What do you mean by 'battery gauge' ? A volt meter? Amp meter? If you have a volt gauge, it should not be lower than the battery's voltage unless you have some big short/load somewhere. The engine itself needs basicly nothing. Could be a real big load and as the alt. spins up, more power is output to fullfill it, and voltage goes up, and that if the load is high enough to keep voltage below 13, you battery will not get charged.
I would get an external battery volt meter, and see what is going on. Check the battery - should be 12.6+, and when the vehicle is running, should be 14 or so. Check to see what the voltage is when the engine is off but key in run position. The glowplugs should lower it but when they are off it should be up to 12 atleast. If not, you have a load somewhere, or your wiring is incorrect.
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