Gear Box Air Pressures


Iím having problems with air leaking into the gearbox when selecting first gear when the gearbox is warm. Iíve changed the piston seals but the problem remains. Someone suggested that I check the air operating pressures. There are two test points, one on the end of the air reservoir, the other pictured here.



In the next picture you can see the narrow thread. Iíve not been able to find a suitable connecter to allow the attachment of a pressure gauge.



So I did a bit of lateral thinking and decided to connect a gauge to the lower flexible hose on the offside of the gearbox compressor to measure the output from the compressor and the upper hose to measure the output from the cylinder, via the reducing valve. These are the components I needed, total cost £30.00 inc VAT. The glass of the gauge isnít broken. It is the glycerine which stops the needle sticking. (If you have a less accurate and less durable plastic gauge you could reduce the total cost to about £15.00)



This is what it looks like made up.



I stopped the engine and depressurised the air system by repeatedly selecting gears and then gently opening then closing the drain plug in the bottom of the air reservoir. I connected up my test rig as in this picture and started the engine. The hose from the T piece to the gauge is long enough for me to view it in the cab. The pressure slowly built up and the red light went out at 60 psi. At 116 psi the unloader valve opened and the pressure in the compressor output fell to zero. This is exactly right.


Then I drained the air out again, refitted the bottom hose and refitted the test rig in place of the top hose to measure the output to the EP valve. The pressure soon built up to a constant 60 psi, which is exactly right. When I stopped the engine the pressure stayed at sixty butfell slowly as I repeatedly selected gears. At 45 psi the red gearbox warning light came on, which is exactly right. So I still donít know why I have air leaks into the gearbox when it is warm.


In the next picture you can see the unloader valve on the left and the reducing valve on the right. You can also see the air reservoir drain plug in the distance. I did the regular maintenance suggested in the New Routemaster Manual anway.


I also had a look at the air filter in the driverís cab to the left of the seat. The manual suggests dry cleaning this (it shrinks if you wash it in water!). I didnít feel brave enough to take it to the dry cleaners! You can get new ones from Filtration Control Limited.



This is what it looks like when you remove the filter.



Please email me with any comments, corrections or additions to this article.


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