How to upgrade a classic car to a modern alternator

By Carl Heideman
Jul 10, 2024 | Restoration, alternator | Posted in Shop Work , Restoration & Renovation | From the March 2016 issue | Never miss an article

Alternator changes and upgrades are among the most common underbonnet modifications performed on British cars. We’ve seen hundreds of these swaps performed through the years. While some are appropriate and effective, we’ve witnessed–and fixed–too many that have ultimately caused some sort of expensive problem: an electrical meltdown, a spinning part thrown from the car, or maybe just something sticking itself into the radiator.

Bottom line: We think most alternator conversions are a waste of time. In fact, they’re often dangerous.

We know, them’s fightin’ words. Everyone wants a reliable alternator, and those Lucas contraptions don’t seem to hold up or generate enough current. To put it simply, they just aren’t the kind of outdated technology we want to see when we raise our hoods. However, anyone who took debate–or made fun of the debate kids–in high school knows that you can’t come out with fightin’ words without backing them up. So, allow us to state our case.

It’s clear that many modern alternators put out more current than their old Lucas counterparts. You could also argue that the newer units hold up longer. However, the alternator itself isn’t the problem in most alternator swaps; it’s the wiring and the mounting that get people every time, making smoke and nasty sounds almost inevitable.

Think about this: If you install a modern, 80-amp alternator and use the stock wiring that was designed for a 36-amp alternator, what’s the inevitable outcome? Many people reason that if the accessories will still require the same amount of current from the new alternator as they did from the original, then it’s safe to use the stock wiring.

This logic fails to take two words into account: current potential. If a component ever fails or a dead short accidentally occurs, something will attempt to draw the full current capacity of the new alternator. What does this mean for a big portion of the wiring harness? Melting, smoking and burning.

Here’s something else to consider: Slapping on that alternator with some hardware store bolts and getting the alignment “pretty close” will work for a while. Eventually, however, something’s going to break, let loose or fly off, and that alternator will be flopping around, waiting to short out or preparing to hit something.