Ignition light does not come on / Alternator Drain

[ham204]

Hi Guys, I am an owner of a '72 white Stag. Please bear with me with my ongoing saga with the car's electrics.
On the whole the car is in pretty good condition and my recent modification to an external electric water pump has made it more reliable. When I bought the car the ignition light did not come on and I did not make much of it, thinking it being a blown bulb. The PO had also told me that the battery would drain overnight if I 'did not close the door properly as the puddle light would still be on'. Again, as i am in the habit of disconnecting the battery when the car is not in use so had no opportunity of having the battery drained in the morning.
When I fitted the EWP, I had to locate it in the place where the alternator is originally fitted, with the alternator being fitted on the other side.  I also bought an uprated alternator from EJ Wards together with an extended harness. The first of the problems started when I replaced the old alt harness with the new by simply using the old connections from the car's harness into the new alternator harness. I also checked the instrument cluster and found that the ignition bulb was alive and well and responded to a 12v burst! When I started the car, I found that the ignition light did not come on when I turned the key and the alternator was not charging (tested with a meter and also through the voltmeter on the dash). Seeing that there was another connection in the car's harness, I switched the brown/yellow thin wire to that connection and the alternator was then charging well >13v. Again the ignition did not light up when I turned the key.
I happened to leave the car 's battery connected one day and sure enough, the next morning it was totally drained measuring 1.5v at the meter! After recharging, I removed the earth lead and checked the amperage between that and the battery terminal and it was reading 3 amps, a very heavy drain. First I removed the fuses, one by one (no change in drain). Then I removed the alternator plug and the drain fell to 0. Thinking it was a faulty diode, I took the alternator to an electrician and he confirmed that the alternator was good, there was no drain problem with it and all diodes are fine!
I am sure that the ignition light not coming on and the alternator drain are somehow connected. I am not sure why there were  two connectors in the car's harness in the part where they connect to the alternator harness. In my first try to reconnect the harness I definitely used the same connectors as were used with the old alternator, but as already explained, the new alternator failed to charge.
So the situation now is that the battery has a 3 amp drain if left connected and the ignition light does not come on when I turnt he key. I am now quite at a loss as to what to do next! The only thing I can think of is to wire directly from the ignition bulb to the thin brown/yellow line and see what happens then. Any other suggestions please?

[tony66]

personally if you are getting so much grief with the charging circuit i would just re-do it all[charging circuit]
but to troubleshoot what you have turn off ignition and check which fuse has current and go from there
it wouldnt hurt to rewire puddle lights either-tbh unless mullered the wiring quality on the stags isnt all bad imho compared to a lot of old cars..

[ham204]

Problem has been solved far more easily than I expected. Yday I decided to isolate the thin brown/yellow wire and checked the battery drain again. This time it went to zero. That narrowed down the possibilities considerably. I then replaced the ignition bulb in the cluster just to make sure that it is a working bulb. Reconnected the br/yellow wire to the original connection in the car's harness (see my post above) - the one which when connected the first time to the new alternator, produced no charge when starting the engine! I was expecting the same result this time round but amazingly, the ignition light came on when I first turned the key and the voltmeter started climbing when I started the engine! I am a bit baffled how it has sorted itself out but am very happy with the result and definitely won't complain! I left the battery connections on and this morning the battery read a healthy 12.6v

[tony66]

glad you cracked it
good stuff..

[RobPearce]

Quoted from ham204

I then replaced the ignition bulb in the cluster just to make sure that it is a working bulb. Reconnected the br/yellow wire to the original connection in the car's harness (see my post above) - the one which when connected the first time to the new alternator, produced no charge when starting the engine! I was expecting the same result this time round but amazingly, the ignition light came on when I first turned the key and the voltmeter started climbing when I started the engine! I am a bit baffled how it has sorted itself out

It hasn't mysteriously "sorted itself out" - you've fixed the problem!

Alternators use a field winding, powered off the charge circuit, to generate the spinning magnetic field that the stator windings get their power from to provide the charge. But note: the field winding is powered by the output of the alternator - if there's no output then there's no field to generate any output... and it just doesn't work.

So, something must be done to get the feedback loop started. There are, in fact, two things done. One is that the rotor on which the field windings are wound will naturally retain a bit of magnetism, albeit not much, to get you started. The other, and much more important, is that the warning light is wired from ignition to the field winding. When running, the charge circuit pulls that up to +12 so there's no voltage across the bulb. When the alternator isn't yet running, though, the current that lights the bulb also passes through the field winding to produce a (small but sufficient) starter field.

This is why you can, sometimes, persuade a marginal charging system into proper operation by fitting a brighter (higher wattage) alternator warning lamp.