Couple of questions

I've been regestered on this board for some time, and have done my best to help others with problems i'm familiar with, and generaly put my 2 cents worth in where GMC'S are concerned. I'm no genious but am considered pretty good mechanicly, but i'm having a few problems that have so far stumped me.
One is the battery light in the dash flickers, the gage shows everything is fine, ie its charging, I took my baby to autozone and had them run a test and the test showed everything is fine, alt charging. but did show a weak battery, i replaced the batt thinking hah! but guess what, the light still flickers. I do have a duel batt setup with the proper isolator etc all in place, the second batt a deep cycle marine/rv was checked and found fine, which is good since it is as well brand new.
im stumped on this because i have no problems with mysterious dead batt situations. Does anyone have any ideas where I could proceed from here to solve the problem?
Also I'm having problems with a slightly erratic idle. its fine when cold the problem only starts when the truck is warm, and once in awhile sitting at a light it will surge, just slightly but its noticable, I service this truck faithfully and have already checked the obvious items, vacume lines etc, at first i attributed the idle to a new cam I installed about 6 mos ago but have since dismissed this theory, it was the largest cam that would work with the computer controls, basicly an R/V style. but not large enough to cause the irratic idle since its not a lope like a rough cam idle would bring. first thing i changed in attempts to solve this was the o2 sensor. as well when i changed the cam i throughly cleaned the egr, so after all this im at a loss, anyone have any ideas where to go from here?
For some background the truck is a 91. 2500 w/a 5.7 170.000 mi. on it.
it does need valve seals,'smoke only when it is first started in the morning' and I have a beautifull set of vortec heads waiting for the $ to get the intake and other goodies to make this swap, but doubt the worn seals would cause the idle troubles.
anyways any help would be greatly appreciated.

First of all, I have to ask, why do you have a dual battery setup in your truck? I would almost be tempted to hook it up to the one battery only and see if the light still flickers. If not, I would guess your altenator might be dropping out periodically. It's charging when the light it off and dropping when the light goes on??? Not sure.

Thanks for the reply, I have a duel batt setup for several reasons, for 1, I have a camper, as well as a high wattage stereo system, as well as a power inverter that I run power tools off of in my line of work, the second deep cycle battery is set up to carry ONLY the load of the camper 'when its on the truck', the stereo system, a backup for the alarm system the power inverter, and a few auxilary lighting systems, 'high watt search lights etc'.
The main battery carries the starter and the basic systems to power the truck. I decieded to go this rout because I got tired of finding myself in need of a jump start while camping in the middle of the boonies, or on a construction site after a days work.
I'm a plumber but also climb and work on hill top towers installing antenas and dish's for RF, communications, I personaly know many people who have duel batterys for various reasons, but these are mine, any way I also thought of the alt fading in and out so I was inside the truck durring the autozone test and though the light flickered durring the test it came up fine, plus I could see if the light flickered and my gauge showed some variation while this was happening, but this is not the case, and why im so stumped. but thanks for the input.
I might just end up rebuilding the alt anyways, but if i do this and the problem persists im really gonna be aggitated. lol. but as a next step im gonna try and disconnect the second battery at the isolator to see if that is the possable csause, though i've spoken to the manufacturer of the isolator unit i installed and they assured me that could not be the trouble.

what is the size of the alternator and the number on the pulley? If you have a 105A change it to a bigger one like 120A or 130A. I would guess your pulley # would be 964 go to a 580 you would have to change your belt I believe. First disconnect the second battery and turn off the stereo see if it still does it.

Might be a loose bulb behind your instrument cluster panel/amp meter

Thanks for the ideas Tweaked & Mud.
I do plan on switching to a little higher output alt, I am running the stock 105 output at the moment, the pully as well is stock, i'm assuming the pully switch you mentioned would kick up my rpm at the alt even at idle,? if so thanks for the info since I was wanting to do just that in the future, this will allow me to give a better charge in a shorter time when camping and just want to let the truck idle to charge the battery, I do have a generator for the camper but many campsites will only allow a very short run time of a generator, but go figure I'm allowed to run the truck pretty much as i wish.
I will be checking the light itself as well since i plan on switching all my dash lights to leds in the very near future, love the look and they are supposed to last forever.
But at the moment the first thing i must tackle is the wierd idle and intermitant surging at stop lights, since this is a drivability issue where as my gauge shows im charging and i dont have a problem with dead batterys, I guess ill start with the egr and go from there for the funky idle problem. it never dies on me, but i hate it when it feels like it just might.
the worst part is the wife getting pissed at me because i work on my truck nearly every weekend where as i seldome address the problems with her toyota, but hell the last time i worked on her car i was in hell, took me a day and a half just to change the rear motor mount and pretty much had to pull the motor just to replace the rear mount, not to mention the frustration, thank god for GMC'S.

1999 GMC Sierra 1500 Ext Cab (old body style) 305 (5.0 liter)

This weekend, I had what seemed like dying battery problems. Turn the key; lights, radio, fuel pump, etc all power up. Try to start, and just clicking. I replaced the battery with a new one (was due for a new one), and still the same thing. Then I discovered that the positive cable had considerable corrosion. Using baking soda and water, and a flathead screwdriver, I cleaned everything that I could down to bare metal on the cable connectors and bare lead on the side post. This did the trick. I had the old battery tested, and the guy at Checker said that it had a charge, but a shorted cell. So, even though I have the new battery and cleaned cables, I plan to replace the entire positive side cabling with new ones.

Hope this helps!

SunDog67

1999 GMC Sierra 1500 Ext Cab (old body style) 305 (5.0 liter)

2 days later: I started having the same problems again; would have to wiggle the positive side cable while my friend kept turning the key to finally get it to start. I replaced the positive side cables with new ones from the dealer and it started right up. Here are the GM part #'s and the before tax prices that I paid for new cables for my particular truck:

Positive:
# 12156900 $56.98
# 12157093 $47.58

Negative:
# 12156901 $26.90

Pretty expensive, but I'd rather go with oem on stuff like that. Also, the pricing above was from a Chevy dealership; the exact same cables (same part #'s) were 25% higher in price at the local GMC dealership!!!

Hope this helps!

SunDog67

Been awhile since ive been able to get online, but! i did solve the charging problem however, turned out when the isolator manufacturer says 90 amp they mean NO MORE THAN 90 AMPS,. i have the 110 amp alt, i found out about this when i noticed the cooling oil leaking out of the jointed areas of the isolator, the manufacter was great though, talked to a live person several times about my problem, and to top it off, they wanted this unit back to run tests, after my screwup in thinking the 90 amp unit would do the job i went to a local stare that had the 140 amp model and only had to pay the diffrence in the price between the two units, not for my mistake, WOW imagin that, the manufacturer called and asked the parts shop to expect me etc, i probably would have used the proper unit except that all of my local big chain auto parts dealers only carried the 90 amp part, with the 140 amp unit installed, no more flickering light, both batt's work great, tried it out last week, went camping "have an inverter installed under the hood" 1200 watt, and had 4 hrs of tv, dvd, surround sound in the middle of the woods, before i had to start and recharge the batt, kids loved it, plus i can run my power tools air compressor anything anywhere i need, and i dont kill my start battery by the kids running the camper too long on camping trips, i put the inverter under the hood because dc back to the camper was not enough to run things properly, so i ran a construction grade cord in the frame rail back to the bedthis way i can work off the back of my truck and plug in my camper as well, finaly my plan came together!!