G20 Diesel starter does not turn despite NEW starter and battery

Hello together unfortunately I am as a loan with my latain at the end howl………. My g20 6,2 diesel no longer jumps on despite new starter and battery. Since in the last time I had extreme start problems at the end he jumped only with bridging thought of I ok battery defective. so new battery installed, nevertheless after the preglow sang the voltage at the start up to 3-4 volts. after that nine more lasser destructed. wiring to possibly short circuit measured all caliber ok. NE UE BATTERY NEW INLASSER new mass cable for the starter with bridge from a running motor the same problem. start pre-glow voltage about 13-14 volts after the pre-glührelais clicks and I activate the ignition also breaks the voltage again up to 3-5 volts. has someone had this error also already already ? has the pre-glührelais or glowing candles something to do with the starter or starter ? does not know any more and get the car logically now also in no workshop u nd am really more than desperate. hope can possibly help with a further or give good tips. many thanks to you in advance ………