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.