ignition warning light problem

[NickWilmshurst]

Morning all,
Can anyone cast a light on my current (pardon the pun) problem?... I have a "dynamator" installed in my S3 which has been there for some time and has, until now, behaved itself. Recently, though, the ignition warning light goes out at idle and comes on with some revs. I have no idea if I am charging or not.
Anyone got any ideas?
thanks
Nick

[promotor]

Always check connections are tight first with anything to do with electrics. Vibration can do funny things to connections.

[Phil.C60]

I'll do my best......
Ignition warning light bulb is fed from the ignition switched live circuit, and when everything is at rest the bulb is grounded through the alternator (Dynator) via the warning light wire (usually brown/yellow on English cars but may not be on your installation. Regardless, it's the thin wire). When the engine is started and the alternator starts to charge, the voltage at the alternator warning light terminal rises to charging voltage, the voltage at the ignition switch rises to charging voltage, and as the voltage balances no current flows and the light goes out. If the light comes on when you rev the engine (and the charging voltage rises) either there is low voltage at the ignition switch side of the bub, or low voltage at the alternator side of the bulb, so current flows and the light comes on. The best way to check it is to check the voltage each side of the warning light bulb when you rev the engine, that will tell you which part of the system you need to investigate further. Usually I would suggest that if it's the alternator side it's probably the unit, but as your wiring will have had surgery to enable the fitment of the Dynator theres a bit more to check first. My (considerable) experience of most amateurs and even "professional" garages is that wiring and wiring repairs are the thing they do least well, and often not well at all!
If you just want to know if it's charging, check the voltage across the battery terminals with the engine revs at around 3000: it should be 13.5-14V. If you want to check if it's charging properly, do the same test but with the headlights on main beam and the heater blower on. It should also be 13.5-14.3v although 13v would do.
It is pefectly possible for it to be charging correctly even with the warning light on, but that's more complicated to explain.
Hope this helps.

[NickWilmshurst]

Thanks Phil. Too cold to go and play cars at the moment but I'll get to it soon.
Nick

[512BB]

'If the light comes on when you rev the engine'

Could be as simple as the fan belt being to loose. As that is the easiest thing to check, I would start there, but do not over tighten.

Leslie