temp gauge not working???????

[coldgin]

I have a 69 gmc 3/4 ton that I have retrofitted tbi injection on the stock 350...out of the blue temp gauge stops working and is pegged to the cold side....I grounded the wire going to the temp sending unit and the gauge pegged to the hot so I thought install new sending unit would fix my problem but the gauge is still pegged to the cold side...only thing I am wandering is the heater hose return hose goes to the pump instead of the radiator like on the late model I got my tbi from...I also changed thermostat thinking it might be stuck open and not letting the engine warm up...any ideas and I very open to suggestions...