Fiat Twin Cam
Alternator Upgrade FAQs By Csaba Vandor
(Note: I wrote this based on my experience with 2 Fiat 124 Spiders. I assume the same info can be used for other Fiats)
Prior to 1977 Fiat Spiders came with an externally regulated 45 amp
(some '75-76s had 55 amp) alternator. Many owners wish to convert to
the later style internally regulated unit, for simplicity and higher
amperage (55 or 65).
All 124/2000 alternators are interchangeable size-wise, regardless if
it was originally mounted on the left or right side of the engine, so
the later type alternator easily bolts on the early cars.
The externally regulated alternator has 3 wires going to it:
-the 'power' wire, usually 3 black wires secured to a 6mm stud on the back of the alternator
-a wire going to the voltage regulator, gray, has female end
-a wire going to the charge light relay, yellow or yellow-red, has female end
The later unit has only two connections:
-one for the 'power' wire, a 6mm stud on the back of the alternator
-a male terminal for the wire going to the charge light
The later type alternator should bolt into place on the right hand side of the engine.
Connect the black 'power' wires to the stud on the back of the
alternator, connect the yellow (or yellow/red) wire to the male
terminal, and leave the gray wire disconnected.
The two alternators use different methods for turning on the charge light.
earlier type alternator gives a +12V signal to a relay when the
alternator light is supposed to be on, and the relay switches a ground
signal to the light. The later type alternators switch ground directly
to the alternator light, so the relay is not needed.
To make the light work properly, disconnect wires 85 (yellow or
yellow/red) and 30 (violet/white) at the relay, and connect them
Start up the car, and measure alternator output voltage at the back end of the alternator.
It should be at least 13.5 volts. The engine may have to be above idle to achieve this.