How to Repair Alternator Problems on Classic Cars

Classic Car Alternator
Classic Car Alternator. Photo by Mark Gittelman

If you own a classic car from the late 50s through the late 70s the automobile should have an alternator. If it's older it could possibly have a generator. If you're interested in learning more about generators we have a popular article about why you should convert your generator to an alternator.

In this article we'll address charging system problems caused by alternator malfunctions. We'll also answer a question that has perplexed car owners for decades. Should you rebuild the original alternator or replace it with a refurbished unit or a new part.

Replace or Rebuild the Alternator

When it comes to classic cars I'm a firm believer in retaining original equipment parts whenever possible. In many cases the alternator provides an opportunity to replace defective components inside while maintaining originality outside. This is a must for people who participate in preservation class car shows.

On classic cars they mount the alternator front and center making it highly visible. The picture above shows the engine compartment of a Porsche 356 1600 Super Roadster. This is a textbook example of why you would want to keep the original component. The patina gained from years of operation is often appreciated by car show Judges. However, there are a few circumstances when saving the factory installed unit is just not possible.

An example of this is case damage. Most alternators are cast from aluminum. It is possible for this strong, but brittle metal to develop cracks. Another problem area is the mounting locations on the perimeter of the case. Threaded holes with soft aluminum threads can strip easily. Integrated mounting brackets can also break off or suffer damage. Welding aluminum is a difficult operation and is not recommended in these situations.

Another issue that can damage the case is a spun internal bearing. All alternators will have a front and rear shaft bearing or bushing set up. If this component fails, it can spin in the aluminum case and wear away material. This damage can prevent the replacement part from fitting properly. If an alternator case is damaged, then replacing it with a new or rebuilt unit is the way to go.

New Alternator Vs Remanufactured

I am not a big fan of remanufactured alternators. This comes from my experience with replacing them multiple times before receiving a good one. With that said, my opinion should be considered jaded as a professional mechanic that's performed the operation hundreds of times. It's human nature to remember the bad experiences over the good ones.

New replacement parts are available for popular model cars from the 60s and 70s. Here is an example of a brand-new alternator on an old 1970 small block 340 CID Mopar engine. Car owners can even go crazy and get a show quality chrome unit for just about any muscle car. A new alternator costs more, but this can be money well spent. They often provide a longer warranty and therefore are better tested before leaving the manufacturing facility.

Rebuilding the Original Alternator

Although there are a lot of parts inside of an alternator, failed components fall into two major groups. Mechanical parts like bearings and shafts can wear out just like engine parts. If a bearing fails, you might hear a squeaking or even a grinding as the alternator rotates. These parts are almost always replaceable. You can buy Timken alternator bearings individually with an average cost of around $20.

The next major failure group falls under the category of electrical components. When an electrical part malfunctions the alternator stops charging the battery. One of the main electrical components inside the alternator is a set of brushes. They continually conduct electricity to the rotating rotor slip rings. These spring-loaded brushes were designed to wear out over time. If you keep your car long enough, it will need a set of alternator brushes. Another common failed component inside is the diode Trio.

This device allows current to pass one way. When it fails, it allows current to pass in both directions. The diode Trio is easily tested with a continuity checker on an automotive multi meter. Another component that’s likely to fail is the voltage regulator. This is a tricky part to locate as they switched from external to internal in the 60s. Here's what an external voltage regulator looks like. Regardless of whether it's inside or outside the case, these parts are readily available for replacement.

Rebuilding the original alternator will retain a valuable piece of your classic cars history. And this is how much money you could save at the same time. A new replacement Valeo alternator for the Porsche 356 Speedster pictured above comes in around $600 to $800 depending on the year. A new voltage regulator and brush kit for the original Valeo 70Amp alternator carries an average replacement cost of around $20.