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MG Midget and Sprite Technical - My ignition light no longer lights!

And the alternator ACR 18 did not charge. I replaced the broken ignition light bulb. The ignition light does not shine and the alternator does not charge. This all began after I replaced the original bulb with a LED, with is not a good idea.
Question: is the alternator gone? The diodes or the regulator?
Flip Brühl

The ignition bulb is in series with the brushes, regulator and field winding so if the bulb does not light then any one of those could have failed. Try earthing the thinner wire connecting to the alt. with ignition on and engine off. If bulb doesn't light it's faulty (or the wire itself). If it does light check the brushes. After that suspect the regulator and finally the field winding.
Bill Bretherton

An LED is a diode so it only allows electrical flow in one direction, an alternator (I believe) needs to flow in both directions (as in alternating current). Perhaps an LED in this application is the wrong way to go. Try an incandescent and see if that helps. Just my guess!
Stan Kowznofski

Fit a filament bulb as it is required for the proper operation of the alternator. One side of the bulb is connected to the ignition switched 12V and provides a small initial current to stimulate the alternator to generate current, when the alternator generates current into the system both sides of the bulb are at the same potential so the bulb goes out indicating a functioning alternator.
David Billington

Although an alternator does produce alternating current, it is rectified to DC internally.
Dave O'Neill 2

An LED with a resistor in series and another resistor in parallel with the LED and the series resistor might work but I see little point in doing that. As David says, the bulb provides the excitation current to kickstart the alternator. I'd assumed that Flip had now replaced the LED with the original bulb but still had a problem. If not Flip, then put the bulb back.
Bill Bretherton

Thank you so far. Yes indeed, I replaced the LED for a 3watt fillament bulb, tested it with a connection between the yellow wire and earth. It worked well but intermittent..... I replaced the generator. The new generator does not work... My youngest daughter will marry on Sunday, I ll be back on Monday to control al the white file connections.

Flip
Flip Brühl

Here is the solution! see photo. The LED did not work as the current it uses is to small or has the wrong direction to start the generator. I screwed in a new 12 Volt 2,2 Watt filament bulb but the bottom terminal contact was flat and made an intermittent contact in the concave fitting.


Flip Brühl

Glad it's working Flip. The LED current is too small, that's why I mentioned a parallel resistor which increases the current. It cannot flow in the wrong direction.
Bill Bretherton

Thank you bill, I could find the fault because of your first answer.

Flip
Flip Brühl

Why is the filament in the ignition bulb broken. The brand new generator gives 15.5 Volt, that is 1 volt more than recomended. Should I go back to the shop?

Flip
Flip Brühl

Do you mean 15.5v BEFORE the bulb blew? The alternator won't work with a broken bulb. But, although high, the 15.5v will not blow the bulb because it is not ACROSS the bulb. When you first switch on ignition you have battery voltage (say 12v) on one side of the bulb and about 0v on the other side (which is why it lights). After starting the alternator generates about 14v so there is now apparently 2v across the bulb except there isn't because the battery is now clamped to the alternator voltage, so 0v across the bulb i.e. it goes out.

15.5v is a bit high, I'd want 14.5v max. Maybe change it if they'll agree i.e. if it is considered above spec. It will have a tendency to overcharge the battery at 15.5v but I'd welcome a second opinion on that.
Bill Bretherton

I'd agree with Bill on this one. 15.5 volts is way too much. It's hard to tell without an oscilloscope, but it's possible that the regulator of the alternator is breaking down and it may be sending an unwanted AC component to the battery. If so that would definitely not be good for the battery - I've seen them boiled dry and I bet Bill has too.
I strongly suggest changing the alternator if you can and aiming for a maximum o/p voltage of 14.3vDC with consumers switched off (lights, fans, stereo and all that malarkey).
Greybeard

It was 15.5 V over the battery poles at 900 RPM. 15.8 V at 2000 RPM. I tested the voltmeter, a cheap multimeter more than 20 years old: It reads about 10% more than it should.... I am looking for a new more accurate one.

Flip
Flip Brühl

Ah, that might explain it. Perhaps your alternator is innocent after all....
Greybeard

Yes, that may well explain it......

And, Grey, I haven't seen a battery boiled dry but am aware it can happen.
Bill Bretherton

Flip,
the last and current cheap (digital) multimeters I've had have been unreliable, stick with what you have and know.

Over the last so many years cheap analogue meter have come back but I've no idea of their accuracy.
Nigel Atkins

Now it is getting interesting: after a day drive - the ignition light worked well- the battery is empty... :no starter motor, no windscreen wipers. Could it be the battery?
Flip
Flip Brühl

Disconnect the battery and take a reading. Then put the battery on a long, low, slow charge preferably with an old low-amp charger.

Once fully recharged do a drain test on the battery by putting the headlights on for about 15-30 seconds then start the car 5 or 6 times off the battery and see what reading you get after this.
Nigel Atkins

First things first Flip. If you can check the fluid level in the battery cells that's a good start.
Greybeard

Excellent spot and advice there, I forgot the very basic, especially if the battery might have been overcharged.
Nigel Atkins

The battery was shot, I bought a new battery. The car started within half a second and the ignition light kept on burning on and off even at 3000 rpm. The fitting of the ignition light was broken. I repaired it. The ignition light kept on burning and the generator did not charge... I replaced the bulb for a 3 Watt example with glowed in the dark. Everting worked fine.

Conclusion: LED means Light Emitting Diode: not enough current and in one direction.
The starting Amperage for a Lucas ACR 17 is at least 3 watt is 0,25 A
Flip Brühl

This thread was discussed between 04/06/2018 and 22/06/2018

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