Joe easy way to check if its a diode on way out remove earth lead from battery then in the dark touch it to the post to see if any spark if so disconect alternator wiring and check again if it goes away alternator diode pack suspect.
 
Blackburn said:
When ignition is on 12v from ignition goes through the bulb to the alternator through a coil to magnetise and excite the alternator once engine starts and alternator is generating there is the same voltage at each end of the bulb and light goes out. Old alternator may not have been getting the 12V via bulb. An earth some where between bulb and alternator would also give similiar symptons.
Click to expand...
Okay - ran these tests last night. Please see if you can help me interpret it all.
Engine off: turned the ignition switch on and ignition light came on - as usual. I then removed the small outermost wire from the alternator and the ignition light stayed on! Hmmmm. I hooked up my red wire on the multimeter to the disconnected wire and grounded the black wire of the multimeter to the car body. No voltage, or something like .05 V.
With the small outermost wire from the alternator still disconnected, I then started the engine. It started right up. I then hooked up the red wire of multimeter to the small outermost terminal of the alternator, and the black wire of the multimeter to the car body ground. No voltage or about .05V

I'm thinking that if my ignition light is always on, perhaps like you suggested, it is grounded somewhere, getting it's power from the positive off the ignition switch, and being grounded it lights up.

Final test. With the engine running nicely, I took a multimeter reading off the battery. It was below 12V. Hmmmm, not good. I increased the idle, and still the same. Scary. A brand new alternator which does not seem to be charging. Maybe my old alternator was fine afterall!

Any thoughts on these things will be appreciated.
Thanks!
Mike
 
You need to get 12V with ignition on at the excitation terminal of the alternator .ie ignition to bulb then bulb to alternator.
If you open rats nest of dash up the bulb holder for the charge light should have two wires whereas the others wil have one and earth through the casing maybe some one has fitted wrong type of lamp and holder.
The wattage of the bulb is also important .
Post a pic of your alternator connections.
 
I had a problem. A few years ago with red light on , and suspected the diodes, pulled the alternator off and desoldered all the diodes and checked them .... all ok , then I noticed a brown wire with spade connection to rear of engine had come disconnected from other end , joined them up and all worked ok so guess this must have been from the ignition light to alternator
 
Hey All -
I figured out my charging light problem!!! It was grounded to the dash itself. The washer that held the bare bulb in place was grounding it. I replaced the assembly with a plastic housing, and works great. Ignition key clicked (engine not on) the light is ON. Engine started, and the light goes off.

Rodeo Joe - if your light gets brighter when revving, it means it's acting like a standard headlight or dash bulb. Power is going in, and it is grounded. Probably not connected directly to the alternator without being grounded first.

I got the idea for this test and solution in reading this thread several times. Blackburn clued me in above. I went home tested the theory, and that was the answer: "An earth some where between bulb and alternator would also give similar symptoms."
THANKS!!

I appreciate everyone's help on this thread and all the others.
Thanks!
Mike
 
Blackburn -
Great question. I also installed a Smiths voltmeter over the weekend, and my volts are 13 ish and above when engine running. When I gun the engine needle moves up. When I turn on lights and everything, voltage drops very slightly.
So all looks great now.
Question - is there a connection between this now good result and the fact that I fixed the ignition warning light problem?
Thanks!
Mike
 
The alternator iron core does not hold much magnetism , the current from the ignition lamp through a winding creates the initial magnetism to get the alternator to work. Once working its own output will keep it generating
 
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