Alternator Light Won't go off

[rindel]

I decided to replace the alternator wiring harness on my 1969 mach 1 with Tach.  The old cable was working but I wanted to put a new one in.  After installing the new one, I noticed the alternator dummy light on after starting the engine.  After troubleshooting i decided to put back the old cable and the light still stays on after starting it.  The battery reading is about 13V with car off and 14V after starting.  Also while she is running, I removed the negative of the battery and she continued to run even on idle.   

I am trying to figure out how the dumm light works.  After pulling the plug to the regulator, i tested 12v on the I side of the plug and 12v on the A side.  The S and F side were 0 volts.  Since I cannot open the regulator without breaking the rivets, i cannot manually close the relays.  Does anyone know how this should work?  Does an energized relay in close position shut off the light?

[1969_Mach1]

Here is a wiring schematic from my 1969 Ford shop manuals.  As you can see:

1.  When the key is turned on 12V are supplied to the bulb and it goes to the "I" terminal on the regulator and grounds through the voltage regulator to light the bulb.
2.  With the engine started, stator voltage (approx half of charging voltage) from the alternator "STA" terminal goes to the "S" terminal on the regulator.  This voltage energizes the coil of a relay inside the regulator closing contacts that go to the "A" terminal on the regulator.  With the contacts closed, 12V from the "A" terminal feeds to the back side of the light bulb.  With equal voltage on both sides of the light bulb, no electrical current will flow through the bulb and it turns off.  So battery voltage has no affect on light operation, 12V, 12.6V, 13V, 14V, etc. doesn't matter.
3.  The 15 OHMS resistor shown below the light bulb is a resistor wire wrapped up in the underdash harness.  It's purpose is such that if the bulb burns out the charging system will still work.

Note: New regulators use solid state devices instead of old electro-mechanical relays.  But the regulator and light operation is the same.

Hope this helps.  You might need a new regulator if all else checks out okay.

   

[rindel]

Thanks for the info and schematic.  I was puzzled why the bulb was getting voltage on both sides but now i know why. since I saw 12v on the I side of the plug, I suspect that the regulator is not feeding 12v to it and thats the issue.  Do you know how i can check the Stator for voltage?  i assumed it should be AC and connected my voltmeter from stator contact of alternator to ground and saw very small mv readding. Its possible that perhaps my stator is not charging the coil.

[rindel]

this may be a dumb question but  how i can check the Stator for voltage? getting nothing when connecting stator on back of alternator to VM and ground.

[1969_Mach1]

Sounds like you are checking it correctly.  Then engine needs to be running.  Per the schematic it would be AC voltage.  But I was taught many many years ago that it should be roughly 1/2 of the charging voltage in DC mode.  Any way, you should see something at the alternator STA terminal and the S terminal on the regulator.  You may have just found your problem.

[rindel]

Makes sense to set VM to DC which should read half the DC voltage since I'm catching half the cycle.  Will try thiis out again to retest.  I thought I read somewhere that I would only see STA voltage on startup but that didnt make sense to me.

[1969_Mach1]

I agree, I think you should see something at the STA terminal as long as the alternator is spinning and charging, and not only at startup.

[rindel]

i tested sta to ground with VM and got 19mv.  so really low voltage.  I suspect thats my problem.  as i was installing the harness originally, the sta screw was loose.  I wonder if something is not making contact on the inside

[1969_Mach1]

I think you are right.  If it were mine, I'd open up the alternator.  You might find something loose or a solder connection broke loose.  Might still need to replace it regardless of what you find.

I have wondered if with the engine running, if you can power the S terminal on the regulator with about 6V-7V and see if the light goes off.  If it does, you definitely know the issue is the alternator.

[rindel]

good idea.  I'm wondering if I can get away with an E 9v batter for test or string 4 D batteries to get some decent current