65 Mustang Alternator light on

When I first installed the 302 in my 65 mustang, it started and after a minute or so, the alt warning light would go off. It would be on - revved the engine, it would fade - and go off. It has a new voltage regulator and alternator. The car sat for about 4 months. Now when I start it, the alt light stays on. How should I proceed?
Thanks - Wayne  

first... put the keys in your pocket...  take your thumb and place it on the alternator fan blade...  push...   does the motor turn with the alternator...  it should...    if it slips without turning the motor the alternator belt is loose...

do you have a digital volt meter..

set it to 20 volts DC scale..

engine off...

measure across the battery...    

12.65 is a full charge...
12.45 is a half charge.
12.25 is a quarter charge..

the battery needs to be over half way charged for a proper charging system test...

now.. start the motor.....  turn on the head lights....

test from the

negative battery post to the positive battery post..   charging..  14.1 to 14.6...

negative battery post to the engine block....  0.04 volts..

negative battery post to the body..................0.02 volts..

engine block to the firewall...........................0.02 volts...


engine block to voltage regulator bracket....  0.02 volts...


alternator output terminal to the battery positive post...    0.04 volts...

if you get more voltage on any of these... post what you get...


now.. if all these test good.... but you have only 12 volts with the engine running...

unplug the voltage regulator...   have a short wire with a pair of male 1/4" quick connects on it..

hook the volt meter to the battery so you can see it..     jump the unplugged connector from the voltage regulator...  from the F wire to the A wire... for no more than 10 seconds...   the charging system should rise to over 15 volts in just a few seconds...   this proves that the alternator works and that some of the wires are working properly..

now   flip your digital volt meter to 20 VOLTS AC scale..    plug one test lead into the back of the S terminal connector.. the other to ground...    then jump the F to A again.. with the engine running.. you should get about half the voltage AC that you got a few minutes ago when you measured the battery...

yes complicated...    but it has to be done...

this should take less than 10 minutes do to....  

how ford alternators with external voltage regulators work....

there are 2 different types of set up depending if you have a alternator light or an amp meter..

since you have an alternator light... i will describe that one...

the I terminal is hooked to the alternator light in the cluster... the other side of that bulb is again hooked to switched power..    

when the system is not charging..    the power flows from the ignition switch. through the bulb to the i terminal which has lower voltage so the light shines...

when the alternator starts working  the i terminal comes up to equal the battery voltage and since both sides of the bulb have the same amount of voltage the bulb goes out...


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the A  terminal on the voltage regulator..  gets full time power on alternator light cars.. and is switched on amp meter cars...   amp meter cars also don't use a wire on the I terminal..

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the S terminal is the stator voltage..  its only there when the alternator is charging... half of what the DC output in AC.......... it helps control the voltage output some how..   this circuit is also used on later model fords to power the electric choke...    it will support the voltage regulator s terminal and the electric choke just fine...

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the F terminal....    this wire runs from the voltage regulator  F terminal to the FLD on the back of the alternator..     this wire gets pulsed positive voltage from the regulator to one of the brushes in the alternator..  the other brush is grounded...  

the pulsed electric current builds a variable strength magnetic field..   when the voltage regulator calls for more output.. the F wire gets more power.. this increases the size of the magnetic field on the rotor that is spinning away inside the alternator..      the bigger the magnetic field.. the field lines pass through the stator windings... more current is produced...   there is a slight delay... as magnetic fields built up and break down slowly.....


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why is that important..   the battery is a load on the charging system... keeps it from running away with high voltage...   once in a great while.. when somebody does a wrong thing and pulls a cable off or the output wire breaks off..  the alternator has a lot of current to get rid of...   the build up of current is called voltage...  when the voltage exceeds  the max rating of the diodes they short... or open...  this reduces the operating output of the alternator..



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so.. please .. who ever reads this.. do not pull a cable to test the charging system...

use a digital volt meter..    set to 20 volts dc...    14.1 to 14.6 is good...  

Thanks Wayne - I'm on it  

I have  a 1965 and the light stays on. The regulator is new, I only get 11.5 volts running across the battery. Does that mean the alternator is not working?  

Correct  
Bill

1966 coupe
1995 Cobra R

Hmmmmm. New regulator, new  alternator and light still on?  

i spent some time on this other thread...

anybody might want to check it out..

http://forums.mustangmonthly.com/70/9200376/vintage-mustangs/alternator-light/index.html

new regulator..  new alternator..   could be a simple broken wire... the push on connectors used on the alternator backs have fairly stiff wire that will break.. leaving you with out a charging system...  

Sometimes if the engine is idling to low the ALT light will flicker or come on and then goes out up as the engine speed increases.