Gauges not working (voltage and temp)

[JACOB1557]

My voltage and temp gauges don't work in my truck. Sometimes they move when I turn the key to the on position but when the truck turns on, the temp slowly goes down to 0 if it's hot and if cold it does nothing. The voltage gauge worked awhile back registering 12v. But just went off all of a sudden. What could I look for as the problem? 

[wreckdiver230]

http://www.southernspeedometer.com/ , try contacting this guy , he  fixes them , hemight be able to send you in right direction

[95k150050]

Temp gauge could be the sending unit and voltage could be alternator or whatever control it