MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MG Y Type - YA ammeter

This is one of my spare 20/20 ammeters just for reference. The one in Y3348, the original, seems to be reverse polarised - ie it reads the opposite + or - if wired according to the WSM. My 3 spares all read correctly. For now, I've simply reversed the wiring locations on the original ammeter so it reads correctly - but is it possible to reverse the polarity in the ammeter so that it reads correctly when wired as per WSM? Hope this makes sense.
John,

J P Hall

John

Is your car POSITIVE GROUND/EARTH still as per original or NEGATIVE GROUND/EARTH?

Paul
Paul Barrow

Sorry, should have stated up front: positive earth, everything per wsm, and new (2022) loom from NTG. The way this and the TD's ammeter is in series hot from the battery, I tend to fret about the fire risk if anything shorts ( including inside the gauge). Am I worrying unnecessarily?? John.
J P Hall

Thanks for confirming that John.

How do you know that it wasn't incorrectly wired before?

The BROWN wire on Terminal A on the back of the Ammeter should be traceable back to Terminal A on the Lucas RF 95 Control Box.

The two YELLOW WITH BLACK TRACER wires on Terminal B on the back of the Ammeter should be traceable back to the starter switch on one wire and the clock on the speedometer on the other.

How WAS your car wired up? I presume it has a proper STANDARD MG Y Type wiring harness in it?
Paul Barrow

Home/Aus import model YA Paul, so white/purple from A terminal to A on Control Box, and yellow/black from B terminal to starter switch and clock.
As said, new (Y Type specific) loom throughout. If no-one has a view on repolarising the guts of the ammeter, I guess I'll put up with reversing the wiring on A and B terminals. I prefer that to having an ammeter that looks nice but doesn't work! John.
J P Hall

I would say that maybe try repolorizing your car to positive ground again as described in https://www.mg-cars.org.uk/imgytr/hints&tips.shtml#Positive_to_Negative.

What you describe you are seeing is almost as if the car has (at some time maybe before your ownership) been flashed to negative ground so flash it again as POSITIVE ground.

Then see what happens :)

Paul
Paul Barrow

Very interesting article from SpriteSpot, thanks Paul. Because it took me 25 years to finally finish the YA resto (don't ask - work, kids, constant sport ...) in 2023 my auto leccy did flash the dynamo on the bench, to make sure it was positive earth; and as far as I recall, the car had run on positive earth from 1984 when we bought it - but your advice holds good. What I'm unclear on, is whether I can put the ammeter on the bench and wave a magnetic field across its innards to reverse its polarity. That's above my paygrade. If not, even in the above article, it seems that reversing the ammeter wires as I have done, is perfectly legitimate, and should not be considered a crime. John.
J P Hall

Ammeters arent polorized to the best of my knowledge (which isn't much). All it does is read current or lack thereof. It wont harm anything to flash Positive earth/ground a second time.
Paul Barrow

Definitely no harm, Paul - I must remember to disconnect electronic/solid state items first.
Apropos magnetic field in the ammeter, I know that as you move a magnet across the face (on the bench), the needle moves first one way, then the other - suggesting a stored field of some sort. That's why I thought it might be possible to reverse its field. As I say, above my pay grade!
J P Hall

Apparently yes a DC ammeter CAN become polarity sensitive!

https://blog.truegeometry.com/api/exploreHTML/325563012aa4dc743c666d8c5714f2b8.exploreHTML

As to changing the polarity, I found this below. Cant say I understand it at all ... but here goes.

Changing the Polarity of a DC Ammeter

Reversing the polarity of a DC ammeter generally involves swapping the positive and negative connections. This action changes the direction of current flow through the device.
Important Considerations

Device Specifications: Before attempting to reverse polarity, it is crucial to check the specifications of the ammeter. Some digital ammeters are not designed to handle polarity reversal and may be damaged if this is attempted.

Potential Damage: If an ammeter does not support polarity reversal, connecting it incorrectly can lead to short circuits or permanent damage.

Methods to Reverse Polarity

Here are common methods to change the polarity of a DC ammeter:
Method Description Best For Complexity
Manual Swap Physically disconnect and reconnect the + Testing, one-time None
/− wires. setup
DPDT Switch Use a double-pole double-throw switch to DIY projects, Low
reverse connections. simple control
Relay Use a relay that swaps connections when Automated systems Medium
energized.

Conclusion

Always ensure that your ammeter is compatible with polarity reversal before making any changes. If unsure, consult the user manual or manufacturer for guidance.


I don't know this for a fact so maybe check it out with an auto-electrician ... but maybe a Positive Ground flashing may change it back??

Check first though.

Paul
Paul Barrow

Worst case ... I have 2 -20+20 ammeters on the shelf in stock to replace yours.

Paul
Paul Barrow

Hmmm, interesting - I would be happy to try it with the Ammeter on the bench. Swapping the supply (Y/B) and ignition feed (W/P) connections hasn't reversed the polarity, but that would be because it's still grounded, so the electrons have somewhere to run. I'll definitely involve my auto leccy mate, and report back.
I also have 2 spares, thanks.
John.
J P Hall

Have (safe) fun!
Paul Barrow

Paul's suggestions are simply different methods of swapping the wires over---I doubt very much that flashing it will do anything.apart from giving the needle a workout.
Maybe the terminals can be swapped over inside the gauge itself but I doubt that as well.
Looks like I'm pulling one apart just to see.
Has this gauge always read back'ds or only since something(what) happened----There must be a reason, none of these cars were ever neg. earth.
William Revit

Dragged a gauge set out today, removed the amp. meter and yeah the front face is riveted on with tubular rivets--Not wanting to wreck a perfectly good gauge really just to see what's inside, so it's gone back to sleep---don't mention it ,happy to help-lol


willy
William Revit

So Willy o' the south, I had already done that on a previous occasion! This is what they look like inside (I have no idea why it comes out sideways, as it's the right way up on my computer). This one has only the stub of the needle (you can guess who broke it) - but it works as it should, so the innards are fine. I gather it sends the electrons across via the thick wire with the 2 turns in it; and that the resultant magnetic force pushes the needle one way or the other, depending on which way the electrons are flowing? Clearly above my paygrade - but as per the original post and my various others, I'm happy to chip away at something until (a) I understand it, and (b) I can get it to work the way it should.
Thanks always. John.


