temp gauge prob and glow plugs

hi i have just put a 260hp om605 merc engine in a disco 2 taking out the td5.the ecu is still in and have most thinks are working,but the temp is not i have the td5 sensor on the wiring and when the ignition is turned on the temp gauge goes up then down and the red overheat light comes on .also think this might be why the glow plugs have no power to them as the ecu thinks its hot ,maybe .any help on this will be good

Make continuity test on the temp sensor wiring to not be interrupted cos it seems that the ECU doesnt get the signal from it, or swap it with a known good one to rule this out first cos that's how it does when it's open circuit on the sensor input to ECU, the sensor if hardwired to pins 7 -18 of the red plug(wire colours the same), you have attached the pin disposals, unplug the sensor and bridge the pins in the plug then you should get short circuit across the two pins in the plug, if you get that measure against earth cos if it's a short to earth on the wiring it will do the same.