Alternator vs Generator

Both devices convert mechanical energy (from the engine) into electrical energy, but they work differently and have very different performance characteristics.

Generator (1964½-early 1965):

• Produces DC (direct current) directly

• Mechanical commutator and brushes

• Lower output (30-40 amps typical)

• Heavier and larger

• Wears out faster (brushes and commutator)

• Poor output at idle (needs RPM)

• External voltage regulator

Alternator (late 1965-1973+):

• Produces AC (alternating current), converted to DC by diodes

• Electronic rectification (no commutator)

• Higher output (40-100+ amps)

• Lighter and more compact

• Longer-lasting (fewer wear parts)

• Good output at idle

• External voltage regulator (some internal on later models)

Why alternators replaced generators:

• More reliable

• Higher output

• Better low-RPM performance

• Less maintenance

• More efficient

The transition:

Ford switched from generators to alternators mid-1965 model year. Early 1965 Mustangs have generators, later 1965s have alternators.

I drove my Mustang with the original generator for exactly one night drive. The headlights dimmed at every stoplight. The radio cut out at idle. I upgraded to a 100-amp alternator the next weekend. Cost: $200 plus $150 install. The difference was transformative—lights stayed bright, accessories worked, battery actually charged. Should have done it immediately.

Charging system performance determines electrical reliability.

Original generator (if you still have one):

• 30-40 amp output maximum

• Barely sufficient for stock electrical load

• Insufficient for modern accessories (stereo, electric fan, etc.)

• Dims lights at idle

• Struggles to charge battery

• Requires frequent brush replacement

Stock alternator (55-amp):

• 55-amp output

• Adequate for stock electrical system

• Marginal for modern accessories

• Good idle output

• Reliable operation

Upgraded alternator (100-140 amp):

• 100-140 amp output

• Supports modern accessories

• Excellent idle output

• Powers everything without voltage drop

• Cost: $150-$400

The modern accessory problem:

• Stock electrical load: ~35 amps

• Modern stereo: +15 amps

• Electric cooling fan: +15 amps

• Heated seats: +10 amps

• Phone chargers, LED lights, etc.: +5 amps

• Total: 80 amps

A generator or stock alternator can't handle modern electrical loads. Upgrade is mandatory if adding accessories.