Dynamo vs Alternator: Charging Systems on Classic Cars

Written by Petro Olekhnovych, mobile mechanic in Aylesham, Kent

In the dynamo vs alternator classic car debate, the short answer is this: a dynamo works well if it and its control box are in good order, but an alternator gives more charge at low engine speeds. Most British cars moved from dynamos to alternators from the late 1960s onwards. Whether you should convert depends on how you use the car and how much originality matters to you.

How a dynamo charging system works

A dynamo is a belt-driven generator that makes direct current (DC). It cannot control its own output, so it needs a separate control box. On British cars this is usually a Lucas unit.

Inside the control box are small electromagnetic switches with contacts:

• The voltage regulator keeps the charging voltage steady as engine speed changes. It does this by rapidly opening and closing its contacts, cutting the current to the dynamo’s field winding when output gets too high.
• The current regulator (on many control boxes) limits how much current the dynamo is asked to deliver.
• The cut-out connects the dynamo to the battery once it is producing enough voltage to charge. It disconnects it again when the engine slows or stops, so the battery doesn’t drain back through the dynamo.

Because a dynamo gives only modest output at low revs, lights can look dim at tickover and brighten as the engine speeds up. That is normal for a dynamo car. It also means a car used for short trips at night, with lights, wipers and heater all on, can use more power than the dynamo puts back.

How an alternator is different

An alternator makes alternating current (AC) and converts it to DC inside the unit. Its regulator is built in and is solid-state, so there is no separate control box to adjust. The main benefit for a classic owner is higher output at lower engine speeds.

Signs of a charging fault

Whichever system your car has, these are the common warning signs:

• the battery goes flat overnight or after a few days
• the ammeter shows a constant discharge, or the needle jumps about
• lights stay dim even when the engine is revved
• the battery needs topping up with water often, which can point to overcharging

A charging system can be checked with a voltmeter across the battery with the engine running. As a guide, a reading consistently above 15 volts suggests overcharging, which makes the battery gas and lose electrolyte. A reading below 13.5 volts suggests undercharging. Exact figures vary by car, so check your workshop manual.

Common causes on dynamo cars include a slack or worn fan belt, worn dynamo brushes, poor connections, and control box contacts that are dirty, stuck or out of adjustment. A cut-out that sticks closed can drain the battery quickly, and the fault is easily mistaken for a bad battery.

On a more modern car with a warning light rather than an ammeter, our guide to the battery warning light explains what to check.

Converting a dynamo to an alternator: pros and cons

Conversions are popular, and kits are widely sold. Here is an honest look at both sides.

Pros

• More charge at low revs. This suits cars used in traffic, at night or in the rain.
• Simpler regulation. The alternator has its regulator and cut-out built in, so the old control box is no longer needed to do that job.
• Room for extras. Owners fitting more powerful lights or a modern radio often convert for the extra output.
• Disguised options exist. Some suppliers sell alternators built to look like the original dynamo and use the original mountings.

Cons

• Originality. Some owners care about period-correct parts. Keeping the old control box as a junction box helps the car look original and makes it easier to change back.
• Polarity. Many dynamo-era cars are positive earth. Alternator conversions usually need the car changed to negative earth first. That means reversing the battery and coil connections, and checking anything polarity sensitive, such as an electronic tachometer, radio, heater or fan motor, and some fuel pumps. Our guide to positive earth classic cars covers this in detail.
• Wiring and brackets. The warning light wiring changes, and the alternator may need a different bracket, pulley and belt.
• It has to be done right. Trade Classics advises that a skilled auto electrician should carry out the conversion where the polarity needs changing.

Another middle ground is an electronic replacement regulator that fits in place of the original control box. These need no adjustment and hold voltage more steadily than a worn mechanical unit, while keeping the dynamo.

Is a dynamo still good enough?

For many classics, yes. A healthy dynamo with a good battery and a correctly working control box will keep up with normal use. Problems usually start when the battery is tired, the belt is slack, or the car is loaded with extra electrics. Fix the cause before deciding a conversion is the answer.

How we can help

We are mobile and come to you across Aylesham, Canterbury, Dover, Deal, Sandwich, Folkestone and nearby villages. If your classic keeps going flat or the ammeter shows a constant discharge, we can come out, diagnose the cause and tell you what it needs. Fault finding is part of our classic and vintage car work across Kent.

Labour is £30-£50 per hour depending on the job and its difficulty. Parts are extra, and we agree the price on WhatsApp before any work starts.

Frequently asked questions

How do I know if my classic has a dynamo or an alternator?
Look for a separate control box near the dynamo wiring. A dynamo needs one, while an alternator has its regulator built in. Some alternators are made to look like dynamos, so check the parts fitted if unsure.

Can I convert back to a dynamo later?
Usually, yes, if you keep the original parts. Keeping the control box in place as a junction box makes it easier.

Will a modern radio work in a positive earth car?
Some won’t. Check the radio is compatible, or change the car to negative earth first.

Need help? Send a WhatsApp to 07563 350335 with your car’s make and model and we’ll come to you.

Sources

• Classic car voltage regulators: what they do and how to adjust them - Classic Car Hub
• Working principles of the voltage regulator - healey.org
• Changing from dynamo to alternator - Mini Mania
• Change to an alternator - Irish Veteran and Vintage Car Club
• Should I upgrade my dynamo with a negative earth alternator kit? - Trade Classics
• Dynamator C39 replacement - MG Bits
• Conversion from dynamo to alternator - MG Car Club Magnette Register