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Assuming that you have confirmed that the bleed off hole in the operating
valve is not blocked, something to try would be to remove the operating
valve plug (cautiously) while the unit is stuck in overdrive.
This would determine if the problem was being caused by residual oil
pressure or a mechanical fault.
If the overdrive does not slip whilst disengaged it's very unlikely that
there is anything wrong with the 8 clutch springs.
Incidentally I have rebuilt dozens of "A" type overdrive units and have
never encountered the need to replace those 8 springs.
M
On Sun., Aug. 13, 2023, 1:33 a.m. Bob Spidell, <bspidell@comcast.net> wrote=
:
> Two possible causes of this that I can think of:
>
> 1) O/D clutch is so worn it becomes jammed together (FWIW my BJ8's clutch
> was still serviceable at 205K miles)
> 2) something is preventing pressure on the operating pistons from bleedin=
g
> down; not sure how this would happen, hopefully someone will chime in
>
> bs
>
>
> On 8/12/2023 2:19 PM, David Wirken wrote:
>
> Now that we are on the OD topic, I have a Healey 100 whose OD
> won't disengage after I switch it off, it's not an electrical problem and
> I'm really cautious about not putting the transmission in reverse. The on=
ly
> way I can release it is to rock the car back and forth in neutral until i=
t
> pops out .
> It's probably time to rebuild, I presume.
>
> Any thoughts would be appreciated
>
> DW
>
>
>
> On Fri, Aug 11, 2023 at 11:11=E2=80=AFPM Bob Spidell <bspidell@comcast.ne=
t> wrote:
>
>> re: "... The health of the 8 springs that keep the unit in direct drive
>> is important to keep the clutch from slipping...."
>>
>> I believe the shop manual admonishes owners to replace these when
>> overhauling the unit, even if the old ones look and feel fine (some say
>> not so). I think my old ones are in a box on a shelf somewhere if
>> anybody wants them.
>>
>> Note the O/D pump is a positive displacement pump that will continue to
>> produce pressure--driven by the full power of the engine--unless there
>> is a pressure relief mechanism.
>>
>>
>> On 8/11/2023 10:34 AM, Hank Leach via Healeys wrote:
>> > Guys-this is a very lively topic and one of sometimes great mystery
>> > and confusion. The unit is complex but the end result is working well
>> > or not functioning at all.
>> >
>> > On the subject of pressure required to operate the shift into
>> > overdrive there are many schools of thought. Some people replace or
>> > stretch springs or add washers to increase the pressure needed. The
>> > health of the 8 springs that keep the unit in direct drive is
>> > important to keep the clutch from slipping. The pressure supplied to
>> > overcome the strength of these 8 spring , and effect overdrive, relies
>> > on adequate hydraulic pressure. The change from direct to overdrive
>> > needs to be smooth, not harsh, and if pressures are high the unit will
>> > snap into and out of OD rather than the smooth interchange we are used
>> > to while driving.
>> >
>> > When applied to the Austin Healey 100, the Laycock overdrive was
>> > initially a 32% reduction unit. It was soon found out in these early
>> > Warwick cars that 32% was too much reduction. DHMC swapped units
>> > three times in the original DMH tour car until they finally found a
>> > combination that produced the smooth shift and complimentary gearing
>> > desired.
>> >
>> > It is difficult, today, to find any information on this original 32%
>> > gearbox, however if you examine the booklet (attached) supplied by
>> > Laycock for the early cars, you will notice that the casting allowed
>> > for a large accumulator piston (seen on p6 in brochure). After some
>> > research and modified engineering, the company came up with this alloy
>> > "sleeve" called the piston housing (photo) which partially filled the
>> > chamber of the original large accumulator casting and allowed for a
>> > smaller accumulator piston that we see in all the later 28% reduction
>> > units. That size piston, in proper condition, produced from 350-450
>> > lbs of needed hydraulic pressure to overcome the springs influence.
>> >
>> > I'm confident that the remark made by Geoff Healey in his writings
>> > about the early transmissions "harsh shifting" revolved around this
>> > huge accumulator piston producing too much pressure. Additionally,
>> > there were three various accumulator springs supplied which adjusted
>> > the pressure. The early boxes produced about 350lbs and the later
>> > about 450.
