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Old 10-18-2008, 12:24 PM   #1 (permalink)
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Alternator/Accessories

2002 Chevy Silverado 1500 120k miles
Battery is 1 1/2 years old.
Hey guys, just thought I would ask before I dish out the $$ for a new Alternator.
While driving at normal speeds the battery light comes on from time to time. Also when I use the power windows, the high beams, blinkers, or shift to reverse (reverse lights + brake light at same time) the in-dash voltmeter goes down. When the blinker is on it ticks with the blinking. At one point I parked the car and went into a store with it running. Came back out and the truck was lurching and eventually died. The battery at that point was dead and I ended up removing the alternator and bringing it to Auto-Zone. The alternator test results were fine and the guy at autozone said it was good.
I put everything back together and 2amp charged the battery for 8 hours. The truck started right up and has been running fine with no battery light now for 3 weeks until yesterday. Currently the truck is starting and running with the battery light on and the dimming of lights and flickering of voltmeter while using accessories. It's bugging the hell out of me and want to fix it withoug bringing it to a rip-you-off so we get our commision dealer.
With the engine running and a buddy holding down the power window button and flicking the lights I was able to determine that the alternator drops to about 10-11 volts while the accessories are calling for a large load. I had him give it gas and it shot back up to 13.5-14 volts.
The serpentine belt seems to be in good shape but squeeks a lot while the truck is warming up in the morning. If the belt was slipping then I don't understand how the accessories effect it.
Any thoughts? All input would be appreciated.
Not sure what to do other than replace the alternator at this point but don't want to spend the money if theres a possibility it's something else.

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Old 10-18-2008, 02:02 PM   #2 (permalink)
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Check for a bad connection somewhere. Also check the voltage regulator.
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Old 10-18-2008, 04:13 PM   #3 (permalink)
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Quote:
Originally Posted by chevysilverado View Post
With the engine running and a buddy holding down the power window button and flicking the lights I was able to determine that the alternator drops to about 10-11 volts while the accessories are calling for a large load. I had him give it gas and it shot back up to 13.5-14 volts.
The serpentine belt seems to be in good shape but squeeks a lot while the truck is warming up in the morning. If the belt was slipping then I don't understand how the accessories effect it.
Any thoughts? All input would be appreciated.
Not sure what to do other than replace the alternator at this point but don't want to spend the money if theres a possibility it's something else.
Not just a possibility it's something else.
It is most likely something else!
The more demand (demand = voltage drop) placed on the alternator, the harder it is to turn. So the belt slips more.

Procedure to fix it -
  1. Take the battery cables loose.
    Use a wire brush on the battery posts and the battery cable ends until everything is bright and shine.
    Reconnect the battery cables.
    (90% likely to cure the problem. Test it. Start up and check voltages with and with out ALL ACCESSORIES ON. If voltage drops below 13.0 you still have a problem.)
  2. Check that all starter cable, ground cable and alternator charging wire connections are clean, tight to wire and securely fastened.
    If you're a belt and suspenders kind of guy do this step even if step 1 fixed the problem.
  3. Replace the serpentine belt and check the belt tensioner while you're at it.
    How many miles/years on the current belt. Might be a good idea to replace it regardless of steps 1 and 2.
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Old 10-18-2008, 08:07 PM   #4 (permalink)
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Quote:
Originally Posted by TestDrive View Post
The more demand (demand = voltage drop) placed on the alternator, the harder it is to turn. So the belt slips more.
If there's more resistance in the circuit, the current will actually decrease, as will the efficiency.
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Old 10-20-2008, 10:18 AM   #5 (permalink)
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Thanks for the feed back guys.
The connections are all clean and tight. I can't replace the belt until Thursday. Hopefully that's all I'll have to do.
Questions:
How do I check the voltage regulator?
I know how to verify that the auto-tensioner is working, but how do you adjust it if necessary.

I know I need to replace my plugs and wires, they are original to the vehicle (120k) would that have an effect on my problem?
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Old 10-20-2008, 08:20 PM   #6 (permalink)
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With a 2002 GM vehicle, the voltage regulator is almost certainly built into the alternator. When you had the alternator checked, it most likely checked the VR as well, as they are one unit.

