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MG MGA - Electrical question

Hi all,
Is there an electrical expert out there? - at least someone who knows more than I do which shouldn't be difficult. This concerns an ammeter which is normally at mid position except when something electrical is on when it dives into the minus sector by varying degrees depending on the load. Headlights on and it goes to max on the minus side. Yesterday I drove the car about 100 miles with lights, wipers and fan on for most of the time. When I got back I connected a voltmeter up to the battery which read just under 13. With the headlights on and the engine revving to about 2500 this crept up to about 14. Have I got a problem or is it just a faulty ammeter do you think?

Many thanks,
p parmenter

Ammeter and voltmeter are not measuring the same things.Ammeter shows current flow to and from the battery. Voltmeter shows the actual supply (emf)from the battery .
Your ammeter seems to be measuring correctly - i.e. large discharge of battery with the headlights on and engine off. If you are using the original dynamo then you will find that even with the engine running - there will be a slight discharge of the battery if you have lights, wipers and heater fan on. An alternator will work better giving a higher output current and so keeping your battery fully charged. So your voltmeter shows this as it is slightly below full charge if it is less than 13v. Full charge of the battery is around 13.8 volts with engine running so the generator seems to be coping with just headlights - but don't forget that when you are driving, the ignition system takes some charge too.
Cam Cunningham

Hi Cam, sorry - I haven't explained it at all well. Firstly forget to mention that I have just fitted a new alternator (the car already had an alternator which I suspected to be faulty). The large discharge shown on the ammeter when the headlights are on is with the engine running and it makes no difference how high you rev the car - it stays exactly the same. I'm thinking that after a run of 100 miles with lots of electrical things on, the battery would show a lot less than 13 if the system wasn't charging it.

p parmenter

Phil, If you drove 100 miles (3 hours?) with all lights, wipers, etc. on (15-20amps) then your battery (50 Amp Hour ?) would be dead if your alternator had not been charging. If you have a multimeter put it on the 20A range and connect in series with your car ammeter to check your charging circuit/ammeter.

PS Have you recently installed the ammeter? ................................Mike
m.j. moore

Have you seen this:

It might be useful in checking all your wiring is as it should be.
Graeme Williams

Simple test to see if the charging system is working. Start the car and let it warm up until it idles properly. Remove ground cable from the battery. If the engine dies, your system is not charging. If the engine continues to run smoothly then it is charging the system to around 12V. Stop the engine... Reconnect battery and take a voltage reading... with the engine running.... should show between 13.8 and 14.5 V.

Make sure the battery connections are cleaned and lubed.... many charging problems are due to corrosion at the posts.

... CR
C.R. Tyrell


It sounds like everything is working properly. What is confusing you is that, most likely, the alternator charging current is going directly to the battery (probably a large diameter wire running from the alternator to the battery side of the starter switch). The charging current does not go through the ammeter (you really do not want all that current running under the dash!). This is why you see the load of the lights, etc. (meter going negative) and never charging current (meter going positive). The fact that you say about 13 volts at rest and 14 with the engine a bit reved says all is right with the world. A voltmeter is a much better indicator of alternator operation.

F. J. Bruns

you should check out Barneys website (Article ET 220) which is all about the benefits of using a voltmeter.

To get an accurate indication of amperage using an ammeter you will have to divert the entire output of the alternator through the ammeter which (as Frank said) is not ideal.

It would need a pretty big wire for this and you would have to be really carefull to fit grommets to the wire as it went through the bulkhead to prevent any short circuits. You would also have to be really careful how you connect it to the meter.

A voltmeter would be much easier to rig up as you can connect it pretty much anywhere in the ignition circuit using relatively thin wires and it will still tell you everything you need to know about how your charging system is working.


c firth


It sounds as if it's not connected correctly, ie it only shows current draw, but not charge.

As you have an alternator fitted you really need a 60-0-60 Ammeter as the charging ammps are much higher.

I think a previous poster has the answer, I bet the alternator is connected directly to the stater soleniod, thus bypassing the ammeter.

Hope that helps.

Regards Steve.
SR Smith 1

If the voltmeter is connected across the battery, does a short on the live cable to the voltmeter present a lower risk than the connection to an ammeter?
Graeme Williams

Thanks very much for all the info. I'll stop worrying now.

p parmenter


Disconnect your ammeter and just drive it. One less thing to worry about! MG didn't fit one so can't be necessary.

Steve Gyles

I bet if he disconnects it the car won't run!

By all means join the wiring to the same post on the back, that'll bypass it.

Better idea is restore the wiring to oe spec, there must be additional wiring installed between the control box and the ammeter itself.
SR Smith 1

Ref my own comment about voltmeter - as a voltmeter draws only a low current I guess it can be protected by a low rated fuse and therefore doesn't present the same risk!
Graeme Williams

Just a thought and to show my total lack of understanding of electrical intricacies, does the ignition warning light provide the same essential information as a voltmeter/ammeter? i.e. if it goes out after starting then the system is charging and all is well with the world?

Steve Gyles

My understanding is that with ignition on/engine off the ignition warning light has a feed from the battery and grounds through the dynamo/alternator. When the engine is started the alt/dynamo also puts a voltage on the ignition warning light so the circuit is broken and the warning light goes out.

With a dynamo fitted the warning light can glow at low engine revs - so I guess there is some sort of 'competition' leaving a small voltage to light dimly the warning lamp?
Richard Atkinson

Happy New Year to the list

Clearly the comments of wrong connection are correct. I have had an ammeter on my dash for several decades and it works as I would expect on my dynamo car. I do glance at it with the other instruments but is it really useful is a good question.

Until recently I had uprated 75 watt sealed beam units fitted but the dynamo always seemed to cope even for many long night drive in the old day. The biggest epic was Gatwick to Edinburgh after a ski holiday all in the dark in the days of the overall 50mph limit of a 1970s oil crisis for those who remembers those days (as we crossed the border at around 3am I then risked putting my foot down a bit to reduce the tedium).

Paul Dean

((1) RE: the Moss document
has an error in the wiring. The MGA has two black wires on the "E" terminal of the control box ("earth" terminal). When converting to alternator, and disconnecting these wires, the two black wires have to be connect4ed together to retain ground connection for the wiper motor. Otherwise that error does not affect the new alternator which is grounded through the engine block.

(2) Do NOT disconnect the battery while running the alternator. This has the potential to fry the alternator.

(3) Anp meter wiring diagrams:
Amp meter with generator:
Amp meter with alternator:

(4) If the alternator output terminal is connected directly to the battery cable, the alternator output wil not go through the ammeter. The ammeter will then register only negative current draw and will not include the positive charging current.

(5) Ammeter used with generator is usually -30 to +30 amps scale. The alternator has higher output than the generator, so the ammeter needs to be -60 to +60 scale, and the wiring to and from the ammeter needs to be much heavier wire to carry the higher current.

(6) The ammeter must use fat wire to carry all of the current in the car, including the alternator (but not including the starter motor). If you get a short to ground on the ammeter wire, you will get a HUGE burned wire that will burn up anything around it very quickly. The volt meter can use a very small wire, carrying only micro-amps of current. If that one gets shorted to ground it will be a much smaller "fire" (small wire burning), and it can be fused with a 1-amp fuse (if you like) so it won't burn.

(7) If you want to have an ammeter used with the high output alternator, there are more modern forms of ammeter that use a "transducer" to sense the high current and produce a small voltage to drive the ammeter (which is then essentially a low current volt meter with amps reading painted on the face plate).
Barney Gaylord

This thread was discussed between 02/01/2016 and 04/01/2016

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