Temperature Gauge issues

[Dale P Hall]

I have a 1996 Gibson Sport House boat with twin Marine Power 350's. I got hit while tied up to the dock and I needed my swim platform replaced. My insurance company sent a captain to drive my boat three hours to a very respectable repair facility. While it was there I had them paint the bottom, install cable cutters, and change the oil in the engines and generators. None of this should have been a factor in my problem. When the captain brought the boat back he said the temp gauges weren't working. I have a fly bridge with duplicate gauges and all four gauges are reading below 120 and stay there. I have been troubleshooting this for three weeks. I have continuity on the ground and sending unit wires from the engine to the gauges. When I turn the key on I get 12 volts to the gauge. I have a good ground. I read resistance from the temp sensor and got around 1200 ohms. I started the engine and watched this resistance drop to around 850. This tells me the sensor is working but my gauges are still reading below 120 and stay there. I installed new temp sensors just for the hell of it with no change. I removed one of my heat sensors and put a heat gun to it and got it pretty hot. My gauges still read below 120. When I turn the key on you can see the gauges drive to the negative side as far down to cold they can go. I am completely lost here and would like any and all advise or troubleshooting techniques anybody can offer.