Having old spark plugs and plug wires should not affect the charging system appreciably.

I would still suspect electrical connections first, then the belt.

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Old 10-20-2008, 08:52 PM   #7 (permalink)
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To accurately check the voltage regulator on the vehicle,
the belt, auto-tensioner and all pulley driven accessories must be in proper working order and the battery fully charged!!!

On vehicle test procedure:
  1. Engine off and all accessories off.
  2. Use an Amp meter to verify no significant parasitic current loss at battery.
    (There may be a few milli-amps for the radio presets, clock etc.)
  3. Briefly turn on the headlights (10 seconds) to remove any surface charge.
  4. Connect a volt meter to the fully charged battery (13.0 - 13.4 volts).
  5. Run the engine at 2000 rpm.
  6. The volt meter should read steady between 13.5 and 15.0 volts.
  7. Turn on: High beams, heater fan on high, radio up loud and rear defroster (if you've got one.)
  8. The volt meter should read steady between 13.5 and 15.0 volts.
As to the auto-tensioner, there is no adjustment. It correctly tensions a correctly installed, proper length belt; or it's got a weak or broken spring and you replace the entire auto-tensioner assembly.

Most GM auto-tensioners have a tension indicator, but that's only valid if the belt is correctly installed and the proper length. (An old belt might be stretched.) A ribbed-belt tension guage is the official testing tool, but I've yet to find a mechanic or shop that owns one. They all just rely on a mechanic's calibrated thumb and seat of the pants experience.

Short and sweet. If the tensioner doesn't feel funky/jerky when you take the old belt off, it's a near certainty the spring is not broken (at least 99.5% of the time.) If you put on a new, correct part # serpentine belt all correctly routed on the pulleys and the tension indicator on the auto-tensioner reads correctly, it's a virtual certainty the auto-tensioner is OK and the belt is correctly tensioned (say 99.99% of the time).

While you have the serpentine belt off, check the other pulleys it turns. Any pulleys that should turn freely (idlers, AC Compressor Alternator) - give 'em a good spin. Listen for noise and that they don't spin down to quickly. Idler pulleys will spin longer than AC compressor and alternator pulleys (drag from the brushes). Turn all pulleys that can be hand turned to feel for bearing roughness as you turn them. (That includes all the ones you just gave a good spin.)
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Old 10-21-2008, 12:39 AM   #8 (permalink)
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TestDrive and others, thank you for the help.
As I am working full time and also go am going for a degree at night my opportunities to work on my truck are limited. For now to further diagnose my problem I am going to thoroughly inspect all connections and fuses possible (agian), check the voltmeter, and possibly replace the serpentine belt.
TestDrive, when I removed the alternator the first time I checked all of the pulley's and they seemed to spin with no unwanted friction.
What does the RPM have to do with the VR? I will preform test but i would assume that the VR should work at idle speeds also.

Again thanks for all of your help.
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Old 10-21-2008, 05:15 PM   #9 (permalink)
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Quote:
Originally Posted by chevysilverado View Post
What does the RPM have to do with the VR?
Anthropomorphizing a bit... The voltage regulator doesn't produce voltage. The voltage regulator requests the alternator to produce one specific set voltage. At low / idle RPMs even a health alternator may have trouble producing all the voltage requested. At 2000 RPMs a healthy alternator will have no problem what so ever producing any voltage in the correct range and beyond. If the voltage regulator has an internal short and is requesting 18 volts, you'll see that (excessive volts virtually guarantees its a bad regulator). If the voltage regulator is weak and only requesting 13.3 volts you'll see that.

Back in the days when voltage regulators were a separate, external component, standard on car test procedures (as opposed to bench test procedures) dictated that when voltages were out of spec. you temporarily swapped in a known good component (usually the regulator, because it was less work) and retested. If the voltage irregularity was corrected, the swapped component was bad. If not, the other, unswapped component was bad.
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Old 10-21-2008, 07:14 PM   #10 (permalink)
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Drive over to autozone have have them re-do the test - they should have a cart that comes out to your car. It will be an entire system test rather than a component by component.

Did you have the battery checked out (eliminating variables)?

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