Carb V12 - Charging Issue

My 1973 DDS has had a recent trip home on the back of a recovery flatbed and another trip home followed by a helpful AA man !
Let me tell you the story and ask for input…
Initially the engine just died on me whilst nearly home (first trip out after hibernation!) It would restart but there was no power and it wouldn’t idle but it would run over 2000RPM but being an auto as soon I selected Drive it died. So I had fuel, battery and sparks but no motion!
The day after recovery she started fine in the garage, just like normal so I took her out around the block a few laps and once she was hot I noticed the battery condition was way OFF-CHARGE. She died again and wouldn’t tick over but after a while fired up normally and I got back home (relieved).
Next day I tried to get scientific and after another perfect startup I noticed that battery voltage whilst running was down around 12.6volts.
So I took out the old 20ACR alternator (thinking 50 years wasn’t bad), took it to a alternator shop, and they said it was working fine! Then I thought the battery must be dying (only 3 years old) but my local Quick-Fit tested it for me FOC and said it was good as new!
I then decided that the alternator must be the issue so I bought a replacement and fitted it. Fired her up - got a nice steady 14.3Volts and thought I had fixed the issue.
A couple of days later we go out for a spin and - you guessed it - she died again with exactly the same symptoms. Waited by the side of the road for 45 minutes for the AA man then she fired up again ! and we drove home waiting for failure that didn’t happen until just pulling onto my drive.
Next day I went to start up and noticed there was no IGNITION light on the dash before starting, checked the wiring diagram and saw that the brown/black wire goes to the small terminal on the alternator but downstream of the lamp. Bulb replacement still didn’t light it up so on further checking I found a bullet connector adjacent to the RH bulkhead stud with a flaky connection!
So was this the “smoking gun” and the cause of all my trouble? I believe this connection excites the alternator and so I presume when this connection went down my charging rate dropped to zero, and then when the RPM dropped too low there was not enough in the battery to keep the ignition firing ???
Does this seem plausible to any auto electricians out there? Since fixing this connector all seems well but I still need to rebuild trust in the old girl.
Sorry it was such a long post but any comments on this failure mode would be welcome!
Mike Badger
1973 DDS S1

i think i managed to absorb all that…kidding…typical response to die if battery low, was the battery put on charger for a day or so before it was load tested?, is the alternator have an a built in regulator or external?.. if you give me the model of you car specs/ i can post you the charging system info in colour, old connectors are a pain at the best of time, often easier to replace with new…not sure why you bought a replacement alternator if the old one tested ok…batteries can be deceiving as well… i have a batter in my work shop that i use for low dray testing, it charges up perfect with the green indicator showing but if i attach the load tester & hold for extended time the battery dies down to 11V…anyway…send me you car info & i’ll post the schematics…

The battery light at the dash connects to the Alt exciter circuit, so if the light or the connection is broken the Alt will not charge the battery. I don’t know about the 1973, but on later models there was a resistance in parallel to the light so if the bulb burned, the exciter circuit would still work.

If the bullet connector you mention had an intermittent connection, when it lost contact the car would run as long there was juice in the battery.
Keep an eye at your voltmeter.

Hi Tom,
Yes I charged the battery each time I limped home, as it always lives on a battery conditioner in the garage… and yes I think I was doing my headless chicken impression, leaping from one assumption to the next! My suspicion of alternator failure was (with hindsight) a bit premature but the guy that tested it and declared it OK seemed to take no time to test it thorougly, just connected a lamp and a few leads and spun it up with an electric drill! After I had the battery properly load tested I thought that the alternator was the stand out culprit.
My car is a Daimler Double Six from 1973 (same as XJ12 apart from trim)
Many thanks,

Mike Badger
1973 DDS S1

Thanks for the info Aristides and you are right that there is no parallel resistor to the brown/black connector so when the bulb fails (or a connector) you are left in the dark - literally!
The IGNITION lamp is therefore only useful as a pre-start check? I had assumed that if the charging system failed the lamp would light up whilst driving but that doesn’t seem to be how it works.
I take your point too about keeping an eye on the BATTERY CONDITION gauge, but thats very slow acting and not exactly in your eyeline on an S1. Maybe I’ll get a low cost digital voltmeter and fit it until I’m sure it’s all AOK again.
Mike Badger
1973 DDS S1

In later cars the resistance is together with the bulb at the instrument cluster.
The ignition lamp is indeed a pre start check, but if you have no resistance and the bulb is burned, or not connected, the exciter circuit will not work and the battery will not charge.