Blowing my headlights.... WHY
OK, before you go buyin a lot of stuff, unlovingly known as "throwin parts at a problem," do some simple tests
The 70/ later alternator is really not much different than the earlier 69/ earlier alternator, except that it has an ISOLATED FIELD, incorrectly called a "dual field."
One field terminal is fed switched 12V from the same wire that feeds the ignition system and one terminal of the regulator. With key IN RUN (engine off) and the GREEN wire disconnected from the other field terminal, BOTH field terminals should have 12V on them
IF not you have found the area of the problem
This could be a grounded field WINDING, meaning the field --which is a simple elecromagnet, is fried to ground in the rotor, OR more likely, you have a damaged brush which is grounding out the GREEN wire connected field terminal. Pull the alternator, and check the continuity to ground from both field terminals (brushes) If you show continuity, remove the brushes and inspect them for broken insulators. Either replace the brushes or replace the alternator. IF you do replace the alternator, and you have the old "round back" style, at LEAST ask for a middle 70's alternator and make sure it is a "square back" style" Photos at bottom
If you DO have 12V on both regulator terminals above, LEAVE the green unhooked and start the engine. The alternator should NOT charge
IF it does, you are "in the area" of the problem
IF it does NOT charge, hook the green wire back up, IE both the blue and green are hooked up at the alternator. It does not matter electrically which terminal is which
Now pull the connector off at the regulator. Start the engine. The alternator should NOT charge. IF it does, you have a short to ground in the green regulator wire
IF it does not, check the GROUND on the regulator. Remove the regulator, scrape the bolt areas and the back of the regulator clean, put some electrical anti--oxidant on there if you have it, and remount the regulator. Hook up everything "normal" and see if it works OK
IF it still overcharges, do one last test
Turn the key to "run" engine OFF. Hook your meter to the blue field wire, and the other meter probe to battery POS post. You should see a VERY low reading, the lower the better. Anything over .2--.3V (three TENTHS of a volt) means you have a voltage drop in the ignition harness
LAST if the above tests show nothing amiss, try another regulator. "NEW" does not mean "working properly"
69/ earlier "round back" style with one factory grounded brush. You should NOT have a grounded brush:
Middle 70's "square back" style which has better low RPM performance, and "isolated field" IE both brushes insulated from ground:
THIS is a 69/ earlier "grounded brush" style which was CONVERTED at some point to "isolated field" by a rebuilder. This has a hole drilled at 12 o'clock which once had a rebuilder insulated brush installed. It now has the original grounded brush at 9 o'clock, and the original single insulated brush at 6 o'clock
Below is an EARLY 70's "round back" "isolated field." It looks just like the 69/ earlier ground brush style, except that it has two factory insulated (isolated) brushes
Below is the mid 70's improved "squareback" style This is the kind you want if you buy a replacement You can ALSO use these with 69/ earlier regulators by grounding one brush with a jumper
Below is the simplified diagram of a 70/ later "isolated field" system, here being incorrectly called "dual field" One field terminal receives 12V and the regulator controls the "ground" on the green connection to control rate of charge
OK, before you go buyin a lot of stuff, unlovingly known as "throwin parts at a problem," do some simple tests
The 70/ later alternator is really not much different than the earlier 69/ earlier alternator, except that it has an ISOLATED FIELD, incorrectly called a "dual field."
One field terminal is fed switched 12V from the same wire that feeds the ignition system and one terminal of the regulator. With key IN RUN (engine off) and the GREEN wire disconnected from the other field terminal, BOTH field terminals should have 12V on them
IF not you have found the area of the problem
This could be a grounded field WINDING, meaning the field --which is a simple elecromagnet, is fried to ground in the rotor, OR more likely, you have a damaged brush which is grounding out the GREEN wire connected field terminal. Pull the alternator, and check the continuity to ground from both field terminals (brushes) If you show continuity, remove the brushes and inspect them for broken insulators. Either replace the brushes or replace the alternator. IF you do replace the alternator, and you have the old "round back" style, at LEAST ask for a middle 70's alternator and make sure it is a "square back" style" Photos at bottom
If you DO have 12V on both regulator terminals above, LEAVE the green unhooked and start the engine. The alternator should NOT charge
IF it does, you are "in the area" of the problem
IF it does NOT charge, hook the green wire back up, IE both the blue and green are hooked up at the alternator. It does not matter electrically which terminal is which
Now pull the connector off at the regulator. Start the engine. The alternator should NOT charge. IF it does, you have a short to ground in the green regulator wire
IF it does not, check the GROUND on the regulator. Remove the regulator, scrape the bolt areas and the back of the regulator clean, put some electrical anti--oxidant on there if you have it, and remount the regulator. Hook up everything "normal" and see if it works OK
IF it still overcharges, do one last test
Turn the key to "run" engine OFF. Hook your meter to the blue field wire, and the other meter probe to battery POS post. You should see a VERY low reading, the lower the better. Anything over .2--.3V (three TENTHS of a volt) means you have a voltage drop in the ignition harness
LAST if the above tests show nothing amiss, try another regulator. "NEW" does not mean "working properly"
69/ earlier "round back" style with one factory grounded brush. You should NOT have a grounded brush:
Middle 70's "square back" style which has better low RPM performance, and "isolated field" IE both brushes insulated from ground:
THIS is a 69/ earlier "grounded brush" style which was CONVERTED at some point to "isolated field" by a rebuilder. This has a hole drilled at 12 o'clock which once had a rebuilder insulated brush installed. It now has the original grounded brush at 9 o'clock, and the original single insulated brush at 6 o'clock
Below is an EARLY 70's "round back" "isolated field." It looks just like the 69/ earlier ground brush style, except that it has two factory insulated (isolated) brushes
Below is the mid 70's improved "squareback" style This is the kind you want if you buy a replacement You can ALSO use these with 69/ earlier regulators by grounding one brush with a jumper
Below is the simplified diagram of a 70/ later "isolated field" system, here being incorrectly called "dual field" One field terminal receives 12V and the regulator controls the "ground" on the green connection to control rate of charge