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1965 Mustang Alt warning Light stays on

New User
Posts: 2
Joined: 11/13
Posted: 11/08/13 01:51 PM

Alternator warning light on my 1965 coupe stays on.  The alternator is fine.  The main under dash harness is the only original wireing/electical component left! In the car everything else was replaced with factory correct reproducitons. My dad the electrical engineer has gone through he said looking at the original schematics hes thinks there a resister with a wire harness some where but cant determine where?  Has any one else had this problem and been able to fix it?  Everything else in the car works perfictly.  

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Posts: 223
Joined: 04/13
Posted: 11/08/13 10:46 PM

first thing.. a few diagnostics... this only takes a few minutes...

was this a Generator car.. that was converted.. probably not.. but .. i had to ask..

next.. takes just a few minutes... will save hours of hair pulling... please print and run this test...

its a JPG image..




post results by number..





now.. lets look at the dash board ground issues...

66 Mustang Cluster Ground Location  

Post Reply
Posts: 223
Joined: 04/13
Posted: 11/08/13 11:05 PM

now... lets look at the positive side of the charging system;
you will have to slide the image to the left to see the far right side.. or right click and open it in a new window and print it...  the problem above is thats a 66 dash with an amp meter..

the other side of the bulb is a red green wire #904.. that goes thru the bulk head connector and to the I terminal on the voltage regulator..

what goes on here..  when the key turns to the ON position.. power on wire 16. goes to the ignition coil and to the charge light...   if the alternator is not charging.. the wire from the voltage regulator I terminal will be LOW.. and the electrons will flow thru the bulb and turn it on...

once the alternator starts charging.. the voltage comes up and equals the  voltage coming from the ignition switch.. since there is equal voltage on either side of the bulb.. there are no electrons flowing across it and it stays dark.. even though there is 14 volts on it..

a trick circuit..

start at the ignition switch #16 red and green wire.. it goes o the female bullet connection and splits there to the charge indicator light..  16A continues and that is the pink wire with the cover that is the resistor wire to reduce current for the ignition coil..


Post Reply
Posts: 223
Joined: 04/13
Posted: 11/08/13 11:21 PM

lets look back at the alternator to regulator wiring..

alternator ----------------------  regulator



ground _____________________________mounting bolt on foot of regulator.

the FLD inside the alternator connects to ONE brush.. the regulator pulses positive and negative voltage to this to control the size of the rotating magnetic field as it spins..

wait.. pulses positive and negative.. remember. this is a coil of wire in the rotor.. spinning away happily.. inside the stator with multiple coils of wire..  when there is positive current sent to the rotor it builds a magnetic field. but to control the output.. you have to have that field wire grounded almost an equal amount of time.. so the rotor magnetic field does not continue growing and creating a run away charging system..  sneaky.. yep.. works.. for hundreds of thousands of miles on every car..

the other brush is grounded inside the alternator...

the STA. is the 3 wires of the stator hooked together.. this ONLY has voltage on it when the alternator is actually charging.. and it will have 1/2 the charging system voltage.. but in AC...  this AC signal is used by the regulator to pulse the contacts to keep the charging output stable independent of alternator speed..  

the ground wire from the case of the alternator to the mounting foot of the regulator.. keeps the pair connected to keep an accurate amount of charging voltage happening..  

Post Reply
Posts: 223
Joined: 04/13
Posted: 11/08/13 11:33 PM

i think that you will have it fixed by now... but just incase..

how to test this....

engine off..  ignition off..

reach over and unplug the positive wire off the ignition coil..

reach over and unplug the voltage regulator connector...

hook your digital volt meter up to ground on one side.. .. turn the KEY on without starting the engine..

measure the voltage at the I terminal of the starter relay...

measure the voltage at the wire you disconnected from the coil positive..

measure the voltage at the wire I wire of the regulator harness.

with the coil disconnected.. you should actually get the same..  

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