does the voltmeter cycle up and down a short while on cold startup?
if yes then it is most likely working , a more direct test would be placeing a inductive
ammameter around the load wire for the grid during warm up, it will draw quite a few amps, if nothing then it is open wich is possible but your truck would really smoke like alot and hard start in the cold for quite a while

good luck
scott

 

Im also leaning twards grid heater as well .... if the grid heater is good then check the grid heater relays ... may only be getting half the power ... I can start my truck and it was 24 out side and fired right up ....
It shouldnt smoke all that bad ... that much unburnt fuel is not real good on rings either ... can start washing cylinders and contaminating oil with the fuel....

 

One question i haven't heard yet is what rpm is your truck ideling at when the motor is cold.

 

I agree check the grid heater.

If you do need to change it out consider going with a 24v style grid heater and a tunnel ram intake plenum for better flow (think cheaper version of the banks twin ram )