Alternator problem, ignition light goes out after a while

[Pete Carr]

Hi,

 

I seem to have exactly the problem recently described in one of the "more electrickery" articles in the Magazine, but have lost the copy with the relevant article!

 

The problem is that the ignition light stays on for a bit and then goes out when high revs are reached. I believe the problem is that that the exciter current isn't being fed to the alternator but after a while the residual magnetism in the rotor gets it going anyway.

 

Trouble is I can't remember exactly what the fault was or if it can be easily be fixed.

 

So if anyone has a copy of the article handy and can supply the relevant information I would be most grateful.

 

It would be great if the technical articles from the mag were also posted on the web site so idiots who lose their magazines can refer back to them. (If they already are posted - where are they?).

 

Thanks,

 

Pete.

[Chris W]

Pete

 

Ref my article in LF on Alternators. The usual cause of the alternator not kicking in until high revs are reached is a problem with the connection to the ignition light. As you correctly state the alternator will suddenly spring into life due to some residual magnetism providing the initial exciter current.

 

However, in your case, the ignition light does indeed come on so it must be connected and working.

 

Questions:

 

1. What is the battery voltage with the engine running at idle? (should be about 14v)

 

2. What is the battery voltage with the engine running once you reach high revs? (should be about 14v)

 

3. Once you reach high revs and the ignition light goes out, does the battery voltage stay high even when the revs decrease?

 

Chris

 

2003 1.8K SV 140hp see it here

[Pete Carr]

Hi Chris,

 

I shall check the voltages tonight and report back tomorrow (hopefully).

 

Great articles by the way, it would be excellent if electronic copies were available on the web site.

 

Thanks,

 

Pete.

[Andy Best]

Whilst the voltage tests are the first point, I had the same problem a couple of years ago after the engine had been out.

 

Eventually I traced the problem to the earth connection on the alternator, nipped the nut up and the problem was solved.

 

Andy

[Pete Carr]

Hi Chris,

 

I tested the car last night and the answers to your questions are as follows:

 

1. What is the battery voltage with the engine running at idle? (should be about 14v)

When the car is first started and idling, the ignition light stays on and the battery voltage is just over 12v.

 

2. What is the battery voltage with the engine running once you reach high revs? (should be about 14v)

As the revs are increased up to about 2500 revs the ignition light stays on and the battery voltage is just over 12v. Once you reach about 3000 revs the ignition light goes out and battery voltage is just over 14v.

 

3. Once you reach high revs and the ignition light goes out, does the battery voltage stay high even when the revs decrease?

As the revs drop back down to idle the ignition light stays out and the battery voltage stays high at just over 14v.

 

Hope this helps with the diagnosis and big thanks for taking the time to help me with this. Tonight I will also have a good look at the alternator connections as suggested by Andy.

 

Thanks guys,

 

Pete.

[Chris W]

Pete

 

From your answers, you have the classic symptoms of no excitation current getting through to the alternator until residual magnetism provides the initial kick, after which it is self-generating.

 

There is often a bypass path for this current which is in parallel with the ignition light (ie: it's not the ignition light itself providing the current). This can be the case if the ignition light is an LED rather than a conventional bulb. The latter will allow far more current, whereas the LED would only allow about 10mA.

 

Carefully check around the ignition light itself to ensure that IF there is a bypass path, that path is still intact. It may be a resistor and the connection may be broken or the resistor burnt out. There may even be a fuse in-line as well.

 

If no luck, try putting a 47 ohm (5 watt) resistor across the ignition light (eg: from Maplin) here. This will provide the necessary excitation current and still allow the ignition light to work as normal.

 

Chris

 

2003 1.8K SV 140hp see it here

[Chris W]

Pete

 

I just checked on the Maplin site. The following resistor is ideal (it's actually a 10 watt version) @ 47 ohm value.

 

Product Code H47R

 

10Watt 47 ohm

 

2608 in Stock

 

£0.19 each

 

Chris

 

2003 1.8K SV 140hp see it here