Regular readers of this eventfull forum will recall that 531 has long had an issue with the red gearbox light flickering on gearchanges. It has happened for so long, and with no other consequences that I had decided it was probably a faulty switch rather than an air issue and put the issue on the back burner, to change the switch next time I was under the bus.

Going to Kelvin Platts London bus day in Manchester changed this. The light has only ever lit after a couple of miles, and on change from 3rd to 4th. It lights for about 5 seconds. After the motorway run to Manchester, and onto the A roads and into the city centre, the light began illuminating on the downward change from 3rd to 2nd. Then on the way up from 2nd to 3rd. Both in Auto and manual. Then it came on when driving in 4th just as we approached the museum. I thought it may have had something to do with the stop start, up and down gearchanges in the city centre.

On leaving, the light quickly lit on most gearchanges in the city centre. On reaching the M56 it lit again, after no gearchanges. For the motorway run home it was lit more than it was out. After leaving the motorway close to home, changing from 3rd to 4th produced gear slip momentarily. Rather like dipping the clutch and revving a little in a proper manual while on the move. This confirmed the switch was not lying and there really was a low pressure issue.

So I made a test kit to test the air system as per the manual. With a line and gauge connected to the test point on the air tank, the following was observed.
1. The red light extinguishes on start up at 55psi. (correct).
2. the red light illuminates at 45psi. (correct)
3. The unloader valve cuts in at 105psi. (correct)
4. Pressure then drops to 88psi when the valve closes and builds pressure back up to 105psi. (correct)

This would suggest the compressor, air tank, and unloader are functioning correctly.

I was unable to get a fitting for the test point after the reducer valve and before the gearbox so made a T piece fitting into a length of flexible air line prior to the EP block. This was more interesting. The pressure, which I thought would be low here was actually too high at 80psi. (should be 60). This seemed to rule out the reducer valve needing to adjusted up to give the gearbox more wind. So i decided to take the bus for a drive on private roads with the test kit attatched prior to the gearbox to see what happened if I could get the red light to come on on gearchanges. With lots of gearchanges the pressure in the line between the reducer valve and gearbox began to drop from the 80 psi it was to more like 60-70psi. A hard acceleration up the gears and maintaining speed sees the pressure drop off to 50-60 psi. Eventually with everything hot the line pressure drops to 45psi on change to 4th, causing the light to come on.

My next course of action is to repeat the road test with the test gear connected to the air tank to see if pressure is dropping here, suggesting the compressor may not be blowing enough when hot, OR, the unloader may be unloading too much. But why only when hot, not all the time???

There are no obvious air leaks, tested eveything and every joint with soapy water. No sounds of hissing and bubbling from the gearbox in 1st or 2nd, unable to check 3rd and 4th due to the auto function. I am still suspicious of the unloader valve and reducer valve. Any ideas???

My bus number (if any): RM531

This is baffling. This area is not my speciality of experience, but If you've gone to these lengths then it's maybe interesting to know what happens next.

Unless someone in the know can tell from your descriptions what the problem is...
I would replace the unloader valve, trial that and record the data.
Then replace the reducer and record that too.
Then reverse the replaced parts and see if it is one or the other.

It's always prudent to keep a spare that at least functions even if not as good as it could.

It's very much trial and error but could solve the problem or eliminate the suspects.

A lot of this stuff used to be bench tested by some of the old hands I worked with, some of whom had made their own testing devices. Many who I knew during apprentice years were former toolmakers, they were very resourceful and kept their 'secret weapons' away from LT officialdom.

My bus number (if any): none at present

Todays roadtest, involving the test gauges fitted to the air tank test point for road test revealed that whatever I attempted, however quickly I attempted to go up and down the gears, high speed, low speed, manual and auto, some real harsh acceleration and deceleration which I would never normally do, the air pressure in the tank remained in the range of 88-105 psi. This would seem to indicate that compressor, tank, unloader valve and assciated pipework are all in order and functioning correctly. So if pressure at all times on the road is 88-105psi at the tank, but only 45-60 reaching the gearbox, this would all point to a sticking or faulty reducer valve OR some mega air leak into the gearbox when in motion. As the light illuminates on the change into second, and no bubbling can be heard with the engine off and second engaged, this would (hopefully) reduce the likelihood of this. So now I need an RML refurb style reducer valve, the one with the steel domed cover covering the adjuster on the top, with the switch incorporated into the bottom. It must be a part common to other vehicles. (Possibly Metrobus which also had a similar flag unit for air and gears??). I need one to trial to confirm or deny this is the cause before splashing out hard earned on a new one, bearing in mind this one has done less than 1000 miles since fitting....(The last one was well and truly knackered, and was leaking badly).

My bus number (if any): RM531

http://www.rmoof.co.uk/Gear%20box%20Air%20Pressures.htm

I did this pressure testing routine with a similar test rig last year because I had similar gear box pressure problems when the gearbox was warm. They turned out to be due to faults in a refurbished gearbox from Queensbridge which they fixed under the guarantee. The compressor, EP valve, pipework and unloader valve where all fine.

Ther are full details and pictures of the test rig on the RmOOF website, link above.

My bus number (if any): RM238, RM471 & RM2213

I have probably got one of those RML reducer valves that you are welcome to borrow, do you have a photo of it?

Rob

My bus number (if any): RM13, RM17, RM1797 and RT2291

Will have a go at putting photos on. Its a square, brass valve bolted to the centre of the crossmember in front of the gearbox. It has a conical steel top concealing the adjustment nut. Air feed in from the left, out to the gearbox on the right. secured to the crossmember with one through bolt. rectangular switch on the bottom with two wires coming out, leading to a junction box on the outrigger in front of the air tank.

My bus number (if any): RM531