Volt and Temp Gauges Fluctuating and Reading High

[dmarty]

My 2012 5.3 Sierra with 195k miles started showing temp and volts greatly fluctuating high while driving at highway speeds the other day when outside temp was -15F. No codes or IP warning light. Tried replacing battery charge sensor, hall-effect unit on neg cable with new GM part, alternator is one year old GM unit but tried switching with known good unit, replaced coolant temp sensor with new GM unit, thermostat is one year old but replaced it as well. Cleaned cable grounds in engine bay. Problem still present. Coolant gauge is wrong, as point and shoot laser gun shows normal engine temp (180-195F). Battery volts measure with meter at 14.7-15.8v at idle so battery volt gauge in IP is correct. If I disconnect sensor wire at alternator battery volts drop to 13.7v and IP warning says to check charge system. High coolant reading causes both rad fans to run so pulled fuse but batt volts still measure high. Coolant is GM orange and one year old. Hydrometer says good to -35F. Now that outside temps have increased to +40F coolant reading is closer to normal but volts are still high at 14.7v. Battery is 2.5 years old so haven't tried replacing it, don't think it's a likely cause, but may try it. I've tried everything I can think of and am starting to think the body control module is failing and causing alternator to overcharge battery which is causing an incorrect temp reading. Any suggestions before I take it to a pro with proper diagnostic equipment? Thanks!

[1970cstblazer]

Battery cables in these trucks are a common failure. Even if they look good on the outside, they are corroded under the insulation.

[Accelo]

Originally Posted by 1970cstblazer

Battery cables in these trucks are a common failure. Even if they look good on the outside, they are corroded under the insulation.

You can check this with a volt meter using a voltage drop test to measure the resistance of the battery cable.
You connect the volt meter to both ends of the same cable. One end of the volt meter lead goes to the cable at the battery, and the other to the starter. Then, crank the motor. (You may have to pull the coil as it will take about 5 seconds) All readings should be less than one volt. Repeat this on the ground cable for the battery and the block.

[dmarty]

1970 cstblazer, you may be right. Crawled under truck today and poked at the battery cable terminal at starter and found lots of corrosion not only under the crimped terminal but also in and around the copper strands half inch from crimp. Ordered a new GM original today from Rock. I'll let you know if it fixed the problem after I get it installed. I would think that it would only be an issue when starting, but it definitely needs to be replaced so no big deal if it doesn't fix the main problem. Acello, tried to measure volts at that end, but couldn't get in too well and got very low reading which I don't think is accurate as it cranks/starts fine. Volts at power center and alternator are good.
Thanks!