L322 diesel gearbox volts

Hi all just wanted to know what the operating volts should be at my transmission,the car has 13.6volt running but only 11.9 at the gearbox,I know it should be the same volts all round but I just wanted to check, because the transmission is doing funny things,also could a bad shift solenoid cause this problem.cheers

have you checked that the earth cables are ok, they are known to cause faults the main engine one is by the passenger engine mount and notorious for corrosion, and the battery terminals not tightening up on the post properly, start with the cheap easy stuff first
and is the battery 100% tip top, as them`s motors are very power hungry.