IGN light on, fitted with alternator

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Tas91
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IGN light on, fitted with alternator

Post by Tas91 »

Hi everyone, this is my first post.

I have a 1962 Minor 1000 that I bought at the end of November. It’s fitted with an alternator and recently the ignition light (red) stays on even at higher revs. It eventually goes off when the car is warmed up after 5-10 mins but when idling, it will sometimes come back on again.

I know that this would mean the dynamo isn’t charging but with an alternator fitted, does it necessarily mean that it’s not charging?

Thanks
les
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Re: IGN light on, fitted with alternator

Post by les »

First check is to make sure the fan belt is not too loose and consequently slipping. Dynamo and/or alternator will not charge if the belt is not tensioned properly.

simmitc
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Re: IGN light on, fitted with alternator

Post by simmitc »

Ign light on would mean not chargiung, it works the smae with an alternator as a dynamo. As above, check fan belt, but it's interestiing about going out when it warms up. Usually with an alternator it will be the rectifier built in to the back of the alternator that fails. These can be repaired/replaced (often just a diode) but it depends on the design, and often easier to just replace the whole unit.
oliver90owner
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Re: IGN light on, fitted with alternator

Post by oliver90owner »

More often it is the brushes that are worn out (only one, usually).

The slip ring could also be in need of attention/replacement.

Loose belts usually tell you they are slipping!

Rectifier packs are usually good or duff.

Alternator bearings could also be in need of replacement - especially if the belt has been tensioned too tight (usually because the belt is worn out). It is generally only the drive end bearing that gets hammered.

Fixing is easy and cheap, provided it is only one part that requires attention. Most pay multiple pounds for a service replacement when the overhaul would be peanuts.
IslipMinor
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Re: IGN light on, fitted with alternator

Post by IslipMinor »

Hi and welcome to the MMOC Message Board.

2 quick checks before diving into fault finding. Both related to is it charging? An essential tool for any classic car maintenance is a multimeter, that includes a DC scale that reads up to 16-20v. With an alternator it is almost certain to be wired negative earth (-ve), but CHECK, so connect the negative lead to either the battery -ve terminal, or a convenient connection on the body/engine.

With the ignition switch turned 'off', connect the +ve meter lead to the +ve battery terminal. You should get a reading of ~12v, or a little more. Much less than 12v, charge the battery and repeat the test.

Leaving the multmeter connected, start the engine., and observe the voltage being displayed. It should drop whilst cranking the engine over, but once started, should increase to at least 13v, and more likely 13.5v. Anything more than 12.5v shows that it is charging, but not necessarily enough.

Turn on the headlights - does the charge voltage stay at 13v+ ?
Richard


Tas91
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Re: IGN light on, fitted with alternator

Post by Tas91 »

Thanks everyone, your posts are quite helpful and I’m going to look in to it as soon as I can.

It’s a negative earth so my first port of call will be to find my old multimeter.

I’ll update shortly.
oliver90owner
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Re: IGN light on, fitted with alternat

Post by oliver90owner »

A possible alternative to Islip Minor’s check for charging, is to start the engine and turn on the headlamps. Then increase the revs and note whether the lights brighten. Not always easy to see the difference if the alternator is already charging, but comparing the lights with engine off, and engine running might shed some light on it (pardon the pun). The multimeter should give definitive results under all circumstances.

The multimeter can, of course be left connected in the car to monitor the battery voltage when the ignition light comes on.

Charge voltage with a fully fully charged battery should be around 14.4V.

The charge light can come on if the engine idle speed is sufficiently low (think here it should most definitely glow when the engine is stopped!).
Tas91
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Re: IGN light on, fitted with alternator

Post by Tas91 »

So I looked in to it further and I decided the fault must be with the alternator itself. Took the Lucas 17ACR out to find that one of the brushes was quite worn.
£4 later, I fitted a new pair of brushes and it's working perfectly.
oliver90owner
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Re: IGN light on, fitted with alternator

Post by oliver90owner »

As expected. Alternator failures/repairs are usually fairly simple and easy fixes at much reduced cost than exchange units. Good to get the final outcome - even if it has taken 6 months.🙂. Anyhow good to know the actual outcome - many disappear, never to be seen again, until the next fault.
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