jamesm113 Posted March 30, 2013 Share Posted March 30, 2013 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. Link to comment Share on other sites More sharing options...
monkeyposeur Posted March 30, 2013 Share Posted March 30, 2013 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. Link to comment Share on other sites More sharing options...
Zues Marine Posted March 30, 2013 Share Posted March 30, 2013 ^ thats probably it... batterys good? Link to comment Share on other sites More sharing options...
Sludgeroo Posted March 30, 2013 Share Posted March 30, 2013 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! Link to comment Share on other sites More sharing options...
jamesm113 Posted March 31, 2013 Author Share Posted March 31, 2013 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. Link to comment Share on other sites More sharing options...
cavemanauto Posted April 1, 2013 Share Posted April 1, 2013 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 Link to comment Share on other sites More sharing options...
Sludgeroo Posted April 1, 2013 Share Posted April 1, 2013 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. Link to comment Share on other sites More sharing options...
Zues Marine Posted April 1, 2013 Share Posted April 1, 2013 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. Link to comment Share on other sites More sharing options...
jamesm113 Posted April 2, 2013 Author Share Posted April 2, 2013 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. Link to comment Share on other sites More sharing options...
jamesm113 Posted April 2, 2013 Author Share Posted April 2, 2013 Maybe it was low 13Vs. The lights dim as the voltage drops. That sounds like an alternator issue, right? Link to comment Share on other sites More sharing options...
Hemingway Posted April 3, 2013 Share Posted April 3, 2013 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. Link to comment Share on other sites More sharing options...
spooln30 Posted April 5, 2013 Share Posted April 5, 2013 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. Link to comment Share on other sites More sharing options...
jamesm113 Posted April 6, 2013 Author Share Posted April 6, 2013 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? Link to comment Share on other sites More sharing options...
jamesm113 Posted April 6, 2013 Author Share Posted April 6, 2013 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 Link to comment Share on other sites More sharing options...
Zues Marine Posted April 6, 2013 Share Posted April 6, 2013 i was running 14 lights off my stock alternator with little issues, your headunit is not the issue Link to comment Share on other sites More sharing options...
monkeyposeur Posted April 7, 2013 Share Posted April 7, 2013 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. Link to comment Share on other sites More sharing options...
spooln30 Posted April 8, 2013 Share Posted April 8, 2013 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. Link to comment Share on other sites More sharing options...
spooln30 Posted April 8, 2013 Share Posted April 8, 2013 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. Link to comment Share on other sites More sharing options...
jamesm113 Posted April 8, 2013 Author Share Posted April 8, 2013 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 Link to comment Share on other sites More sharing options...
spooln30 Posted April 8, 2013 Share Posted April 8, 2013 I have a oreilly reman. No problems in 2 years. Have you check out the power and ground cables? Link to comment Share on other sites More sharing options...
Sludgeroo Posted April 8, 2013 Share Posted April 8, 2013 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. Link to comment Share on other sites More sharing options...
Sludgeroo Posted April 8, 2013 Share Posted April 8, 2013 i was running 14 lights off my stock alternator Because rallycar Link to comment Share on other sites More sharing options...
jamesm113 Posted August 13, 2013 Author Share Posted August 13, 2013 Replaced the alt with an OEM reman. No more dimming Link to comment Share on other sites More sharing options...
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