Headlights dimming while stationary

[eric42]

I have a regular issue with my 1996 Grand wagon. My headlights are fine until i pull up and sit idle, then they both dim after i'm stationary. On take off they both come to life again. Why is this and can i fix it? Any ideas?

sounds like teh alternator

[legacy98gt]

maybe a grounding kit will make it go away for a while

[Robbks]

1) Battery is probably getting low on capacity.
2) Alternator is not charging fully

3) do you have upgraded lights? lots of other electrical accessories turned on?

[eric42]

Lights and fittings are all standard.
Battery is only 2 years old - lights have behaved this way for a while now.

[ouch1011]

Potentially the diodes in the alternator are failing. As long as only 1 diode has failed (there are 3) the alternator will continue to charge but at a greatly reduced capacity. The stock alternator should be able to keep up with the headlights and heater, but if you have the headlights, stereo, and A/C all blasting at the same time, the headlights will probably dim a bit at idle.

I'd suggest having your alternator checked out.

[Matty2Hotty]

Interesting... So my car done this since I got it, had a brand new battery in and also new alternator since the regulator failed the bench test. Still dims on idle with just headlights on. Any suggestions?

[ouch1011]

The only really reliable way to check this is to test the output of the alternator at idle. The older Subaru alternators don't really put out that much juice to begin with, I think they are rated at 90A, and at hot idle, its only going to be about 1/2 that. That doesn't amount to much., especially when you figure that the engine electronics will draw 10-15A, the headlights and exterior lights will draw about 15A, stereo draws about 5-15A depending on what it is and how loud you have it, heater and fan will draw around 10A on high, and if you have the A/C or defrost on, the cooing fans will draw about 30A.