Quote Originally Posted by Slither View Post
Not sure I follow this... aluminum is an outstanding conductor, commonly used in the power-distribution industry. In fact, my house runs on it... underground aluminum cables.
Paul, you are right: aluminum is a fine conductor. But, the sticking point is that aluminum sucks in regards to making a good connection to it. It is extremely reactive - it is more quick to oxidize than iron. Every piece of exposed aluminum in your engine compartment has a thin layer of non-conductive aluminum oxide on it; if that's not penetrated in the connection, no conductivity. (Conversely, this thin oxide layer protects the aluminum underneath; rust does not do this for iron. That's why an iron piece will rust away faster, even though it's not as reactive as aluminum.)

And then there's the issue of keeping the connection once you've established it. Remember the fiasco of aluminum building wire in the '70s? The connections loosen up over time, corrode, build resistance, heat, arc, burn the house down... I know the industry tried various types of special connectors and even copper-plated aluminum wire in an attempt to get the connections to last over time, all without much success.

You service drop cable (about a 40% chance it is Southwire cable manufactured on a production line that I installed, BTW! Please go buy more! We need the business!) is indeed aluminum, but you'll see that it stops at your breaker box. From there on you'll have copper. You'll also see that where the aluminum is lugged into the box that it has gobs of conductive joint compound on it to increase condutivity, lube the lugs, and protect the connection from corrosion.

So, even though the aluminum itself is great, the various intermediate connections can kill you. A ground wire straight to the alternator case is an easy way to avoid a lot of potential headaches caused by the connections. (Also, always use a serrated lock washer for your ground wire connections - you know, the washers that look like a Skil saw blade. At the alternator case, these will cut through the oxide layer; at the frame, they'll cut into the steel for a better connection as well.)