cheers (a novice trying to learn)

Steve
Steve Gyles

Hi Steve - a lot depends on the meter you are using. Good quality meters will have an overload protection in them just in case you connect to an overload source. Connecting 12V to a 5V setting shouldn't do any damge to a good meter providing that it isn't left connected for too long.
There should be an Ohms (omega) setting on your meter for resistance.This can be set to a number of ranges. You can click through the ranges until the meter indicates a reading - make sure the circuit is disconnected from the supply first as the meter uses its internal battery for this function.
Cam Cunningham


Steve,
Don't worry about the leads melting, they won't do that.
Your biggest caution with the meter is placing it in the milliamp position (mA) and checking across a motor or other equipment that will normally draw Amps rather than milliamps.
Voltage drop is not a good way to test for resistance in a circuit, unless there is a load attached to the circuit. Ohms is a much better and more reliable method.
If you have the manual that came with your meter, it would be good to refresh yourself.
Good luck.
I learned the hard way, and yes, I have smoked a couple of meters. It doesn't hurt anything but your pocketbook.
Mike
Mike Parker

Steve. Re the resistance measurement bit, and unfortunately it can get a bit technical now! I'm an electrical engineer by training so here goes.
You can indeed measure resistance with a test meter directly, as you query, but you would probably be looking for quite small values of ohms which is often technically difficult and inaccurate because it starts conflicting with the actual resistance of your test leads, the resistance of the possibly "dirty" connections you are trying to locate and eliminate, how effectively you can get a solid connection with the ends of your test leads, etc., etc.
The best way is to measure the actual voltage, under load, directly across a problematical piece of equipment (eg. the horn saga discussed earlier) and compare the reading with that across the battery, also under load. There should be very little difference. If there is, the effect of some offending resistance somewhere in the loop from battery to switch to, say, horn and back to the battery can be diagnosed best by measuring the voltage drop piece by piece in the circuit. That is, best to diagnose series resistance by measuring voltage across each part of the series path. A simple voltmeter, initially on the 12'ish volt range, and then progressively ranged down to a much lower full scale reading to read small voltage drops, is quite feasible but measuring low resistance in ohms, as said above, can be more difficult and inaccurate.
Re the 12 volts applied to a test meter on a 5 volt full scale range, if it's an analogue meter you could possibly bend the pointer as it bangs across and it certainly would not want to be connected for very long either. There might be an overload protection device in the meter but it's always best to start on a high voltage range and progressively range down to the level you are reading. The big no-no is connecting a meter on a current scale (amps) across a voltage source like a car battery! Not a good idea!
I hope that might help, but probably confusing? If so, sorry!
Bruce
B Mayo

Cam, Mike and Bruce

All good stuff so far. Very many thanks.

The problem I am actually trying to address is with one of my 12v batteries.

They are both about 3 years old and I have had them tested. Both ok. Each one sits in a cradle, wired in parallel, each with an isolator. I only use one at a time, normally rotating their use every other day or so. One battery spins the starter motor fine, the other is extremely, and I mean extremely, sluggish. It just turns the starter but no oomph left for a spark. I have cleaned the connections several times but the problem exists. So it is now meter time. I am suspecting the isolator itself as I have been through all other options.

If I try to measure under load (engine running?), won't the alternator out put negate the load? Or should it just be with lights on etc? When you say measure across the junctions, you presumeably mean first one side, then the other, with the other test lead to earth?

Photo of my very expensive(?) meter attached, with backdrop for size.

Cheers

Steve


Steve Gyles

Steve
If both batteries are solidly and similarly earthed at the same point, it would suggest that the earth point and route to the starter motor body must be OK, if one of the batteries performs OK. This does assume though that both share exactly the same good earth point. Both presumably share the same positive (assuming you are negative grounded) route to the starter motor switch and on to the starter motor.
If that's all so, it sounds like you have suspicion in the right area - the only other non common item, the isolator. These things do often give trouble.
Your meter looks fine. Just put it on a dc voltage range and connect it across each isolator in turn with all your lights on - that should be enough to see if one has a large volt drop under some reasonable load, and the other not. As you say, no point in running the engine.
As a general point, this is why measuring resistance is not always a good test because a dodgy earth, dodgy connection, or whatever, is often unstable and affected by how much current you pass through it. Best to load it up with as much current as possible and measure voltage across it.
For really good measure, you could isolate the ignition and run the starter motor off each battery in turn with someone to help you measure volt drop across the isolators in turn. That should prove it one way or another!
Good luck
Bruce
B Mayo

I concur with Bruce about isolating the ignition and just spinning the starter, with the multimeter leads attached on either side of the isolator (doesn't the MGA, like the T series car just use a cable operated starter switch that can be pulled to spin the starter without having the ignition switch turned on?). This will give you a more positive indication of any voltage drop across the isolator than the headlights will because of the larger current draw of the starter. Cheers - Dave
David DuBois

Bruce/David

Many thanks. The confusion in my mind is do I connect the leads to either side of the isolator or do I have one lead permanently grounded, then put the other lead on the 'in' side, note the reading, then the 'out' side and note the reading? I have always looked on electricity as taking the easiest route so I am struggling to understand what the reading will mean by bridging the isolator with with both leads.

Steve

Steve Gyles

David
Correct! Just leave the ignition switch off and spin the starter. My bad memory!
Steve.
You are looking to measure the voltage drop (a bad thing!) directly across the "in" and "out" terminals of each separate isolator switch in turn, with the starter motor drawing serious current. This gives you a subjective, but adequate, indication of how much resistance (a bad thing!) each isolator has across its contacts. A good isolator will only show a fraction of a volt across the contacts so most of the 12/13 volts from the battery gets to the motor. A bad isolator could show several volts drop.
Give me a call if you want to discuss further, any time.
Bruce
B Mayo

Steve, V=I x R. If you have all lights on, say I = 20 amps and the resistance of the isolator (or whatever you are testing) is 0.1 ohms, you will read 2 volts across it. Forget about grounds. NB 0.1 ohms is too much. If the starter should draw 100A, you have nothing left!.
Art Pearse

Steve, the meter takes negligible current to work itself, so it does not provide "the shortest path". On voltage scale, the meter has very high resistance. NOT SO if you are measuring current, where the meter is configured to have very little resistance.
Art Pearse

All

On spinning the starter the volts dropped to 9.5. I went through all the connections and finally found an oxidised (rusty) earth connection on the chassis end of the earth strap from the isolator. Cleaned it up and hey presto, spinning a dream. The bad connection had to be the one that was most inaccessible and must have been why I missed it in my last checks.

Thank you to everyone for the instruction on using a meter. Much happier now.

Steve
Steve Gyles

This thread was discussed between 22/05/2009 and 23/05/2009

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