I was driving around last night. Every single light in my truck was dimming really fast, not flashing, but dimming up and down really fast. Headlights, dash lights, radio, everything. It only happens when I idle, when I'm accelerating or driving it does not do it. When I got home, I opened the hood and started it, then engine idled like it should then bogged down real low and stayed there. Thats when everything started dimming. When this does this, my battery gauge moves back and fourth about 2-3 volts each way from where it normally stays. I'm assuming this is a bad alternator, but I wanted someone to confirm or disprove this. I'm going up to Auto Zone later today to have it checked and buy a new one if needed.

 

Battery and alternator tested fine. Does Anyone Have Any Ideas?

 

1st guess is lose serpentine belt. you should have gauge on a belt pully showing if it's within the factory specs for the belt. looks like a notch opposing 2 other notches on the mount, with about half an inch space between the 2. notch on the pully should be inbetween the 2 stationery ones, or the beslt is stretched and needs to be replaced. i have 5.3L V8 in mine, pully is on the right hand side of the air intake.
2nd guess is - you got dirty belt and it slips on alternator pully. try Beltease or similar belt treatment, it might help. oil on the belt or something.
3. yes, your alternator is going bad. let them check it, but keep in mind - they are interested in sale. sorry, i missed that you had it checked out.
4. you have electrical drain somewhere. lord save, those are very hard to spot.

 

Zach, what is a STS Remote Turbocharger? Does it have any electrical connection to the factory engine controls? If so I would recommend disconnecting it temporarily to see if the problem goes away.

 

Check all & any ground straps you can find going to your frame from engine, frame to firewall etc, and make sure they are clean & tight, as well as alternator & battery connections.

 

Schroer, Sorry, Different truck. This is a stock 5.3L Silverado that I am having the problem with. I guess I should have mentioned that. It seems to only do it when the headlights are on. Could this be the voltage regulator on the alternator, causing the voltage to rise and fall rapidly?

 

You said the bat checked OK, but it sounds like a problem I had with the Delco bat. The positive terminal(I think) had loosened inside the bat and mine just fell out. Found out it was a common problem with Delco side mount bats. Take another look.

 

my guess, for what it's worth, is a bad diode in the alternator. i have had to chack an alt.three or four times to get it to show up a problem. i am presuming they checked it under a load!! Where is carguy?

 

maybe this will help...this is my 5 step charging system and battery evaluation. it works well but may require tools that you don't have or aren't willing to buy (i wouldn't buy a conductance tester to test 1 battery) so you may have to have some of this done at a competent facility. the mass merchandizers unfortunately (in my area, that is) don't use proper techniques and therefore alot of batteries and charging systems are misdiagnosed by AZ and AAP. sorry guys...that's the way it is with them.

ok...here goes
a fully charged battery is 12.66 volts
take a reading at the batt terminals with the key off, write it down

start the engine a take another reading at the battery, engine running 1500 rpm or so (at least off idle) write this one down

now load up the electrical system (headlamps on high beam, rear glass defroster, wipers, for example) take another reading engine above idle at the battery and write this one down.

next test is with the engine running (idle is ok here) and an electrical load (headlamps on high, etc) and measure from the alternator case to battery ground and write it down

do the same test again but your meter test points are the output terminal on the back of the alt to the battery positive and write this one down.

ok...here is how you evaluate.

first reading around 12 and a half volts is good (12.66V is not absolute but from an electrochemical perspective it means 100% charged)

second reading ideally shouldn't be MORE than 2 volts above the first reading, close to is fine

third reading must be at LEAST one half volt higher than the first reading...elctricity flows from most positive to least positive for all intents and purposes.

the last two measurements should be very low, they are dynamic readings of resistance on the positive and negative sides of the charging circuit (.2-.3 volts is ok...lower is even better) a reading of higher voltage here (on the positive one) would show the bad wire that was referenced in the post from the guy with the old honda. we call these last two reading the positive and negative side voltage drops.

aftre you have done this a couple times, the actual test only takes a few minutes and a few more to sort out the readings...this will catch most all charging system problems. as was mentioned in your original post, most all voltage regulators are located inside the alternator so if there is a charging system test failure, it means replacing the unit or at least removing it and taking it to a local rebuilder. i agree with your assessment that she was being taken advantage of...but i couldn't tell exactly how.

just for the hell of it, the most accurate and dependable way that batteries can be tested today is by conductance...a conductance tester looks like a multimeter...it's hand held and just hooks to the battery with two small clip leads. the cold cranking amps are set on the tester and then the tester sends a small current through the battery and actually measures the internal resistance of the battery. this relates to how easily the battery can turn chemical energy into electrical energy. it's very accurate, in fact new car dealers MUST use this method to get reimbursed for any warrenty claims on batteries.

one other thing i should add here is that there is a GM service bulletin (not a recall, just additional service information to help techs with accurate diagnostics) that describes your condition and basically says it's normal. you'd, of course, have to apply it to your particular scenario and determine if it really fits your truck, but it is possible that it's an acceptable condition.

 

Well...it was the alternator! I was leaving work last night, I hit a stoplight. RPMs almost dropped to 0 and it was shuttering and about to die. Luckly I was only 2 blocks away from the dealer. I made it there just before they closed. They called me back today. The alt was bad. I also had them put a new batt in because the one that was in there, the CCA was only 550. Now all is well! Thanks for all the help!