Hi, i took my commer for a service this morning. While i was driving in the rain i started to think. What would happen if i reversed with overdrive still on? Would it make a terrible noise? I have put a sticker on the dash to remind me.
It's a very bad thing - overdrive kaputt[xx(]
Like Baz says there should be an inhibitor switch, if there isn't it's easy to fit one. I wouldn't rely on a sticker - a switch is essential.
The switch is screwed into the side of the gearbox, near the top on the drivers side. If there isn't a switch there will be a blanking plug in it's place, simply unscrew it - screw in the switch and attach the wire that normally goes to the solenoid to one terminal - then run a new wire from the other terminal to the solenoid. This will prevent overdrive being engaged in 1st, 2nd and reverse.
Here's a pic of the switch in position, the one on the right is the inhibitor, the one on the left is for a reversing light connection - they are identical.