hook a volt meter to the batt.,unplug the voltage reg put a jumper wire to the first and third connetors after the space it will be terminals A and F,if the volt meter pulses,it is the alt.if not it is the reg. we are talking about a classic mustang correct?
hope this helps you.
The Alternator light or headlights do not not pulse at higher revs, just on idle which is about 750rpm in neutral.
It has been suggested that I take the belt off the Alternator, start the car and check for pulsing of the headlights.
The only thing you have to remove from the alternator to take it out of the loop is the wire going to the field. No field current, no out put. The biggest tell as to what is going on is an Oscilloscope. Just measure the voltage on any circuit in the fuse box or off of a cigarette lighter. You should see very little ripple and for sure nothing going to zero or even negative. Most common problem is the diode rectifier having one or more opens which will be very apparent on the scope.
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2008 manual GT with deluxe trim, FRPP CAI, FRPP GTB Mufflers & 91 All Out Tillman Tune. 3.73 gears, Coast Ultralight Drive-shaft. 18" OEM Polished Bullitts, Webelectric Sequentials, XM built into the Shaker 500. Alarm and Escort 8500.
Guys, i'm not an expert but if a diode is bad, its bad, it either works or does not work. since its a classic mustang and he says it "pulses'. These cars have a built in circuit breaker and if its not handling the load (pulling to much current) with the headlights on, it will kick out and then kick back on as if you had the emergency flashers on. Its happened to me. Check your switch contacts, plug connections and your grounds. Mine was a bad connection.
Pulsing headlights and Alternator light 
