I have a 2005 Chevy Silverado that for some reason has started showing signs of an electrical problem I cant figure out. The battery gauge goes up to 16v get a red bat light. But my lights tend to dim while driving at night,almost to the point of going out.I have had both the battery and alternator checked twice at 2 separate locations and they show fine,no problems.It has no trouble starting up or running.

The voltage for the truck is controlled by the ECU. Older vehicles used a replaceable voltage regulator to control that. This hints at an ECU problem.

Is your check engine light on?

The battery and the alternator could each test out OK and the problem lie in connections between them, and grounds. If not something like that, then ECU is the top suspect.

The check engine light comes on every now and then.Only when the battery voltage goes up.The meter on the dash only climbs to about 16v and stops.it doesnt keep going.I used a volt meter have and everything checks out. I checked the wiring and plugs and they are clean.

I tried to reset the ecu but nothing changed. Engine is charging normal.Battery is fine but the service electrical and batt light came on the dash. Could it be a bad sensor somewhere?

So, you verified that the battery charging voltage is actually 16V when this happens?

Ok I checked using a volt meter at idle on Batt I get roughly 13 to 14 volts. Look at the dash gauge it says over 15 with bat light and service electrical on. One of them is wrong. Second problem, when driving at night all lights dash, radio and headlights tend to dim for a second while I drive,not going out but flicker dim. no warning.3rd problem.fuel economy is tanking. Sensor says im getting 15.4mpg but I know better. I have to put gas in every other day

Sounds like an instrument cluster problem and not a charging problem. I trust your voltmeter over the one in the dash all day long. The other issues lead me to believe the cluster is lying to you.

There is a ground connection right under the driver’s left foot on a bracket that goes to a body mount on the frame. There may be a few black wires grounded to 1 or 2 screws there. Crawl under and take a look at how corroded they are. Remove the screws, clean the terminals and install nice shiny new sheet metal screws to hold the wires. I think it may solve your problems

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Just to add to that, there could also be a wiring issue where the cable connects to the main circuitboard of the instrument panel. I’ve seen situations where years of vibration have broken soldered connections between the female connector and the board. But either way, a bad ground somewhere in the instrument system could explain a great deal.

OK I cleaned the ground wires. results TBD until I get it on the road. If this is the fix.Thank you

Overcharge problem hopefully solved. I cleaned the grounds and reset the ECU. everything looked normal after a 40+mile drive.Now I have the problem of flickering lights. Dash and headlights .

The battery ground is attached to the chassis but the alternator ground is attached to the engine. Good idea w/this symptom to make sure you have a solid low resistance connection between the engine and the chassis. You should see one or more wires from the engine/transmission to the chassis. Remove the clean and tighten those connections at both ends of the wire. Use an ohm meter to make sure the wire hasn’t broken inside the insulation too.