Check the fuses and this might be worth a try to.....the braided ground cable going to the stud on the firewall in the engine bay just to the left of the brake booster is the ground for the dash gauges check it and clean it at both ends.

 

I never figured it out.

 

X-2 ! ! ! yes check this out.. i was having the same problem at WOT and i was getting codes also at WOT due 2 this. happen that my MAin fuse box located on the firewall was half melted and cable was loose...

 

the cable looks fine i think its the cheap advance auto parts alt. not made for the high rpm. AS far as the gauges go the volt gauge went out a year ago the temp gauge just went a few weeks ago i think its the gauge motors truck has 140.000 on it.

 

I went through the same exact thing awhile after this thread ran, a brand-new, not rebuilt alternator solved the problem. It has been said that the springs on the brushes on cheap rebuilds are week, allowing the brushes to bounce at high rpm. Even after I told the rebuilder that this fixed the problem, he said he never heard of it, and insisted there was nothing wrong with the unit he sold me, because it tests very good on his machine. Buy a new one, they just can't test them at high enough rpm to uncover this problem.

 

Truck is gone now so it's no longer an issue for me. The alts I had were Delco remans, didn't buy any cheap ones or anything.

 

I have also run across several reports exactly like this mostly with the 145 amp alt for some reason. Don't buy a rebuild. Besides the new OEM take off alt I got was almost half the cost of a rebuild at the major parts stores and it actually works at all rpm ranges.