Gear Box Air Pressures
I’m having problems with air leaking into the gearbox when selecting
first gear when the gearbox is warm. I’ve 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. I’ve 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 isn’t 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 but fell slowly as I repeatedly selected gears.
At 45 psi the red gearbox warning light came on, which is exactly right. So I
still don’t 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 driver’s cab to the left of
the seat. The manual suggests dry cleaning this (it shrinks if you wash it in
water!). I didn’t 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.