Ignition light glows faintly after startup

[AndyG]

Just when you think everything is going ok another issue raises its head

Took the Stag out at night for the first time since rebuilding the engine. Up until now only been out during the day in case I broke down!

I noticed that once it started the ignition light was glowing very dimly which got dimmer the higher the revs go. I'ts a new alternator and I checked the belt which was tight. I have an external water pump so the alternator is the other side of the engine. I'm assuming its a resistance problems due to the extended loom but could be wrong.

I'm planning to clean up all the connectors from the alternator back to the ignition light to see if that helps. Is there anything els it could possibly be ?

I could just drive it during the day as you can't see it glow then

[wilf]

Use a voltmeter to see what is going on. the voltage at the battery should be around 14.3V with the alternator charging (i.e. with a few revs on) and around 12.6V to 12.8V a short while after the engine is off. If it is, relax.

Keep youself, your clothing, and the voltmeter leads well clear of the drive belts while doing this check.

[NeilR]

Not another 'Chinese Alternator' is it ??

Regulator / diode pack dying.

Neil.

[AndyG]

I bought the alternator from ANG Classic Car Parts, not sure of its origin, I'll check to see if I can see any labels as to where it's made.

I'll get the volt meter out and have a check, thanks for the quick responses

[sunnystag]

mine has always done that. very faint glow to ign light. can not see it at all in the daylight. but hold your hand over the cluster and it is glowing. battery charges fine, long or short runs and at night with lights on. mine is a mk1 and I had understood most mk1s did that? is that correct? I have a later alternor fitted with no external regulator ( the black box on mk1s inner wing. so might be something in the mk1 wiring. I thought it should not happen in a mk2.....anyone know if that is right? but on all cars something happens with the circuit to the "ign" bulb as taking the bulb out is not a good idea, so maybe a current does flow on all models?

[AndyG]

mine has always done that. very faint glow to ign light. can not see it at all in the daylight. but hold your hand over the cluster and it is glowing. battery charges fine, long or short runs and at night with lights on. mine is a mk1 and I had understood most mk1s did that? is that correct? I have a later alternor fitted with no external regulator ( the black box on mk1s inner wing. so might be something in the mk1 wiring. I thought it should not happen in a mk2.....anyone know if that is right? but on all cars something happens with the circuit to the "ign" bulb as taking the bulb out is not a good idea, so maybe a current does flow on all models?

That makes me feel a lot better knowing others have the same issue. Not going to worry so much now

[Stag About Town]

mine has always done that. very faint glow to ign light. can not see it at all in the daylight. but hold your hand over the cluster and it is glowing. battery charges fine, long or short runs and at night with lights on. mine is a mk1 and I had understood most mk1s did that? is that correct? I have a later alternor fitted with no external regulator ( the black box on mk1s inner wing. so might be something in the mk1 wiring. I thought it should not happen in a mk2.....anyone know if that is right? but on all cars something happens with the circuit to the "ign" bulb as taking the bulb out is not a good idea, so maybe a current does flow on all models?

+1 me too

[Roy_S]

I read that the bulb is part of the circuit to enable the alternator to push out a charge to the battery, it's the thin wire that comes out of the back of the alternator alongside the two thicker charging wires.
That being the case there must be some current then going through the bulb so a low glow could be normal.
Volt test at the battery would confirm everything, especially with headlights on as well.

[wilf]

The current flowing through the "ignition" bulb is used to "excite" the alternator, i.e. initially start it charging. Once it has started, it self-excites, and the voltage it produces stops the current flowing through the bulb, so it goes out. Or very nearly out, lol.

It could well be that the bulb is "seeing" the volt drop along the main alternator output cable as current flows through that, and that is why it glows very dimly. As I said, if the voltages check out, relax.

[StagJonno]

My understanding is that the IGN light circuit ensures the alternator is excited on start up. Initially, the circuit is fed at battery voltage to the bulb and goes to ground via the alternator rotor windings allowing the bulb to light. Once the alternator is charging, the wire to the alternator rises to battery voltage, so no current passes through the bulb.

If your battery voltage is rising to 14 - 14.5V when you put a few revs on the engine then, as others have said, your alternator is charging OK. The light may be coming on because the voltage at the alternator is higher than the voltage to the ignition side of the bulb. Causes for this could be bad connections in the alternator power output (brown wires) to the battery - terminals at both alternator and battery and possibly the big insulated connector mounted on the inner wing on the right hand side. Also, possible (but less likely) on connections between this connector and the Ignition switch, or in the ignition switch itself.

Disconnect the battery before checking these circuits to avoid the possibility of short circuits and big sparks.

A further check worth doing is to see if there's a dim glow with the engine off. This would point to an internal problem within the alternator, most probably the internal diode pack. If this were the case, then it could easily result in a flat battery after a few days of not running.

PS - Took too long as got distracted - I see Wilf has covered it much more succintly

[wilf]

No problem - if a thing's worth saying, it's worth saying twice, lol.

[DJT]

Many modern Lucas replacements are ‘self-exciting’ so don’t need the small brown/yellow wire. The Q-H one I have is like that. The exciter wire is not connected and is taped up, but the ‘ignition’ light behaves normally.

[KOY 23]

Many modern Lucas replacements are âself-excitingâ so donât need the small brown/yellow wire. The Q-H one I have is like that. The exciter wire is not connected and is taped up, but the âignitionâ light behaves normally.

Hi Dave.
Slight confusion above about exciting and sensing, must be the lack of playing with your little white balls
Many modern alternators are machine sensed and do not have a terminal for the thin brown wire, which was used on earlier battery sensed alternators. The wire must be taped up as it’s permanently live and unfused.
The brown yellow wire feeds the warning lamp to excite the alternator and must be connected.

[DJT]

Hi Dave.
Slight confusion above about exciting and sensing, must be the lack of playing with your little white balls
Many modern alternators are machine sensed and do not have a terminal for the thin brown wire, which was used on earlier battery sensed alternators. The wire must be taped up as it’s permanently live and unfused.
The brown yellow wire feeds the warning lamp to excite the alternator and must be connected.

Thanks John. I knew it was something like that. I shouldn'™t post late at night after a few glasses of Stellenbosch Cabernet Sauvignon Merlot........

[AndyG]

Checked the voltage at the battery 14.4V at about 1500rpm, turned engine off and dropped down to 12.6V.

From what you have all said looks ok to me

Thanks for the great advice again