My 10 year old MTD why does the new rear drive belt to the gearbox fall off when I take the mower out of drive?

My 10 year old MTD why does the new rear drive belt to the gearbox fall off when I take the mower out of drive?
There should be an idler wheel that is spring loaded near the middle of the belt. Make sure the belt is on th inside of that pulley so that the spring is pushing inwards on the loop to tighten the belt. If you have it on the outside, the belt falls off as soon as there is no tension from the mower being in gear.

Thank you Grasswhacker

The spring had fallen off and only when I saw your reply did I see the problem again thanks for your excellent and accurate assistance

robomower

By the way anyone with experience of robomowers? I just bought one and so far it is excellent, it mows between 1/2 and 3/4 acre and maintains it perfectly Im not sure which is the bigger attraction the lawn or the mower!