I asked a while back about the cause of vibration experienced when the
overdrive on my BN1 was engaged and though there was no definitive
answer to my question I decided to engage in a little forensic
expedition to see if I could correct the problem.
Once I had removed the transmission, detached and dismantled the OD the
problem(s) became quite apparent.
One item was the excessive wear of the thrust surface of the annulus
and the way it undercut the bronze thrust washer (behind the one way
clutch) and mixed the filings into the lubricating oil.
I replaced the annulus with a spare and installed a new thrust washer.
The other problem was that the sun gear had about .090" of end float as
opposed to a spec of .008"-.014" . I replaced the steel spacer washer
and brought the end float within spec.
I also took the opportunity to change the annulus bearings and replace
the output seal with a double lip seal.
When I installed the transmission and took the first test drive I was
in heaven.
The car is as smooth as a babies bum up to the speed where I chicken
out. (not as fast as it used to be, but still a bit above the statutory
limit.)
Then as the transmission oil warmed up I was overcome by a black cloud
of frustration. The OD began dropping out when I tried to accelerate.
After several runs starting with a cold car and allowing the fluids to
warm up the problem always recurred. The OD had always been a little
slow to engage,
but had always before responded to its controls even if sluggishly.
Now, however, it refused to remain in OD when I stepped down on the
throttle.
I know some listers have experienced this problem and asked about it
here, but again I don't think I ever saw the problem solved.
First I went through the electrical controls and determined that
everything was working correctly (or as correctly as they ever do on 50
year old Healey).
Then, suspecting a problem in the hydraulic circuit I started by
removing, checking and resetting the operating valve and the check
valves. Then I checked
the movement of the operating valve and the setting of the solenoid and
operating lever. Everything seemed to be in spec.
At that point I asked Dave Porter for his advice and he pointed me at
the O-rings on the accumulator housing (Moss part number 866-690,
number 76 on the exploded drawing of the BN1 overdrive).
These rubber rings are original to the unit and are thus over 50 years
old. They are probably one item that no one replaces in a rebuild, even
if they know they are there.
If they have deteriorated they will not hold pressure in the
accumulator and the OD will operate erratically.
Dave told me the secret for removing the housing in-situ (Hold the
piston in the housing with a dowel or some other solid item while you
apply air pressure to the operating valve port
and pop the housing out of the case.) and he sold me an O-ring. I went
home and after a couple hours of messing around managed to remove the
housing and replace the O-ring.
The result, once again, was encouraging, but I was still to be
disappointed because although the OD engaged much more positively, it
still dropped out on a long run (30 miles yesterday)
when the lube had a chance to soak up a lot of heat and the viscosity
dropped.
So last night I removed the housing again and arranged with Dave to
pick up another O-ring after work. Tonight I replaced the other O-ring.
Dave says he knew some cars used two, but forgot which ones.
After replacing the remaining O-ring I drove a ten mile loop during
which the OD performed flawlessly and engaged much more quickly and
with a much more positive feel. For insurance I also exchanged the
original wimpy pressure spring (Moss 866-670, NLA) with a later item
(Moss 866-680, which is still available). Together I think these
changes will correct my problem.
For those with overdrives which drop out under acceleration, after you
have tried everything else check out the o-rings on your accumulator
housing.
Here's to the road...
Bill Lawrence
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