Temp gauge problems

[franke08]

Sorry for the bad english, Im from Germany. I have a problem with my 2006 H2. Since i have built in a WEBASTO engine-and inside-heating system, wich is also connected with the fan for inside heating, the temp. gauge is not working correctly. Every time i have used the heating system the temp gauge only works for 5 to 10 minutes and then it goes back to zero,even the Webasto heater is already off.. If i dont use the Webasto heater everything is normal. After two days without the Webasto heater, the gauge is working good. If i use it again, then the gauge is not working after 10 minutes, even the Heater is shut down befor I start the engine. When I erase the failor code in the bordcomputer, just in this second the temp gauge is working normaly. Everything is good when I dont use the Webasto Heater. But this Heater ist powered by fuel and only conected with 12 V power and the fan for inside heating by using a relay. When the Webasto system is on, then the wire wich comes from the climatecomputer to the fan is hanging in the air, there is no connection between any module and the Webasto system. When the Heater is off, then the relay connects the climatecomputer with the fan and than is there no connection with the Webasto heater. GM Germany dont know why it is so, and they can not help me.
Does anybody have an idea?
Thanks for reading and for your help