Technical ignition light on under revs

[sprie]

A shortcut on the battery can get a diode toasted. Touching the jump cables between them while connected on the battery does that.
AC voltage that comes out of the alternator should be under 50 mili Volts.

If even one diode fails two things will happen.

• The DC field will be out of balance with one side of the bridge producing more current than the other. This can be heard and felt as a low pitch vibration originating from the alternator (different than a bearing noise).

• The out of balance field on one side of the rectifier bridge will begin to overload the remaining diodes on that bridge. This overload will cause a DC voltage increase on the failing side as the remaining diodes overheat and begin to allow AC voltage to bleed through. An oscilloscope can verify this condition.

Somehow your voltage regulator cuts the charging too at some point, when you have 12.2 V with the engine running.

I am pretty sure i haven't shorted between the battery nodes.

However...

Whilst it is possible i have been unlucky with the alternator and it was/is faulty, before i go to the trouble of taking it off and sending it back to the supplier to try and get a replacement, I feel i ought to investigate the current setup in case something has caused the issue.
What should i do as part of the investigation?
Is it just to look for a short?
I did replace both the vehicle battery and the leisure battery, so they might be worth checking somehow?

 

[Mike1alike]

Considering the alternator just cuts out whenever, 12.2 V with the engine running, there isn't much you can check. Just that, read the voltage on the alternator post. If there is 12.xx with the engine running alternator is the problem, send it for replacement.
Did it do this light on thing as soon as you bought it (new alternator) or was there some time when it was ok?

 

[bugsymike]

I am pretty sure i haven't shorted between the battery nodes.

However...

Whilst it is possible i have been unlucky with the alternator and it was/is faulty, before i go to the trouble of taking it off and sending it back to the supplier to try and get a replacement, I feel i ought to investigate the current setup in case something has caused the issue.
What should i do as part of the investigation?
Is it just to look for a short?
I did replace both the vehicle battery and the leisure battery, so they might be worth checking somehow?

Would it be an idea to isolate leisure side when testing new alternator and if OK, then see if still good with leisure battery reconnected.
In the past I have gone to start a car with flat battery and it wouldn't turn over even with a good battery until I disconnected the original battery and bypassed it and car started perfectly as original was shorting internally.

 

[sprie]

Update on alternator (maybe good news, maybe not):

I have got agreement to return the alternator to get a replacement, so I started on the removal job today.

Needed to remove the cables first before removing the attaching bolts.

There are 2 cables – which are attached with nuts onto their respective threads on the alternator – which are nestled inside a plastic protective cover/shroud (can’t get a photo, too many engine bits in the way).

The thick cable came off fine – this is a 13mm nut, and I have a long socket that fits nicely.

The thin cable: I eventually got the nut off. It is a 8mm nut. I don’t have a long socket for this (got 9mm to 13mm), so when I put this on, I had to manage with normal sockets. And this nut was loose.

Looking on the web, this thin cable is the one that goes to the ignition light on the dashboard. So now I am thinking because the nut was loose the connection was poor and maybe under revs and with a bit of corrosion from damp air, this caused the issue with the ignition light. Which is potentially good news.

However, when I tested at the battery (as described above), it wasn’t charging.

Is it possible that the two things are linked: if the circuit to the ignition light is faulty, would that stop it charging (I am guessing not)?

Regardless, I guess I need to get myself a 8mm long socket, put the cables back on, tightly, using a bit of contact cleaner and see what happens

 

[bugsymike]

The thin cable: I eventually got the nut off. It is a 8mm nut. I don’t have a long socket for this (got 9mm to 13mm), so when I put this on, I had to manage with normal sockets. And this nut was loose.

Looking on the web, this thin cable is the one that goes to the ignition light on the dashboard. So now I am thinking because the nut was loose the connection was poor and maybe under revs and with a bit of corrosion from damp air, this caused the issue with the ignition light. Which is potentially good news.

However, when I tested at the battery (as described above), it wasn’t charging.

Is it possible that the two things are linked: if the circuit to the ignition light is faulty, would that stop it charging (I am guessing not)?

Regardless, I guess I need to get myself a 8mm long socket, put the cables back on, tightly, using a bit of contact cleaner and see what happens

Certainly well worth checking as it can affect charging rate also when charging starts.
I recall when fitting an alternator to a slow revving diesel I was told by increasing the wattage of the charging light it could make the alternator start at lower rpm by an Auto electrician.
Another point if wire loose or off for a while it can damage the diodes inside the alternator.

 

[Communicator]

For the M5 (8mm A/F) nut, a 1/4" drive socket would be my choice of tool. A loose wire should not cause the warning light to illuminate when engine is running, as the circuit for the bulb is through the loose wire described. The warning light provides the initial excitation for the alternator from the battery, and as Mike has stated, a larger bulb will help the alternator to "build up" at lower revs by providing a higher initial field current. Conversely a failed or disconnected bulb may prevent the alternator from generating, or make it need revving to start generating.

I am trying, but failing to see the logic in @bugsymike 's final statement when applied to the basic warning light circuit. Minimal current flows through the warning light once the alternator is generating, so what would the loss of connection do in either circumstance? Perhaps I am overlooking something? An open circuit on the main alternator output could damage the main diodes due to excessive voltage.

 

[bugsymike]

I am trying, but failing to see the logic in @bugsymike 's final statement when applied to the basic warning light circuit. Minimal current flows through the warning light once the alternator is generating, so what would the loss of connection do in either circumstance? Perhaps I am overlooking something? An open circuit on the main alternator output could damage the main diodes due to excessive voltage.

Re the diode bit , it was advice from an old auto electrician, but I haven't verified it although he was quite highly experienced and although well into his 80s has only just sold up his business and retired.
Thinking a bit more and it may be the senile decay kicking in, he may have said if battery lead to the alternator disconnected.

 

[sprie]

Ahh

Now I am feeling a bit stupid.

The socket arrived. I cleaned up both connectors, and reattached them - including making sure the connector for the smaller cable was firmly attached.
Started the engine - and as it should, ignition light on prior to starting, and ignition light off when idling and under revs. And measuring the volts on the battery i can see it is charging (e.g. 14.5v at idle).

So my conclusions are
1) the loose connection (plus maybe a bit of light corrosion in the damp weather) caused the light to come on, and more so under revs
2) somehow this issue stops the alternator from producing charge.

There was no other tool i can think of that would fit in the space other than the 8mm deep socket i didn't have.
I (and my wife even more so) hate to buy yet another new tool for just one job, but with hindsight, that was exactly what i needed to do.

live-and-learn

 

[Mike1alike]

The thin cable: I eventually got the nut off. It is a 8mm nut. I don’t have a long socket for this (got 9mm to 13mm), so when I put this on, I had to manage with normal sockets. And this nut was loose.

I was gonna say that that is the exactly cause of your problem, but I saw @Communicator had other opinion so I waited for you to try things and tell us how it gows.

A loose wire should not cause the warning light to illuminate when engine is running, as the circuit for the bulb is through the loose wire described.

Good thing you fixed it!
We live and learn, my friend. We live and learn!