Truck Trend Garage: 2001 GMC Sierra Voltage Issue
Question: I own a 2001 GMC Sierra. It has 49,700 miles and I have noticed a charging system problem over the past 12 months. My volt gauge seems fine on startup, but while coming to a stop at a sign or light, my dash and headlights dim and the gauge drops to 11 volts. After fully stopping, the gauge jumps back up and the system acts fine until the next stop or deceleration. Master techs from my local GM dealership tell me it's a normal occurrence, but now the gauge is dropping lower, and now the idle has been affected in addition to the lights dimming. I had an Exide orbital battery installed when this first started, so I replaced it with an Optima Redtop. This didn't help. Do you have any suggestions?
Answer: It seems that almost every manufacturer has a technical service bulletin describing "volts dropping" and "lights dimming" at a stop to be a normal characteristic of the charging system. And the majority of the time this is true. In your situation, we know the battery is good after two replacements, but you also want to be sure the technicians have accurately tested the charging system, and there are no poor battery or alternator connections (high resistance) giving you grief. After that, and before marking it off as normal, be sure a low-idle speed condition is not reducing engine rpm enough to give you the extra-dim lights and low volts. It's possible there's excessive carbon buildup on the back of the throttle plate, inside the throttle body assembly. This is a common situation that can cause a stall and/or idle speed dipping down extra-low, before the Idle Air Control valve can compensate.
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