Any life after 150K on an alternator is a day with sunshine.

The only way to test Alt output is to load test to specific AMP value and read voltage,total amp output, and diode results. Reading volts will show regulator control/alt output but not total capacity and capabilities of the said unit. Testing must be done at 1500 RPM and again at 2500 RPM. The Autozone tester will find dead ones, not intermittent functional Alternators (brushes, reg, winding etc). They only do a pass fail test based on the user's skill set - poor at best.

I personally would recommend replacing the Ign SW (if it is the OE one)and the Alternator, as they both are due. When these two parts start to go away, they will give you all kinds of weird failures and operational errors. Cheap insurance and cost of normal ownership.

Both are easy to do with the right tools. Nothing fancy just hand tools.

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2005 X5 4.8IS
The Blue ones are always FASTER....

Current Garage:
2005 X5 4.8is
2002 M5 TiSilver
2003 525iT
1998 528i
Former Garage Stable Highlights
2004 325XiT Sport
1973 De Tomaso Pantera, L Model
1970 Dodge Challenger T/A 4 sp Alpine White
1970 Dodge Challenger T/A 4 sp GoManGo Green
1971 Dart Sport, “Dart Light” package
1969 Road Runner 383
1968 Ply Barracuda 340S FB Sea-foam Green