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Hello i was wondering if i start my 335i and then disconnect the battery while its still on will it stay on like how it does on different vehicles? , just want to see if my alternator is good just so i dont start taking things apart if its not the alternator.
 

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I highly recommend that you do not disconnect the battery while the engine is running. These cars are extremely complex electronically so and disruption or change in power could be costly. To check how well the alternator is doing connect one lead of a digital volt/ohm meter to the jumper bolt under the red plastic cover under the hood. Connect the other lead to a metal part of the engine. Place the meter where you can see it through the windshield. Check the voltage with the:
- ignition in the run position but with engine off
- engine idling
- engine revving at 2000rpm.

For the last 2 checks vary the electrical load by enabling high current items such as the rear windshield defroster, seat heaters, etc.

The voltage with the engine idling should be in the high 13VDC range. The voltage with the engine revving at 2000rpm would be about 14.4VDC.
 
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