J P Hall

Ahh---amp. meter---so from what I understand that dark piece down under the (broken) needle with the rounded triangle ends is in fact a swiveling permanent magnet--pos. one end neg. the other. Depending on whether the current flow in the gauge's coil is pos or neg(charge or discharge), the appropriate end will be drawn in towards the gauge's coil-moving the needle to show charge or discharge---For your gauge to be reading backwards then that swiveling magnet would have to have been magnetised in reverse which is almost impossible in normal service---except--If the gauge had been completely dismantled it would be possible to fit that magnet and shaft in there half a turn out maybe--apart from that it's a bit of a mystery how it could get like it is.
William Revit

Interesting, Willy - your description tallies with what happens when I poke the stick-magnet towards each little pointy bit behind the needle. It pushes the needle away. Also - looking at the innards more closely, it's more understandable (to me) that there's no obvious electrical problem or risk when I simply switch the input wires over. So, since the ammeter/amp meter is functioning perfectly, I shall now leave it as-is for another 25 years. That'll make me nearly 100.
John.
J P Hall

Yep, fairly basic old gauge--absolutely no harm can be done reversing the wiring----Maybe the gauge was assembled incorrectly new and has always had to have had the wiring switched---rest easy young man there will be no smoke from there.
William Revit

Roger that. Good conversation as always. John.
J P Hall

Interesting outcome there John and good story!

I would take one of your spare meters and wire it in correctly and then see if the deflection differs.

If it does not, then maybe there is a fault in your control box in the engine bay.

If it does deflect correctly (goes to + when you rev) then you definitely have an anomaly in that gauge so I would fit the correct on and mark the incorrect one as reversed if you intend keeping it ... or toss it.

Paul
Paul Barrow

I might just do that, Paul - be interesting to see. Will report findings.
This is what I like about this forum - it's well suited to more depth, which helps those of us who are not trained, but keen to wield a spanner and keep on learning.John.
J P Hall

Well now. Curiouser and curiouser. Today, my dear wife of 42 years (who's counting)rang me from town to announce that the Y was refusing to even crank to return home - even though it has behaved faultlessly until now. On test, she (the car) fired up immediately with my leccy mate's jump starter. BUT - the meter indicated no charge from the dynamo; the battery voltage rose to an alarming 16+ as soon as we carefully upped the revs. The car did drive home, but at rest the battery only indicates 9.9 volts.
Given the above discussion about my ammeter reading negative when correctly wired - is it possible that in fact the ammeter was telling the truth? - ie that with increased revs, more strain was being put on the battery because unbeknown to me the dynamo was failing?
The battery is no more. It has ceased to be. Bereft of life, it has gone to meet its maker...
All thoughts gratefully received. John.
J P Hall

Update: new battery, auto leccy checked dynamo polarity and operation of dynamo (original 1949, but new bearings, brushes & insulation) and control box - all functioning well, so it was just the battery deciding to suddenly die minutes after its 3 year warranty period. Typical.
Now back to testing a second amp-meter to see if it too reads in reverse. John.
J P Hall

Maybe John - just maybe, all your supply of gauges are working correctly and it was your car itself playing tricks all along--?
William Revit

I must agree everything points to that, Willy - I just don't get why, when the dynamo is correctly flashed to +ive ground, coil connected likewise and control box is functioning perfectly (solid state from UK specialist), the ammeter when wired according to wsm reads backwards - including spare ammeters connected the same way.
Solution: reverse the ammeter connections, watch the ammeter like a hawk, and enjoy our early winter days of 24 in the sun, 12 at night.
Onward and upward.
John.
J P Hall

Just to add.--Last year I had a mate travelling on the mainland (should have known better, it's dangerous over there) in his MGA. The genny spat a bush and he bought a new/old stock generator from an auto elect. and had it fitted by the auto guy who said it was good to go. All the way home his battery only just held up but was flat the morning he had to get off the boat here. Had a look and it wasn't charging and discharged with any load applied.
decided to give it a reflash and bingo, charging like a trooper and hasn't looked back since.
Maybee your generator didn't get flashed properly originally and now with it done it's ok. If they've been apart or stored for ages and lost their magnetism they just sit there in no no land--It's important to always give them a jolt so they understand--

willy
William Revit

Agreed - that's one of the things that stood out in my readings about this ammeter caper - most of us use these old bangers intermittently- life gets in the way and they sit idle from time to time - giving the dynamo the opportunity to lose its residual magnetism and its polarity. Apparently there's no harm done in freshly polarising from time to time - although in my case the job has to be done at the dynamo, not at the regulator since mine in the Y is solid state, so it's polarity specific. As said, Willy - I keep on learning!
Grateful, Atherton.
J P Hall

The norm would be to do it at the generator(dynamo) It's pretty simple really ,wires off and zap, wires back on. I've never had one lose it's memory in the car it's usually when they've been on the shelf or dismantled, but thinking about it yeah it would be possible I guess. As you say it doesn't do any harm, bit like shock therapy-lol
William Revit

This thread was discussed between 20/04/2026 and 28/05/2026

MG MG Y Type index

This thread is from the archives. Join the live MG MG Y Type BBS now