If it hasn't happened to you already, you probably know somebody who's experienced an alternator break down. This occurred recently to our Project M3, which had covered around 130k miles, and it certainly isn't an uncommon problem.
In a nutshell, the battery starts the car. The alternator continually charges the battery while the car's running. If the alternator goes out, the battery will eventually die as well, and so will all your electrical accessories.
We guessed our alternator was on its last legs when the battery light came on whenever engine speed fell below 3000rpm. In less than a week, the entire electrical system suddenly went out while driving (thankfully at neighborhood speeds), leaving us with no speedo, no lights, nothing.
Instead of ordering a battery through the dealer and paying somebody to install it, we decided to check Bavarian Autosport's catalog and tackle the installation ourselves.
Bavarian Autosport sells a 140 amp Bosch alternator, which is a hefty upgrade over the standard 80 amp Bosch unit that came with the car.
The new alternator is a remanufactured unit, not rebuilt, and there's a big difference. Rebuilt means the broken pieces were replaced, and maybe a few other easily replaceable items like the bearing or connector. Remanufactured, on the other hand, means every single piece, except for the housing, is replaced. It's essentially a new unit.
The alternators you get from your BMW dealership are remanufactured Bosch units, but they run over $325 for the 80 amp and $400 for the 140 amp units. Our 140 amp alternator retails for just $269 from Bavarian, and you get $40 back when you send them your old one.
There are advantages to upgrading to a higher amperage, especially in extreme cases. Let's say you're driving at night listening to a sound system with several amplifiers, you're lighting the road with high wattage lighting, you have the air conditioning on and it's raining, so your windshield wipers are also on. Let's add a set of underdrive pulleys under your hood, which reduce parasitic horsepower losses through underdriving some accessories, including the alternator. However, this means your alternator needs to turn at a higher rpm before its rated amperage is reached, which may require over 1500rpm from the engine. Lastly, let's assume you're usually in stop 'n go traffic, and your car idles at around 800rpm. Can you see where this is going?
You could end up drawing more amps than your alternator will put out and eventually draw off the battery, and that won't last long.
It's not uncommon to see even one good-sized audio amplifier draw over 50 amps alone. Fortunately, a higher output alternator will offset these adverse affects, and we're glad ours puts out nearly twice the amperage of the stock unit.
Another advantage is what happens when your battery goes dead overnight after you've left the headlights on and you try to jumpstart. With a larger alternator, the battery will charge more quickly, making it unnecessary to "drive around" before being confident you can shut the car off and restart it again.
If there's one thing that could be classified as a "disadvantage" of going to a larger alternator it would be that it takes more horsepower to create the greater amperage if the draw is significant. But if that draw is greater than 80 amps, you'd be stuck on the side of the road with the original unit anyway, so who cares?
Some of you might express concern that this alternator might be too much for the stock electrical system. However, the car's regulator is supposed to prevent any amperage spike, and even the dealers sell them! What's more, we haven't had any problems on Project M3 to date.
It's good we got this alternator because we plan to test a set of underdrive pulleys soon. Stay tuned.
Author's Note: If you don't have (or don't trust) the battery light to warn you, Bavarian Autosport offers a simple way to diagnose alternator performance: Place a voltmeter on the battery while the car is running, which should read at least 12.5 volts, although 13 volts or more is preferable. Now turn on all your accessories, including the stereo, windshield wipers, high beam and A/C to draw lots of current.
If your voltage reads lower than your previous measurement, try bringing the revs to at least 1500rpm. If it's still lower, the alternator is either on its way out or it generates insufficient amperage for your current draw.