Temperature gauge in the red on start up

Hi, I have just finished the rebuild of my series 3 with 2.2 diesel engine. When I started it up all was fine but once on the road the needle went into the red. It is not boiling up so I think it is electrical? I swapped the temp sender for another new one and it made no difference. Fuel gauge is ok. Could it be the voltage regulator on the back of the speedo? TIA as usual