Rhumb Do Date:
Whilst flushing the engine cooling jacket
following an overheating problem, the engine revs suddenly shot up to
2000rpm and steadied there. Subsequent engine starts had the same
result of high revving idle speed.
My engine, a Mercedes OM636, is fitted with a
pneumatic governor which is attached to the end of the fuel injector
pump, so I thought the diaphragm inside the governor must be split,
damaged, or brittle through age. Fortunately, they are still
available from Westfield 4x4 in the UK, so I ordered one and it was
delivered to Labuan in about a week.
Access is the main problem for me, the engine
sits snugly in the bay with very little room to get down either side, so
it was a case of using mirrors, lights and doing most by 'feel' rather
than being able to see.
A cut-away view of the governor showing the
inside workings. The diaphragm is the copper coloured part (it's
actually leather and aluminium), the black part attaches to the fuel
rack control arm. Left of the diaphragm is the vacuum part and
right is the atmospheric part. The pipe into the top of the housing is
the vacuum pipe from the air inlet manifold.
Remove the vacuum pipe from the top of the
cover - banjo bolt and two copper crush washers.
Cut and remove the security wire.
Then remove the four machine screws, taking
care to hold the cover in place as it will be forced away from the
governor body by the spring inside. Inside is a spring and one
or two shims which you don't want to lose!
With the cover removed, you can see the
diaphragm. Carefully remove the outer aluminium ring from the
housing using a screwdriver on the inside edge. If you are
replacing the diaphragm, it doesn't really matter if you damage the
actual leather part but if you plan on re-using the same one, then you
have to be very careful.
Once the outer ring is out, you will be able
to pull the diaphragm outwards a small way to allow access to the
securing pin inside. On the cut-away photo, you can see the
horizontal pin of the control arm, secured by a split pin to the
diaphragm body. Remove the split pin and washer, and slide the
diaphragm towards the outside. That black part of the diaphragm body is
actually a hollow cylinder, inside is a piston with spring and shims, so
you have to put a finger over the end as you slide it off the control
arm, so the insides don't fly out!
Once it is off, it's just a case of putting
the new one on by reversing the process, then replacing the cover and
This photo shows my new one in position.
It's a fiddly job when you are hanging upside
down with very little room to get your hands or tools in. Much
easier of course if the engine were removed!