The difficulty in diagnosis arises because the only gears that can be selected from stationary are 1st, 2nd and reverse. The other two gears are inhibited until the panel receives a speed signal from the transducer on the rear axle. This and the wiring to it have been the subject of a fair number of posts calling for gear loss help on various Fora over the years, however none of these have been recent.

It is possible to adjust all the gears but without the necessary go no-go gauges,a source of compressed air at the constant correct pressure and knowledge of the proper procedure it is possible to either create problems or cause damage. The garage electricians had dedicated test gear to carry out fault finding, without this things become a lot more difficult and time consuming.

Excellent check list for future reference thank you Michael.

My bus number (if any): RTL 960, RM 1585 and several RTs

Some people are asking about the by=pass plug used for getting round gear problems and test boxes. From what I understand the original LT plugs and test boxes are only suitable for the AEC or Leyland engine version, they are not suitable for the refurbs with later gear control panels. the feed arrangements and polarities differ.

My bus number (if any): RML2532

Thank you to everyone for their useful advise and help. I now have the problem of the lost top gear sorted.
On closer inspection of the gearbox area, I noted a disconnected pipe on the nearside of the gearbox. Just a 5 minute job to reconnect the pipe and tighten the nut. Everything working correctly now!

My bus number (if any): RML2672

Makes a change to have a simple fix! Are they metal or plastic pipes from the EP block to the gearbox?

My bus number (if any): RML2532

Metal pipes

My bus number (if any): RML2672