Alternator Light Won't go off
#1
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?
Reply
#2
Photo 
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.

   
Reply
#3
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.
Reply
#4
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.
Reply
#5
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.
Reply
#6
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.
Reply
#7
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.
Reply
#8
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
Reply
#9
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.
Reply
#10
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
Reply


Possibly Related Threads...
Thread Author Replies Views Last Post
  map light 60FN9 5 4,639 02-03-2017, 10:02 AM
Last Post: ohiomustang
  Need to install original trunk lid light on my 1969 mach1 rindel 8 11,187 02-09-2015, 03:26 PM
Last Post: Steven Harris
  Help Identifying Mystery Light Behind Dash rindel 7 7,595 01-03-2015, 11:45 PM
Last Post: rindel
  TECH - Repairing Map light switch Mach 1 Club 2 4,126 08-12-2011, 10:37 PM
Last Post: Rare Pony
  TECH - Install a Trunk light Mach 1 Club 1 3,924 07-01-2011, 08:17 AM
Last Post: Rare Pony
  LED Tail Light Technical Manuals DAKOTA DIGITAL SIOUX FALLS Mach 1 Club 0 2,767 01-23-2010, 03:38 AM
Last Post: Mach 1 Club

Forum Jump:


Users browsing this thread: 1 Guest(s)
[-]
Latest Threads
Repair a 1971-1973 Mustang urethane fron...
Last Post: muskan123
Yesterday 06:30 PM
» Replies: 10
» Views: 17217
Is this noise normal?
Last Post: 1969_Mach1
Yesterday 03:36 AM
» Replies: 11
» Views: 787
Rear brake issue
Last Post: Mustangmike
04-18-2019 09:46 AM
» Replies: 8
» Views: 370
hdshadpar
Last Post: snah04
04-14-2019 10:47 PM
» Replies: 4
» Views: 997
70 Boss 302 - Night Mission build blog
Last Post: SWPruett
04-14-2019 01:29 PM
» Replies: 117
» Views: 93000
69 Mach 1 Headlight switch reinstall iss...
Last Post: combat32
04-12-2019 10:10 PM
» Replies: 9
» Views: 720
One piece headliners?
Last Post: SWPruett
04-11-2019 12:28 PM
» Replies: 9
» Views: 700
Garage Shots
Last Post: JTS71 Mach1
04-05-2019 09:51 AM
» Replies: 24
» Views: 15187
WELCOME ALL NEW MEMBERS INTRODUCE YOURSE...
Last Post: 1969_Mach1
04-03-2019 11:59 AM
» Replies: 63
» Views: 57126
Why are Americans so stupid? Stop buying...
Last Post: mymachone
04-02-2019 09:14 PM
» Replies: 9
» Views: 9160
Need Data plate decode help pls.
Last Post: Aris
03-30-2019 05:33 AM
» Replies: 6
» Views: 1082
1969 Mustang Paint Codes & Colors
Last Post: BLACK JADE 69
03-28-2019 02:46 AM
» Replies: 25
» Views: 15078
Been Awhile... '71 Rustang
Last Post: Mister 4x4
03-25-2019 07:22 AM
» Replies: 144
» Views: 116961
Throwback Picture
Last Post: Oztrailer
03-22-2019 07:05 PM
» Replies: 2
» Views: 641
1973 Mach 1 parts
Last Post: Oztrailer
03-21-2019 07:39 PM
» Replies: 7
» Views: 1737
Will buy parts in real good condition or...
Last Post: shopcarpro
03-21-2019 10:39 AM
» Replies: 2
» Views: 657
Manual Trans Gear Oil
Last Post: JTS71 Mach1
03-21-2019 10:21 AM
» Replies: 11
» Views: 1501
"Jacobra"
Last Post: Mustangmike
03-07-2019 09:03 AM
» Replies: 69
» Views: 27308
My Newest Project - My Youngest Child
Last Post: Steven Harris
03-05-2019 11:56 AM
» Replies: 39
» Views: 11642
1971 moch 1
Last Post: JTS71 Mach1
03-04-2019 07:11 AM
» Replies: 16
» Views: 2015

>