1999 S70 headlamp voltage question

Mirrored · Post by **Mirrored** » 13 Mar 2020, 05:46

Hello all, I have a question about the headlights on my 1999 S70. I had this car given to me by my dad 6 months ago. Not long after I got it, I noticed a headlight pretty dim. After investigating, one of the low beam sockets was pretty smoked, so I decided to buy new headlight assemblies, both sides. All has been fine until recently, my wife was driving behind me and the low beams were bright as a heck. So much so I called to ask if the high beams were on, so she flicked the highs on to show they were not. I checked voltage at one of the bulbs and saw 14.4 vdc. The car had an alternator replaced a couple months before I got it. Seems like high voltage to the low beam bulbs? Sorry for the long winding posted, but any input is appreciated, thanks.

Post by **abscate** » 13 Mar 2020, 05:49

That does sound a bit hot. How old is the battery on the car? If it is weak, the alternator might be pushing hard to try charge it and running the voltage up higher.

Most of us are surprised how bright our lights are when we replace the 20 year old reflectors, though

Mirrored · Post by **Mirrored** » 13 Mar 2020, 05:51

The battery is pretty new also, about 1 year old.

Post by **abscate** » 13 Mar 2020, 05:54

Try comparing lights with engine off, key in POS II, and then running. Check voltage at lights under same conditions.

If you alternator is overcharging you want to deal with it as it can cook the battery if you run it for months like that. Concern, not panic.

The voltage from the alternator might spike after a start if the car has been sitting, to recharge the battery. Maybe do the above checks after a 30 minute drive to see if it changes?

Mirrored · Post by **Mirrored** » 13 Mar 2020, 05:57

Thanks abscate, I will check that tonite when I get home from work.

Mirrored · Post by **Mirrored** » 13 Mar 2020, 19:47

Ok here are some numbers:
Key off, VDC at battery-12.2
Key in position 2, VDC at low beam bulb-11.7
Car started, VDC at battery-14.4
Car started, VDC at low beam bulb-14.4
After 45 minutes drive, VDC at low beam bulb-13.95

I guess the question is the voltage regulated down before it hits the low beam bulb? I have yet to look for a schematic to see if this is the case.