From my experience you must to find where is problem with low voltage because charging of 40v is bed.If is battery 48v I thinks it is you need about 48-50v on +pole of capacitors than you will not have error 51 and you will get -minus pole from electronic card for main contactor.I dont know tcm and this solution but i will try to look in documents to se did i have wiring for this unit.Try to solve low voltage first,that is my proposition.And good luck of course!
One more thing starting up electronic goes like this.You have 48v on main switch where goes two wires one on power part one on control,control check for errors and if everything is ok gives signal to contactor to close and second + wire goes to power part and charge capacitors.Most electronic units works like this.I think that you understand me.