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Old 01-30-2013, 08:16 AM
bimmerfan52 bimmerfan52 is offline
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Location: Arizona
Join Date: Feb 2012
Posts: 1,255
Mein Auto: 545i
I recently changed the alternator on my 545i, and given the number of other components that need to be removed in order to pull out the old alternator and fit in the new, I decided to go with a brand new unit.

My problem was wildly swinging voltage output, a function of the voltage regulator, which can be replaced separately from the main unit.

The alternator is basically a large electric motor which is wired to produce electrical current when the serpentine belt spins wound coils that are attached to the pulley shaft within an a magnetic field, rather than accept electrical current and produce work. The internal coils are wound using magnet wire, which is bare wire that is coated (insulated) with varnish so that it can be wound tightly with little gap between wires. You can look in the front of most alternators and see this wound wire. It looks like bare wire, but it is not. This varnish can eventually break down due to heat and over voltage. When it does it can short to a wire next to it, removing some of the turns from the coil and changing the coil's electrical characteristics. Typically there is high resistance to the current at the short leading to high heat in the coil. In order to correctly test that these coils are fully intact and that the electrical insulation has not broken down, the coils need to have a high-potential test run, which subjects the coil to several thousand volts to see if the current bleeds to other components in the assembly. Coils can also be subjected to inductance tests.

Brushes, voltage regulators and other components can also be tested or examined and replaced.

In my case, while I knew how to fully test the alternator electrically, I did not have the proper hi-pot equipment and have heard bad stories about the variations in quality of rebuilt alternators, where sometimes short cuts are taken. A rebuilt alternator is typically not rebuilt from the ground up. Coils are not always removed and tested. Bearings are not always replaced. Sure, almost everyone gives a great warranty (some lifetime), but if the alternator goes bad it still has to be pulled from the vehicle to be returned.

As far as I knew the alternator in my 545i was original, meaning it was eight years old, with 85K miles on it. The entire alternator, not just the one failed component, had been stressed with heat and duty the whole time. I was not willing to risk either just replacing the regulator, or purchasing an unknown rebuilt unit (that might have 125K miles on it before being rebuilt) given the amount of work that needed to be done to replace it if it failed shortly after installation.

I purchased a brand new Valeo unit for $283 delivered. Given the critical nature of the electrical supply in the computer laden BMWs I felt a new unit was peace of mind and worth every penny.
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