Battery light on but very dim

the_sarge · Post by **the_sarge** » Fri Jul 17, 2009 11:40 am

I've been having fun this past week...here's what's been going on. First, the battery light came on bright. I could make the battery light go out by keeping engine RPMs over 2500. I made these checks with a meter across the battery terminals:Car off: 11.3 voltsIdle: 11.3 volts3000 rpm: 11.3 voltsThe car is a 2005 GT with 110K miles on it, the battery is about two years old, and I recently moved from Tampa to Montgomery, Alabama. So it sees a lot of hot hot weather but no winter to speak of.I replaced the alternator today. The battery light went from bright to very dim, and it doesn't change intensity with rpms. Now here's what I get:Car off: 12.5 voltsIdle: 12.5 volts3000 rpm: 13.6 voltsThese alternators are internally regulated, right? Think my battery might be to blame? Help?

jimincalif · Post by **jimincalif** » Fri Jul 17, 2009 2:16 pm

With the car off, 12.5v is good voltage on the battery. Let the car sit overnight, then check the voltage, see if it is holding at 12.5v, if it is, and the battery easily starts the car, very unlikely it is the battery.12.5v at idle is a bit low, the alternator should be putting out something at idle, and 13.6 sounds low at speed, I don't have a meter handy right now to check mine, but I would expect more like 13.2 and 14+. Yes they are internally regulated. The new alternator/regulator could be faulty. Check all connections, make sure they are clean and bright.

keithvibe · Post by **keithvibe** » Fri Jul 17, 2009 8:24 pm

sounds like the alternator.With my scanguage I see 14+ volts after starting the car.

Sublimewind · Post by **Sublimewind** » Sat Jul 18, 2009 1:59 am

the battery light indicated that the alt is not charging the car... The car runs from the alt while the engine is ON... not the battery, unless the alt is going, THEN the car uses the battery... run ot of battery at that point and the car no longer runs... Your alt and/or battery is shot, if you can only maintain 11.5v... You should have at least 14v at the battery with the car running... The only way to charge a battery is to feed it a voltage higher than that of the battery... Have you tried a load test?... you already know it's doing 11.3v with the car running... now, do the same test and turn ON everything electrical in the car you can... I bet it won't look good, if it doesn't kill the car.. lol.. Alt time.. possibly battery

Old Tele man · Post by **Old Tele man** » Sat Jul 18, 2009 8:31 am

...sounds like one of the alternator diodes is shorted (not open), because a dimly lit alternator lamp is a typical indicator....the shorted diode is letting electricity flow backwards (against the battery voltage) for a small portion of the AC-voltage, which is what's making the lamp dimly light up....have your technician check (o'scope) the voltage at the alternator, a small recurring "dip" should be seen, which will confirm the problem.

Sublimewind · Post by **Sublimewind** » Sun Jul 19, 2009 3:17 am

Quote, originally posted by Old Tele man Â»...sounds like one of the alternator diodes is shorted (not open), because a dimly lit alternator lamp is a typical indicator....the shorted diode is letting electricity flow backwards (against the battery voltage) for a small portion of the AC-voltage, which is what's making the lamp dimly light up....have your technician check (o'scope) the voltage at the alternator, a small recurring "dip" should be seen, which will confirm the problem.NICE, I just learned something about the diode pack(rectifier)..THANKS..!!

the_sarge · Post by **the_sarge** » Sun Jul 19, 2009 5:48 am

Thanks all you guys for giving me some things to look at!

the_sarge · Post by **the_sarge** » Sat Aug 01, 2009 12:44 am

Update: I had my avionics guys put a scope on it, didn't see the dip. So the new alternator seems to be OK. That's good because I stripped my tensioner hex-head a long time ago and had to get creative compressing it. Picture a cargo strap and a basketball goal....next time I have to mess with it I'll just cut the belt and put a new tensioner on.SO...I cleaned the alternator connections with DeOxit D5 and put some DeOxit Gold on the pins. Same thing with the battery posts and connectors. I now get 14+ volts with the engine running and no battery light.Thanks again to everyone for the help. Cheers--Steve

Sublimewind · Post by **Sublimewind** » Mon Aug 03, 2009 1:20 am

so you cleaned the battery posts and problem solved? Are you using the stock battery? Battery might be leaking.. around the posts..

the_sarge · Post by **the_sarge** » Mon Aug 03, 2009 6:47 am

Yep--that's all I did. Cleaned the alternator connector and lug, and the battery connections. I did put a drop of deoxit gold on each alternator connector pin, the mating surfaces of the lug, and smeared some all around the battery posts. I didn't know batteries leaked at the posts. I'll check for leaks, the connections weren't covered in corrosion or anything but they did look 'dull'. A dremel with the little wire brush cleaned them right up.

Battery light on but very dim

Battery light on but very dim

Re: Battery light on but very dim (the_sarge)

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