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MG Midget and Sprite Technical - Ignition Light

I've just noticed as I was garaging the car after a run today that the ignition light isn't going out until about 3000rpm.

All I've done so far is check that the fan belt is tight, which it is. What else should I be looking at?
Roger D

Does the car have an alternator or dynamo?
Bob Beaumont

Do you mean after first start, or all the time? On one of my classics the old alternator won't start charging at first until the revs are over 2000, but from there on it works as normal. The car (a Triumph) has a voltmeter and on first start it only shows battery voltage. Once I have blipped the throttle it rises to around 14v. I believe this is normal for alternators of that period (late 70s) - it's something about the alternator needing a kick up the behind!
Mike Howlett

Bob, It's an alternator and Mike I noticed it after a 30 minute run. I am not sure if the light was on during the run, I don't think so, bur I noticed it as I backed into the shade of the garage.
Roger D

Check voltage across battery at, say, 1000 rpm and 2/3000 - should be about 14v. If much lower but you get a red light with ignition on, engine off, it could be a failing rectifier in the alternator. Fixable cheaply if you can solder. If no ignition light with engine off, probably a failing regulator in the alternator. Replaceable with screwdriver/ small socket.
Bill Bretherton

Roger, I should have added, remove and clean the alternator connector first to see if that helps.
Bill Bretherton

I have cleaned the alternator connections and checked the voltage at the battery. There is a good 14v from 1000rpm upwards however the ignition light glows up to about 3000rpm.
Maybe it's not really a problem.
Roger D

I had similar and according to the people that had reconned it, it was the diode pack going down - they replaced the unit.

Worth checking that the battery is not self discharging when parked up.
I always disconnect my battery when garaged - have simple disconnect switch thing on battery.

R.
richard b

I agree with Richard. Set your meter to AC volts and place test leads across the battery with engine running. If the AC volts reading is several volts, then that indicates a faulty diode pack (in the alternator). It is replaceable but soldering is needed with a 50 or 100W iron preferably using a heat sink to protect the new diodes.
Bill Bretherton

I'll do that. I'm not a dab hand with a soldering iron, actually I don't have one!
I notice that MGOC spares have alternators at £59, maybe that's an easier option.
Roger D

Roger
The accuracy of the AC reading depends how good your meter is but it might give a clue.
Bill Bretherton

I checked with the voltmeter on A/C and it appeared to show 30 volts!
Roger D

That's a lot Roger! The actual voltage reading, as I said earlier, depends on how "good" your meter is but it does look like you have AC leakage - a shorted diode will cause that.

I'll measure mine tomorrow both with a digital and analogue meter and post back. The £59 may be a good deal - if you were nearer I'd happily take the faulty one off your hands if it's not an exchange deal and fix it. You don't really want to use the car in that state if it's a true AC reading imv.
Bill Bretherton

Roger, tested my battery voltage with engine running on AC volts earlier with my DMM and an old analogue meter. The analogue one gave a reading but the meter could be faulty - I don't generally use it anymore. However, the newer DMM gave virtually 0v on AC volts setting which I'm happy with. So you do appear to have a faulty diode pack (rectifier).
Bill Bretherton

I have ordered a new alternator from MGOC. They haven't yet told me how much it will cost to send to France. That could possibly be a deal breaker.
Roger D

Just disconnected the battery and checked it with the multi meter and it shows 12.5 d/c but 26.5 volts a/c !

I think I need a better meter, I might just save myself the cost of a new alternator.
Roger D

Roger
My everyday DMM is nothing fancy, purchased about 30 years ago, but probably better than those cheap things you can get on hebay. The top quality ones are made by companies such as Fluke but are expensive - I have an older bench model.

A decent meter should "block" DC when measuring AC imv but some might not.
Bill Bretherton

Bill,
I'm pretty confident that my meter is at fault so I'm going to get the voltage checked by my garage. However if I do need to get a new alternator the one supplied by MGOC has a connection requiring an ARC plug, they can supply this plug and it comes with instructions for wiring which I have attached.

My problem is,apart from old age and incompetence, the existing wires on my car don't really tie up with the instructions. I think the second pic show fairly clearly the brown and green and the brown and yellow wire but in the first pic there is a third wire (white/black or maybe white/blue) which doesn't seem to be mentioned in the instructions.

What to do?






Roger D

Using a meter on AC range on the battery with engine running is a bit of a guess in a sense because it's difficult to know what a particular meter is actually measuring. The only true indication of waveforms and voltages is that measured with an oscilloscope which most people don't have of course.

IF your meter has a "diode test" function you could try connecting between the alternator terminals and ground, with connector removed. The larger terminals (which are connected together) should give somewhere between 0.5 and 0.8v with negative test lead to terminal and no reading with positive. For the smaller terminal you should get a reading above 0 (0 point something) either way round.
Bill Bretherton

Roger
Just seen your last post as I'd gone to the garage during writing my last post to do a test (so post started before yours)!

However, the pdf seems to be written for an MGB? Your car will have a thicker or two thicker wires for the larger terminal and one thinner wire for the warning light but I'm not certain what your specific colours will be, depends on car/ modification to wiring etc. Someone else might know the colours better than me.
Bill Bretherton

Bill,
I think I have sort of solved the mystery of the third wire to the alternator. I have removed the alternator and taken off the back cover, this showed that the white wire was connected to a terminal on the rectifier pack. I then traced back the white wire and found that the other end was not connected to anything!

This doesn't explain why the wire was there in the first place but clearly it was performing no function so connecting the new alternator should be no problem, just the thick wire and the thin one.

My dealings with the MGOC were very good, the order was despatched very promptly. Because they didn't charge VAT I knew I would have to pay French TVA which is the same as the UK, however what I was not expecting was a €15.90 brokerage charge on top of that. Not quite sure who made that charge, customs or UPS.

Isn't Brexit great!
Roger D

This thread was discussed between 15/04/2023 and 30/04/2023

MG Midget and Sprite Technical index

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