68 Barracuda: temp gauge to max with key on

I stopped by a friends house yesterday to see his progress on his 68 Barracuda. It's really starting to take shape! He was having a problem though. When he put the instrument cluster in, he got all the gauges and lights to work fine. Now when he turns the key on the temp gauge goes to hot and stays there. It worked before but now it doesn't. All other gauges work fine. Any suggestions?
Those clusters are such a ***** to remove he might just put a manual gauge in. Thanks, Mike