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signs and syptoms of Alt. failure?

when I start my truck, the voltage seems to fluctuate noticeably, you can see all the lights dim, off and on, pardon me for being a bit dense, but is this normal? A coworker said it could be a sign of the alternator going out...
 

Austin

FTF's #1 Knob Polisher
10,350
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Cumming, GA
Could be the alt... But it would be safe to have the batteries checked.
 
If it's is only happening while you crank the truck it will most likely be the batteries getting down but you need to make sure that you alternator is charging also - if you put a multimeter across the battery while the truck is running you should see at least 13.8 volts - any less means you have an alternator that at the least will need a set of brushes.
If the dash lights are pulsing while the truck is running then i would suspect that the internal regulator in your alternator has a blown diode.
First thing to do with trucks that are getting to this age is go through all the electrical connections and clean them thoroughly ...... make sure you pay special attention to ALL the earthing points as a lot of electrical gremlins are caused by poor earthing connections.
 
thanks for the info, guess this will be a work in progress, electrical and mechanical not exactly my forte, lol. mostly seems to do this (voltage pulse for a 10 seconds or so), when it's started after sitting for awhile (more than 8hrs). Took her over to Napa auto parts earlier today, and they tested batteries and alternator, with multimeter, said everything's good, didn't get specific numbers, but I trust the guy, he's an actual mechanic I know. Course then I broke one of the bolts that holds the negative terminal on, lol, bolt was pretty corroded, and snapped in half, got one end removed, other is stuck. Just put the connection back the battery post loose, and it started up fine, but will try to see I can get 'er fixed on Monday. Anyway, thanks ya'll have a good night.
 

primetime

sawmill slave
I could be wrong, but don`t the glow plugs continue to cycle on cold starts? The draw from those will definately dim the lights.
 
went over to my mechanic today, pulled the broke bolt out with some pliers, lol, no charge. I got two new bolts in both negative terminals, those where the only pieces that were corroded, so greased the 2 new pieces up with that red gunk. need to look at the positive terminals a little better sometime, but didn't feel like removing the air intake again. Anyway, just wanted to follow up, so now ya'll can sleep better ;) Will see if the voltage/light fluctuation has changed at all...
 
Just a shot in the dark here.. My lights were pulsing as well. If you stick a voltmeter/testlight on the single/skinny wire that plugs into the alt with a plastic conector , you should not get any voltage with the key off... (not sure if your year has the same alt/reg setup?)
My ext regulator was sending juice to the alt when the ignition was off...It was also draining my batt...After I changed the regulator fixed my drain and pulsating power..of course this was the 85 6.9 setup/ external regulator and the speed of the dimming was close to that of a blinker or faster ..
 
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