I think I have an alternator problem, but would appreciate your opinions before I fork out 230 bucks for a new 160 amp AC Delco (and that's a good deal!).

All of the lights on my 2008 Silverado sporadically flicker. Not to the point of totally going out, but they'll brighten and dim, especially when apply the brakes or adding an increased electrical load by activating the power windows or rear window defogger.

I checked/tightened the battery connections and cleaned the ground contacts on the engine block and chassis a few weeks ago, but the problem continued.

Battery with no load measures 12.7 volts. Turn the key to "on" without starting the engine and it goes to 12.0V and remains steady. Cranks up just fine, then fluctuates between 13.8V and 15V while revving the engine to 2500 RPM. The lowest readings occurred while activating the power window.

Based on all of that, I'd say the alternator looks good...however, when I set the multimeter to AC volts to check for AC ripple, I'm consistently getting 250 - 350 mV, occasionally spiking to over 1 volt. Does that sound like a bad diode?

ttt

Any thoughts on this issue? If anyone has knowledgeable insight into this issue, your help would be appreciated.

Have the alternator tested.

That generation of chevy/gmc trucks had a problem with the light switch corroding. Caused a similar flicker in my 05 Sierra. If nothing else, it's a pretty cheap part.

Thanks for the info. Are you referring to the switch on the dash? It's a lot cheaper than a new alternator, so I might try replacing that first.

Oddly enough, noticed this same thing happening to someone else the other night. Didn't get a make or model because he was a 100 yards or so ahead of me, but I could see all of his lights flickering. Thought he had hazards on until I noticed the headlights were too.

The Wonderer said:

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Have the alternator tested.

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This.

Had an 01 Silverado that had a bad alt from the factory that did this.

Open the door and the dome lights would pulse.

Mike88Ag said:

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Thanks for the info. Are you referring to the switch on the dash? It's a lot cheaper than a new alternator, so I might try replacing that first.

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Yes.

I had it tested last week at Auto Zone and wasn't too impressed. They hooked their tester up to the battery and told me the battery was fine but the alternator was bad. All of this before I had even started the truck. <eye roll>

Do you know if the bad switch causes flickering all the time, or just when the engine is running? My lights only do this when the engine is running, which seems to point back to the alternator.

Mike88Ag said:

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Do you know if the bad switch causes flickering all the time, or just when the engine is running? My lights only do this when the engine is running, which seems to point back to the alternator.

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I would think so but I have no data to that effect.

Mike88Ag said:

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I had it tested last week at Auto Zone and wasn't too impressed. They hooked their tester up to the battery and told me the battery was fine but the alternator was bad. All of this before I had even started the truck. <eye roll>

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Then go to a different store...

I had this issue happen in a 95 Mustang 5.0 and it was a dead alternator.

How fast is the voltage fluctuating?

If it coincides with the different electrical loads, then yes the alternator looks to be ok.

However fluctuations like that with no load usually indicate the voltage regulator is going out.

You can check to see if this part alone is replaceable.

Not sure if this is applicable.

Quote:

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Subject: Information on Voltmeter Gauge Fluctuation Due to Regulated Voltage Control System and Concerns About Camper/Trailer Battery Charging #07-06-03-009 - (12/05/2007)


Models: 2005-2008 Cadillac Escalade Models

2005-2008 Chevrolet Avalanche, Silverado, Silverado Classic, Suburban, Tahoe

2005-2008 GMC Sierra, Sierra Classic, Yukon Models

Voltmeter Fluctuation Condition
Some customers may comment that the voltmeter is fluctuating between 12 and 14 volts on their full size pickup or utility vehicle. Starting with the 2005 model year, light duty full size pickups and utilities are equipped with a new Regulated Voltage Control (RVC) system. This system reduces the targeted output of the generator to 12.6-13.1 volts when in "Fuel Economy Mode" to improve fuel economy. The generator may exit "Fuel Economy Mode" if additional voltage is required. This will cause the voltmeter to fluctuate between 12 and 14 volts as opposed to non-regulated systems that usually maintain a more consistent reading of 14 volts. This fluctuation with the RVC system is normal system operation and NO repairs should be attempted.

Camper/Trailer Battery Charging Concerns
Some customers may comment that when towing or hauling a camper/trailer, the auxiliary battery for the camper/trailer will not stay charged. In most cases, this concern is blamed on the new RVC system. While the RVC system does reduce the generator's targeted output voltage to 12.6-13.1 volts when in "Fuel Economy Mode", this feature is bypassed if the tow/haul feature is enabled. With the tow/haul feature enabled, the RVC system will stay in "Charge Mode" and the targeted generator output voltage will be 13.9-15.5 volts, depending on the battery state of charge and the estimated battery temperature. To keep the generator in the "Charge Mode", use either of the following two methods.

The first method is to use the tow/haul mode when towing or hauling a camper or trailer.

The second method is to turn on the headlights, which will increase the generator's targeted output voltage to 13.9-14.5 volts.

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Based on this bulletin, it looks like your charging system is ok.

I would looks for a wiring system fault.

Check to see if any rodents have got to any wiring, check trailer wiring connections (assuming they are in same circuit), check to see if any relays have poor connection, and also fuses.

Further internet sleuthing on GM boards indicates the problem is inherent into the voltage regulation strategy programmed in the ECU.

Some folks indicate reducing the dimming by getting a slight overdrive pulley, and also increasing the gauge of wire to the grounding (or adding additional grounds).

Thanks for the info. I sprayed some wire dryer/contact cleaner into the trailer harness connection and the problem isn't as severe as it was before. I'll look into it more this weekend, and may check all of the bulb connections to ensure they're clean.