From your description of the operating fault Mr Pell it seems the problem occurred all of a sudden with no advance warning of any impending fault.In my opinion I believe the fault is a electrical one. The cause could be that the EP unit is not receiving a electrical signal to energise the 4th speed solenoid.


Possible causes are for a Wilson GB32 gearbox:-

a) Broken wire connection at the EP terminal block cover therefore the solenoid is not being energised

b) Dirty connection between 4th gear terminal post and the spring loaded solenoid plunger at the EP unit cover plate

c) 4th speed solenoid valve jammed or seized in the closed position mechanically

d) Internal wiring fault at the 30pin plug control panel or the loom wiring to the EP unit

c) Sudden air loss when 4th speed is selected, but red warning light would show

If top speed adjustment was pending you would have experienced a slipping speed

My bus number (if any): Ex LTE Bus Engineering

Check where the 7 core cable comes out of the conduit near the gearbox. Our bus lost all gears a few years back. that cable had worn thrugh due to exposure to oil from the compressor and heat. Otherwise maybe a dry joint somewhere in the control board.

My bus number (if any): RML2532

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