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Alternator, lights dim some just above 900 rpm, otherwise fine


DChronos

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Hey all.

 

My car is a 1996 Monte Carlo LS 3.1 liter, alternator part #10480119 (internal fans)

 

I had an alternator that was going out (brushes worn way down, copper cylinders they rest on worn into)... at certain RPMs (like around 900, 1400, 2400, etc), all lights and electrical inside and out would flicker (dim on and off), and I could hear the engine whine. I grabbed a replacement alternator from a junkyard and replaced that other, and those problems are mostly gone, no more driving around having my headlights flicker.

 

However, when I'm idling (about 700-900 rpm), if I put just a touch of gas on to get it hanging around 900RPM, just under 1000 RPM, the lights all flicker a little bit at only that RPM. The engine loses a bit of RPM as well, then it speeds back up. It's difficult to hold it at that exact point where the electrical all does that, and all other RPMs, there's no variation in lights or anything, and it all runs fine.

 

Is it normal on my car with that alternator for the lights to do that at right above 900 RPMs or so? When I checked out the alternator, the brushes are worn some, but not too much, and the copper cylinders they rest on definitely weren't worn that much, either.

 

 

I can give more details if needed. I'm gonna be checking the voltage tomorrow by placing a lead on the red line from the alternator and another on a bolt in the frame.

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First Guess: Erratic operation of the voltage regulator. Could be a defective voltage regulator, or compromised wiring/wire connections in the vehicle harness. Seems kinda unlikely that a wire problem would have a specific RPM where the problem occurs.

 

Second Guess: Erratic operation of the brushes and slip-rings. If the brush (either one) loses contact with the slip-ring due to the slip-ring being worn out-of-round, the voltage produced by the alternator will drop and spike. This could easily confuse the on-board electronics. MAYBE ~900 rpm is where the natural frequency of the brushes/springs interacts worst with the slip-rings.

 

I'd just love to see the voltage/amperage pattern produced by the alternator as displayed on an oscilloscope.

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Yeah... if the engine block ground wires are the ones that bolt in with the transmission bolts, those were all fine. I even cleaned them before replacing after replacing my transmission.

 

For the alternator, I guess I am noticing now that at idle, at the top RPM speed of the idle, it does hit that spot where the lights flicker just a tiny bit and the idle drops back down. The idle doesn't seem as good as it was when I first hooked it up now, too.

 

One thing I can note is that the pulley wheel looks like it got hit and has just the slightest left/right wobble. I can see where the belt used to rub against the side from being slightly off like that. But that doesn't seem like it's doing much when seeing it run.

 

I tested the voltage using an analog multimeter, and it holds its voltage at right about 14.7 - 15 volts at any RPM.

 

 

One other question I could ask is about the belt itself... Since the pulley wheel wobbled and I didn't want to put any damage to the new belt I have, I stuck my old cracking up belt back in (belt works ok, notched grooves are cracking). Would the belt do anything that could case that stuff? I was waiting till I switched the pulley wheel to the perfectly straight one on the old alternator before putting my new belt back in.

 

 

 

EDIT: Ok, I can notice it a lot more right at about 950 - 1000 rpm... if I hold it at 1000 rpm, especially with the headlights or the dome light on, it's really noticeable. The lights all sit there half flickering... like I keep turning the dimmer switch up and down on everything including the headlights. Is it a bad alternator? I got 2 weeks to bring it back.

Edited by DChronos
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Anyone? Kinda taking it back out tomorrow and going to be looking over the brushes this weekend, as well as deciding whether to exchange it and find another or not.

 

It's right about 950 - 1000 rpm... any other RPM, above or below, haven't seen any dimming at all. I only see the dimming happen if there's anything more than the dash lights on (dash + headlights or dash + dome light, for example). It's obviously losing voltage at that point, just don't know why only that point.

 

If it was a bad voltage regulator or rectifier, wouldn't it not matter what RPM you're at and have the issue all the time?

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yeah the wiring to the dimmer or switch is probably where I would look into next. I have an issue with my truck thats similar but the headlights momentarily turn off and on randomly when driving. I have replaced all the switches and crap in the circuit and it didn't fix it.

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If it was a bad voltage regulator or rectifier, wouldn't it not matter what RPM you're at and have the issue all the time?

Maybe that's the alternator speed where the regulator has to begin reducing voltage; below that speed the alternator is pumping out all the power it can--the only thing preventing over-charging is the slow alternator speed.

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I dunno... The old alternator doesn't fluctuate the lights at that speed, it's smooth all the way up to around 1400 rpm, then the lights constantly dim on and off while the engine whines, and that happens at multiple rpms on the old alternator.

 

 

I have the replacement alternator apart now to clean it up and regrease the bearings. This alternator has been rebuilt before, I can tell by the cold rolled lip being knocked out for the front bearing and that metal clip in its place. The bearing seem in great shape, just that dirt seems to have gotten into them a bit... slip rings are in great shape... the regulator and rectifier appear in great shape, but can't say if they really work properly or not.

 

 

I've read that a bad diode could cause my issue... and read that at 1000 rpm is when the rectifier clicks the alternator on / off, but can't confirm that source for certain... anyone know for sure? Still doesn't tell me why the old alternator has no trouble or flickering at 1000 rpm or at idle.

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Is there a way to pull the rear bearing out without damaging it? At the moment, the only method I can see is using a 2 / 3 arm puller tool, except that will put all the force on the outer ring of the bearing, to the balls, then the stuck inner ring, and I feel like the balls and the edge lips of the bearing will get dented in from the force.

 

I want to wash out the old grease / fine dust that accumulated a bit inside these and regrease and reuse them since they feel almost new.

Edited by DChronos
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I got another question...

 

Is it possible that the slip rings having grooves worn into them and being dirty with all the dust from the worn brushes could cause the voltage switch to be sluggish at 1000 rpm when the regulator kicks in? Not deep grooves the width of the brush, just scratched-in grooves under where the brushes ride.

 

 

I just sanded the rings back to near perfect smoothness today... haven't tested as I've got a dilemma to solve that requires pulling my old alternator first.

Edited by DChronos
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