The alternator also grounds through its case to the engine block - if somehow it couldn't (powder coatings, paint, corrosion) you might see some weird things like this.
I thought this was a new alternator? Have you had two do this?
When my alternator died it was the diode pack so the alternator was just shorted right across the battery which was.... quite smokey.... So even though you've swapped the alternator, this diode issue seems to be pretty common, ie, your new alternator could have the same fault as your last.
Rob , the alternator is not a new one i have 2 x second hand ones i bought from forum members with cup brackets. The white car starts but i only get 12.6 v at battery while running. Ive checked the output at alternator and dont get anything , also noticed i dont get the battery light illumination on pre ignition and when i tested between battery negative and casing i was getting 12v.
The brackets are powder coated for the mounts.
I took the alternator to be checked off the car and told me they were ok. One alternator was worn but operational he said so that why i was convinced of a wiring error not causing the alternator to start charging.
Ive checked from the exciter wire cable to the white plug of death for a break in wire and all appears ok. But need to check from there into the car then , any ideas or anyone have a diagram where it goes next to test ?
I didnt want to just buy a brand new alternator unless i was 100% sure but i know if i did i would be 100% thats it is a wiring issue.
Ive removed the regulators from both alternators just now , one set looks like brushes are near end of life and the ones on the other look quite new so i could have 2 bad alternators.
I will test the diodes individually to see what i get , i know its going to take time to figure out.
My positive lug also just snapped to the alternator , i think im going to fit a fresh cable.