Question: I have a 2004 Chevy Suburban with the 5.3-liter. When stopped, the volt gauge drops down to about 11 volts and the lights dim. I replaced the battery and the alternator, but neither fixed the problem. The engine runs fine and as soon as I start moving, the voltage goes all the way up to 14. What else could it be?
Answer: GM has a technical service bulletin (TSB #02-06-03-008D) that covers all models dating back to 1990. It simply states that low volts/dim lights at idle may occur normally under certain conditions. The most common factor is a good battery being at a low state of charge due to infrequent use of the truck, or extended idling (low engine rpm). An alternator puts out its lowest amount of amperage (current flow) at idle, because it's spinning so slowly. At idle, the engine and alternator are at their highest operating temperatures. The heat also normally hampers alternator output. Another biggie is the load on the charging system. What do you have turned on? Some high loads come from the headlights, blower motor, rear defogger, heated seats, and so on. Problems to look out for include poor electrical connections anywhere in the system. This includes the battery cables, wiring to the fuse box, circuits to the alternator, and engine-to-chassis grounds. Also, be sure you’ve installed the correct capacity battery. If the specification for your truck is 550 CCA (Cold Cranking Amps), use 550 or better. Last but not least are aftermarket accessories. When a charging system is designed, the engineers determine the size of the alternator (amperage rating) to deal with the needs of the specific vehicle. For example, one model from the factory leaves the assembly line with a 95-amp alternator installed. Meanwhile, the same exact model with the navigation option gets a 105-amp power source. This illustrates how much load aftermarket add-ons can add.
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