Headlights dim with high revs?

My Legacy's headlights have started dimming when the revs get over 4K. When the revs drop below 3K, the headlights brighten up. I have no issues under normal cruising conditions. Could the alternator be going bad? It was last replaced by the PO at 83k in 2005. Alt belt is new.

 

Thanks.

Check the voltage on the alternator. Could be going bad. Running a few more ground lines from the negative terminal of the battery will only help things. Your local auto parts store will check your alt for free.

^ thats probably it... batterys good?

Check battery and alternator like these gents said.

 

The more grounds, the more stable the charging/ignition system.

 

Also... I had this issue and the fix was so easy it made me feel dumb, but it can go overlooked because of the simplicity.

 

Check to see if your belts are very tight with a maximum of 1/2" deflection. When the alternator is under electrostatic load, it puts resistance on the pulley; and likewise at higher revs.

 

I had an issue where my alternator was only putting out 12.9 v and wasn't recharging the system enough. I tightened it and I got 13.9 ~.

 

The belt has a tendency to slip past the pulley without making a squealing noise.

 

Mind the tensioner pulley bolt, because they like to rust, and if it snaps youre SOL until you can grab one from a dealer.

 

Also just quick check connections on the backs of your lights. Just to denounce every possible avenue of failure.

 

Good luck brother!

Now that I think about it, it's probably just a loose belt. Strange that i didn't hear it slip though. But I've been noticing a slipping sensation while parking with the power steering so....

 

Anyway, ran some tests today to check.

 

Battery is an AC Delco, put in in 9/2010 with 124k. Tensioner bolt is new. Headlight connections look good, no rust or or corrosion. No exposed wires either.

 

Car parked:

battery voltage (across posts) = 12.7

alt voltage (top terminal to intake manifold) = 12.7

 

Idling:

Across battery posts = 14.4

top terminal to intake manifold = 14.4

 

med revs (no load)

across battery posts: 14.4

top terminal to intake manifold = 14.5

 

The belts had just over 1/2 inch of freeplay. Following the haynes manual suggestion, I tightened them to just a hair over 1/4 inch freeplay and remeasured voltages, with the same results (12.7V all around parked, 14.4V all around at idle, and 14.4/14.5 at medium revs).

 

I'll give this a try tonight before pulling the alt to take to O'Reillys.

You should also check from alt to batt.

I had an issue where the PO had used a poor ground choice and dispite good v numbers I was not getting the current through, guess they lost the ground bracket for at the starter and ran the main ground to a small m6 hole in the left head.

 

I have also seen carosion affect alternator output at both the ground bracket, as well as St the alt mount points.

 

A loose main alt bolt will also give poor alt performance.

 

Good luck, hope it's a easy cheep fix

Was that your issue? Those volt numbers are perfectly fine, which has me thinking its not alternator it sounds like a grounding issue.

 

Get the wiring schematic off an FSM somewhere and find where the power supply for the headlights are grounded through and check it, if not just replace them all.

Get the wiring schematic off an FSM somewhere and find where the power supply for the headlights are grounded through and check it, if not just replace them all.

Ok, had an assistant help me today. Voltage drops from around 14.2 with headlights on to mid to high 13V's at around 4250 RPM. Had a few brews so i don't remember the exact numbers Doesn't matter if i measure across battery posts or from alt pos to battery negative. Headlights dim with voltage drop.

Maybe it was low 13Vs. The lights dim as the voltage drops.

 

That sounds like an alternator issue, right?

Yes it has from your first post. If you can, Autozone (or another shop) can bench test your alternator ... just need to remove it and bring it in. Call ahead to confirm that they can at your local Autozone.

More than likely its the alt, remove it and take her in to get tested. Their testing will show more than just the voltage output. I had the same issue on my old 96 L and it ended up being a bad alt. What took my alt out was the 2000 watt stereo system I had installed. No more boom in my 99 LGT it got annoying again. LOL.

Tested it O'Reilly's. Tested fine. They said the bearing might be going bad though. I think I heard a little roughness from it when I was testing voltages. Would a bad bearing cause the voltage drop?

Would the sound system cause it die prematurely? It's nothing crazy, a basic JVC digital media head unit, 4 Polk audio db651 speakers & a subaru sub. I don't have a high output alt, but never thought i'd need one either

i was running 14 lights off my stock alternator with little issues, your headunit is not the issue

The ground for your head unit (hu) could be bad. The stock ground on my hu burned out an caused it to short out. For good measure you could run a ground from the hu to the chassis.

You know the bearing, if bad and semi seizing up when hot could cause the output to drop off. Though I would think you would hear a belt screech. Have you looked over the cables from the positive side of the battery to the alt or the main grounds. Sometimes it's just something simple.

Btw how much do the lights dim? I have a newer rebuilt alt in my LGT and my lights will dim a little at night if I close two windows at once. More cars will have a slight drop off when more items are being used. If your lights are going really dim then yes you have a problem.

They dimmed enough that it was difficult to see the road with oncoming traffic. I think I'll run it for a little while, and if I start hearing more noise, or they start dimming more, I'll replace the alternator.

 

I found a subaru reman'd for $115. Also, O'Reilly has lifetime warranty reman for $130, but I don't know if those are any good. Recommendations are appreciated

I have a oreilly reman. No problems in 2 years. Have you check out the power and ground cables?

Check those cables like spooln said, and the bearing in the alt would sound like a shot idler. The design in that pulley system has the most force, as the adjustment of tension is in the alt adjustment.

i was running 14 lights off my stock alternator

 

 

Because rallycar

Replaced the alt with an OEM reman. No more dimming