I have a "gearbox fault" light appearing.
Garage have said its a voltage drop from Becm to solenoid.
Anybody had similar ??

What does the solenoid do ?.
Now it seems the fault light appears if first manuover is reversing and then swithcing to drive.!!!

 

Start by checking the battery and alternator for proper voltage, if the truck does not have the proper voltage it can throw a fault and make the electronic transmission shift funny.