jdub907

2005 SL 500. After 5 hours lights left on not running, won't start. road assist says alternator bad and not charging batteries. Is this common or does alternator stop charging batteries if they get very low?
Thanks

MarkCan

Just test the alternator yourself. For the basic test, inexpensive multimeter is all you need.
With the engine off you should see just over 12V on the terminals and then around 14V after you start the car. Check both batteries, front and back to make sure there is no break in the connection somewhere along the way. Not getting the 14V, not to worry. Check the brushes they are probably worn out, or replace the whole regulator for few more bucks. You can do it! Even if you have never done this before!

A1EK

Just to add, do not jump start the rear battery, just the front.

bobterry99

I think usually it is the voltage regulator which is in fact bad. It can be installed fairly easily with the alternator in the car.

jdub907

Thanks. Where's the regulator? Ha.

bobterry99

The regulator is attached to the back of the alternator.

jdub907

Thank you. It's a $35 part at Pelican Parts. I bought an alternator assembly. Maybe I'll replace both to ensure internals of alternator are good (brushes, bearings). I have a mechanic checking it for me. He's not a Merc tech but a former engineer and very good mechanic who researches well. I recommend him to anyone in south mississippi. Worse case, I'll take it to my indy for further diagnosis.
Thanks for the help.

jdub907

After diagnosing current alternator it was bad, voltage regulator was bad, rectifier was bad. My mechanic replaced alternator assembly...voila. I have proper charging of batteries. The accessory battery may be getting a little weak. But is ok. My little racecar is back in service.

Thank you all for your responses.

poppyboy

Unless you drive the car daily, I highly recommend keeping it on a battery maintainer. I rarely drive mine and keep both the front and back batteries on separate CTEK maintainers.

jdub907

I drive it everyday. I will still look into the CTEK line of maintainers.
Thanks.