alternator? - BMW Forum

bigred318 · 07-17-2004, 02:06 AM

okay well heres the deal i just got my alternator and battery replaced about a year ago, so i dont know if its that but seems like it could be.... tonight (ive noticed it before but tonight too) when i walked my G/F to the door i heard my car seeming to just up and down on idle, like maybe up 100rpm and back down, going right along with that was my headlights dimming and going back with the up and down of the engine! do you think it oculd be my alternator again? i do have an a ftermarket sound system, and i know that can drain power! any help would be great!

David

ausie320 · 07-17-2004, 02:45 AM

Hey Bigred,
Was your airconditioner on at the time? When the compressor kicks in it will lower the rpm of the engine due to the additional load. When this happens the combination of lower rpm and running compressor will lower alternator output, especially when you also have the lights on, causing the lights to dim slightly. All this is normal, if you are worried about the alternator output check it with a multimeter. Read the battery voltage with the ignition off, should be between 12-13.2 V, start the vehicle and check again, should be 14.4 V. Readings slightly lower/higher (0.5V) are OK.

ausie320

bigred318 · 07-17-2004, 03:02 AM

Quote:

Originally posted by ausie320@Jul 17 2004, 03:25 AM
Hey Bigred,
Was your airconditioner on at the time? When the compressor kicks in it will lower the rpm of the engine due to the additional load. When this happens the combination of lower rpm and running compressor will lower alternator output, especially when you also have the lights on, causing the lights to dim slightly. All this is normal, if you are worried about the alternator output check it with a multimeter. Read the battery voltage with the ignition off, should be between 12-13.2 V, start the vehicle and check again, should be 14.4 V. Readings slightly lower/higher (0.5V) are OK.

ausie320

thanks dude the ac wasnt on at the time bit i know it does it when the AC is on! but i will go get it tested and see whats up do places like shucks do that?

David

bitcore · 07-17-2004, 08:27 PM

Even if the engine RPM drops significantly, your lights shouldn't dim very much unless your battery is low. How many amps does your alternator produce and do you rev your engine very much?