How to Test Alternator Output, BMW and MINI DIY
GOT A TON OF FAULT CODES, “LIMP-MODE” OR A DEAD BATTERY?
Late model BMWs and MINIs are very sensitive to system voltage. All of the electronic control units must run at a specified voltage. If the system voltage is low, this can often cause various odd fault codes, system malfunctions and engine running problems.
Testing the battery health and the basic alternator output is a first step in diagnosing suspected low voltage or battery charging issues. All that is needed is a basic multi-meter (using the DC voltage range of the meter) or a higher featured electrical circuit testing tool like the Power Probe III.
Test the battery voltage after running the engine (this can be done right at the battery or via the under-hood positive and negative jumper terminals). It should be at least 12.7 to 12.9 volts (it may higher, right after charging). Check the voltage after a couple hours or the next day. The voltage should be 12.5 (75% charge) to 12.7 (100% charge), at this point. If the voltage is lower, the battery may be weak and incapable of holding a full charge. If the voltage drops overnight (or in a couple days), charge the battery and disconnect the battery cables. If the battery holds the charge over the same time period (same period that it was dropping voltage, with cables connected), test for errant current draw.
Test the alternator output by connecting a digital voltmeter across the battery terminals or the under-hood jumper terminals. Start the engine and check the voltage with the engine at idle. You should see at least 13-volts (more is preferable). Turn on the heavy current draw accessories (headlights on high, rear defroster, heated seats, blower motor, etc.). The voltage should still be 13-volts, or more. If the voltage drops a bit (but not below 12.5-volts) as it sits and idles, rev it to about 1100 to 1200 RPM. The voltage should come up to above 13-volts.
If the above test looks suspect, the alternator output may be low and this certainly can cause all form of faults and system malfunctions on the later BMWs (about 2000-on), not to mention simple battery charging issues.