Low Volts

I have a 93 SN eng. runs great and the alt. puts out 14.1 volts. but at the dash after the pefect pass kicks on I only read 12 volts and this drops as I cut more things on. If I blow the horn it will drop below 10. volts and cut the perfect pass off. Also, when i cut my nav. lights on the temp gauge will jump up form 160 to 180. The other guages will also move. I have spent hours with a volt meter trying to find the problem, shacking wires etc in the dash pod. The only thing I have found is that when I have the clip on my volt meter on the neg. bat. post and touch the other side to a neg on the dash pod grounds it shows a .5volt reading. Shouldn't ground always be O. ? I have cleaned all the grounds on the motor and the bat. terms. Again motor is fine and i know the perfect pass takes about 1. volt. ?????

Correct Craft daisy chained the ground wiring from gauge to gauge using crimp connectons. As the crimps corrode, you get the problems you described. You can fix this by running a separate ground wire to each gauge. You can also try pulling out the old ground wire and soldering the connectors.