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  #1  
Old 03-10-2011, 11:19 AM
Skylark65 Skylark65 is offline
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Unhappy lights dim above 3k RPM

so last night im driving to work, the same route i run every day. about 3 miles from work ( a half hours drive) i have to run up some hills. the truck downshifts like always, i start climbing the hills, but then i notice that the headlights and dash lights are getting really dim. next thing i know the idiot light on the dash that looks like a battery is lit. i reach the top of the hill and back off the throttle, and everything goes back to normal, battery light goes out, ect. next hill, same thing. i noticed the truck seems to do this only when the tach swings past 3,000 RPM, but this is the first that i've had this problem. and as far as i could tell, the amp. guage wasin't doing anything screwy eather.

i've got no real idea what's going on, but i don't like it. does anyone think my alternator could be going bad, or mabe my battery? the battery is about 3 years old, it's an interstate and other than this i've had no problems with it, cranks great, no corrosion on the termanals.

BTW, the truck's an '00 XLT 4x4 with the 3.0 v6 and an auto trans, at about 75,300 miles.

does anyone have any ideas what's going on???
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  #2  
Old 03-10-2011, 11:50 AM
remark123 remark123 is offline
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Default Re: lights dim above 3k RPM

Check the belt tension... possibly time for a new belt, belt is slipping at high r's, can cause the alternator to not charge...
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  #3  
Old 03-10-2011, 11:52 AM
Doctor Wily Doctor Wily is offline
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Default Re: lights dim above 3k RPM

when mine did this it was due to corrosion built up between the post and terminal, loose terminals and an old battery. it would come on any time i ran the engine up thru the rpm range and if i stepped on the brake it would buck and almost stall then shoot RPMs thru the roof (pcm rebooting).

start by cleaning the terminals and making sure they are tight. It may not look corroded, but it takes very little corrosion (resistance) to cause issues, especially right at the source of everything (battery)

Give that a shot and go from there. best part, my recommendations cost you nothing! (as far as parts, testers, etc etc i mean, Not saying you should be paying me, lol :P)

Cheerz!
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  #4  
Old 03-10-2011, 03:26 PM
HazardousRanger HazardousRanger is offline
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Default Re: lights dim above 3k RPM

had this problem like 2 weeks ago where i couldnt start the truck (looked clean) and i cleaned it with terminal brush and it started up faster and better
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  #5  
Old 03-11-2011, 07:01 AM
Skylark65 Skylark65 is offline
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Default Re: lights dim above 3k RPM

i hit the battery termanals with my little cleaning tool while i was at work tonight, but it still did it on the way home. too bad, i love free fixes...

Quote:
Originally Posted by remark123 View Post
Check the belt tension... possibly time for a new belt, belt is slipping at high r's, can cause the alternator to not charge...
come to think of it, that sounds like a good possibility. the belt squeaks like crazy, although from what i've read on here that's very common, and the belt looks to be in good condition, with no cracks, ect. mabe my tensioner is not where it should be?
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  #6  
Old 03-11-2011, 11:20 AM
Doctor Wily Doctor Wily is offline
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Default Re: lights dim above 3k RPM

check voltage drop across the battery terminal to verify everything is good. use a DVOM and put one lead on the battery post, and another lead somewhere along the terminal (i just placed it on top of the terminal connector)
You should get a reading of 0v. If you get a reading then there is still corrosion somewhere causing resistance. That being said, I'm leaning more towards a possible belt slipping at higher rpms (as stated previously) if you are hearing squeaking and such as well. Also if it is only happening at the higher RPM that to me seems to point towards that as a possible issue. Perhaps your tensioner is not doing its job?

Keep us posted on what you find out!
Cheerz
Wily
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  #7  
Old 03-12-2011, 06:33 AM
Skylark65 Skylark65 is offline
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Default Re: lights dim above 3k RPM

well, i think my truck decided to clue me in on what was wrong yesterday, i went up to hang out with some friends a couple of miles up the road, pulling the last hill the truck downshifted and reved up to about 4,200 RPM, this time the little battery light came on and never went off, and the volt guage dipped down below the 1/4 mark to low.... so im fairly sure my alternator crapped out.

got the truck home no problem, gonna call around and see what kinda price i can get on an alternator. damn glad i got my backup car!

oh, and thanks for the help everyone!
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  #8  
Old 03-12-2011, 06:42 AM
FireRanger FireRanger is offline
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Default Re: lights dim above 3k RPM

How you even looked under the hood to see what the condition of the alternator and belt are before you go buying parts?
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  #9  
Old 03-12-2011, 09:28 AM
Skylark65 Skylark65 is offline
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Default Re: lights dim above 3k RPM

Quote:
Originally Posted by FireRanger View Post
How you even looked under the hood to see what the condition of the alternator and belt are before you go buying parts?
yes, multiple times. the battery is fairly new, no corrosion anywhere on the termanals or wireing, and after looking the belt over it appears to be in good condition with no visable cracks. pretty much the only thing left that would cause the truck to not charge at all is the alternator.
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2000 XLT 4X4, 3.0, Auto, regular cab - throttle cable mod, K&N drop in filter, taylor thundervolt 8.2 plug wires, autolite sparkplugs, Accel coil pack, AVS Ventshades, Rampage LED tailgate light bar, BFGoodrich All Terrain T/A tires, Trail F/X toolbox, cruse control pod light fix, Cobra 19 ultra II CB, lil wil antenna, pioneer door speakers, expo woodgrain radio bezal, 06+ mirrors & tails, powerdome hood

2000 XLT 4X4 Offroad, 4.0 / auto, supercab - the project truck!
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  #10  
Old 03-12-2011, 09:35 AM
Doctor Wily Doctor Wily is offline
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Default Re: lights dim above 3k RPM

Pull it and take it in to be tested. Pretty sure auto zone tests them.
Wily
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  #11  
Old 03-12-2011, 11:55 PM
lovemud lovemud is offline
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Default Re: lights dim above 3k RPM

i have all the same problems and thats what i was thinking that my alternator was going bad keep us all posted
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  #12  
Old 03-13-2011, 02:19 AM
TomN TomN is offline
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Default Re: lights dim above 3k RPM

I had this Exact problem with my truck not to long ago. Alternator is done. Pull it out and pull the voltage regulator out and check the brushes to see if they make contact. I had this happen, replaced regulator and brushes, put it back in, worked fine and only cost about $50~$60. Only problem was about 2 months later, the problem came back. Checked the brushes again and they were ground down again not making contact at higher RPM's. I ended up replaceing the whole Alternator. Its really simple to swap out. I did it in advanceds parking lot.
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