alternator question

[floyd]

Evening all,
I am experiencing problems with my charging system. I have had this 75 TR6 for a few years now and have recently changed the battery because it wouldnt hold a charge after sitting in between drives. As long as couple of weeks. Being the battery looked old I went ahead and bought a new one but had them test the old first with the test results being "bad". What part of the test was "bad"?. He said the print didnt say, it just said "bad". After couple of weeks of starting the car I noticed each time it sounded a little slower turning over and then finally nothing. I started a little on-line reading and then headed out to the garage. I put the charger on it and got it started, battery voltage was about 12 volts, not 14ish. During this whole time the ign. light has never lit up even with the car running or with just the key on. I took the bulb out and it was bad. Put a new one in and it glows faintly. I measure 3.3 volts between the white power wire and the brown/yellow at the alt which is the ign lamp wire. My alt. is a lucas 18 acr and all the workshop manual has is 15 and 17. I have a 1- 3 prong plug (1 little prong, 2 big prongs) with 1 big brown wire and a little brown/yellow wire. Then a single prong plug with a little brown that heads over to the battery. I know the 2 big tabs in the alt. are connected together. Here is my Question.....are the internals of the 15,17 and 18 the same, just a differnece in output mine being 45 amps.And any advice beofre I tear into this thing. Please excuse this book I have just written on my alt.
UPDATE......I have got down monkeying with this alittle more, now...the ign. light will increase in brightness, and then sometimes if I rev the engine the light goes out and my meter will go up 13.5 v across the battery. Stay that way for awhile the drop back to 12.5 and the light comes on. very unstable. Maybe a bad connection in the alt.....

 

[TR3driver]

I measure 3.3 volts between the white power wire and the brown/yellow at the alt which is the ign lamp wire.

Sounds like a bad diode to me. Here is a fairly decent article on repairing them:
https://www.vtr.org/maintain/alternator-repair.shtml

As I recall, the early 5 terminal version is basically the same as the later 4 terminal, except it needs an external jumper of some sort.

Here's another article with more photos (tho it still shows the 3 terminal variant)
https://www.oldclassiccar.co.uk/forum/phpbb/phpBB2/viewtopic.php?t=4513

 

[floyd]

Thanks for the links. Very useful info. I'll probably tear into this weekend and see what i can find in there. It's acting pretty erratic. Sometimes when I rev it up the meter jumps up to 13.5 then stays there for while then drops back down to 12....as far as i can find all connections look good outside of the alt. Thanks for your reply