>> >
>> > Today, Overdrive Spares supplies that sleeve housing with only one
>> > rubber O-ring as it is basically a filler part and not under extreme
>> > pressure. The accumulator piston within is under that pressure. They
>> > supply a modified piston using O-ring technology. So, it is not
>> > unusual to get good pressures over the normal 450 lbs required. If it
>> > works well don't fix it. The only concern is if the pressure does not
>> > release when overdrive is not called for. A clean unit, with proper
>> > 30W oil, will allow that to happen. All this some old theory and
>> > history to ponder. Hank
>> >
>>
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>>
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>>
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--00000000000023d2710602cbddb5
Content-Transfer-Encoding: quoted-printable
<div dir=3D"auto">Assuming that you have confirmed that the bleed off hole =
in the operating valve is not blocked, something to try would be to remove =
the operating valve plug (cautiously) while the unit is stuck in overdrive.=
=C2=A0<div dir=3D"auto">This would determine if the problem was being cause=
d by residual oil pressure or a mechanical fault.</div><div dir=3D"auto">If=
the overdrive does not slip whilst disengaged it's very unlikely that =
there is anything wrong with the 8 clutch springs.</div><div dir=3D"auto">I=
ncidentally I have rebuilt dozens of "A" type overdrive units and=
have never encountered the need to replace those 8 springs.</div><div dir=
=3D"auto"><br></div><div dir=3D"auto">M</div></div><br><div class=3D"gmail_=
quote"><div dir=3D"ltr" class=3D"gmail_attr">On Sun., Aug. 13, 2023, 1:33 a=
.m. Bob Spidell, <<a href=3D"mailto:bspidell@comcast.net";>bspidell@comca=
st.net</a>> wrote:<br></div><blockquote class=3D"gmail_quote" style=3D"m=
argin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
=20
=20
=20
<div>
Two possible causes of this that I can think of:<br>
<br>
1) O/D clutch is so worn it becomes jammed together (FWIW my BJ8's
clutch was still serviceable at 205K miles)<br>
2) something is preventing pressure on the operating pistons from
bleeding down; not sure how this would happen, hopefully someone
will chime in<br>
<br>
bs<br>
<br>
<br>
<div>On 8/12/2023 2:19 PM, David Wirken
wrote:<br>
</div>
<blockquote type=3D"cite">
=20
<div dir=3D"ltr">Now that we are on the OD topic, I have a Healey
100 whose OD won't=C2=A0disengage after I switch it off, it'=
;s not an
electrical problem and I'm really cautious about not putting th=
e
transmission in reverse. The only way I can release it is to
rock the car back and forth in neutral until it pops out .
<div>It's probably time to rebuild, I presume.</div>
<div><br>
</div>
<div>Any thoughts would=C2=A0be appreciated</div>
<div><br>
</div>
<div>DW</div>
<div><br>
<div><br>
</div>
</div>
</div>
<br>
<div class=3D"gmail_quote">
<div dir=3D"ltr" class=3D"gmail_attr">On Fri, Aug 11, 2023 at
11:11=E2=80=AFPM Bob Spidell <<a href=3D"mailto:bspidell@comca=
st.net" target=3D"_blank" rel=3D"noreferrer">bspidell@comcast.net</a>>
wrote:<br>
</div>
<blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex=
;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,20=
4,204);padding-left:1ex">re:
"... The health of the 8 springs that keep the unit in direc=
t
drive <br>
is important to keep the clutch from slipping...."<br>
<br>
I believe the shop manual admonishes owners to replace these
when <br>
overhauling the unit, even if the old ones look and feel fine
(some say <br>
not so). I think my old ones are in a box on a shelf somewhere
if <br>
anybody wants them.<br>
<br>
Note the O/D pump is a positive displacement pump that will
continue to <br>
produce pressure--driven by the full power of the
engine--unless there <br>
is a pressure relief mechanism.<br>
<br>
<br>
On 8/11/2023 10:34 AM, Hank Leach via Healeys wrote:<br>
> Guys-this is a very lively topic and one of sometimes
great mystery <br>
> and confusion.=C2=A0 The unit is complex but the end result =
is
working well <br>
> or not functioning at all.<br>
><br>
> On the subject of pressure required to operate the shift
into <br>
> overdrive there are many schools of thought. Some people
replace or <br>
> stretch springs or add washers to increase the pressure
needed. The <br>
> health of the 8 springs that keep the unit in direct
drive is <br>
> important to keep the clutch from slipping.=C2=A0 The pressu=
re
supplied to <br>
> overcome the strength of these 8 spring , and effect
overdrive, relies <br>
> on adequate hydraulic pressure. The change from direct to
overdrive <br>
> needs to be smooth, not harsh, and if pressures are high
the unit will <br>
> snap into and out of OD rather than the smooth
interchange we are used <br>
> to while driving.<br>
><br>
> When applied to the Austin Healey 100, the Laycock
overdrive was <br>
> initially a 32% reduction unit. It was soon found out in
these early <br>
> Warwick cars that 32% was too much reduction.=C2=A0 DHMC
swapped units <br>
> three times in the original DMH tour car until they
finally found a <br>
> combination that produced the smooth shift and
complimentary gearing <br>
> desired.<br>
><br>
> It is difficult, today, to find any information on this
original 32% <br>
> gearbox, however if you examine the booklet (attached)
supplied by <br>
> Laycock for the early cars, you will notice that the
casting allowed <br>
> for a large accumulator piston (seen on p6 in brochure).
After some <br>
> research and modified engineering, the company came up
with this alloy <br>
> "sleeve" called the piston housing (photo) which
partially filled the <br>
> chamber of the original large accumulator casting and
allowed for a <br>
> smaller accumulator piston that we see in all the later
28% reduction <br>
> units. That size piston, in proper condition, produced
from 350-450 <br>
> lbs of needed hydraulic pressure to overcome the springs
influence.<br>
><br>
> I'm confident that the remark made by Geoff Healey in hi=
s
writings <br>
> about the early transmissions "harsh shifting" rev=
olved
around this <br>
> huge accumulator piston producing too much pressure.
Additionally, <br>
> there were three various accumulator springs supplied
which adjusted <br>
> the pressure. The early boxes produced about 350lbs and
the later <br>
> about 450.<br>
><br>
> Today, Overdrive Spares supplies that sleeve housing with
only one <br>
> rubber O-ring as it is basically a filler part and not
under extreme <br>
> pressure. The accumulator piston within is under that
pressure. They <br>
> supply a modified piston using O-ring technology. So, it
is not <br>
> unusual to get good pressures over the normal 450 lbs
required. If it <br>
> works well don't fix it.=C2=A0 The only concern is if th=
e
pressure does not <br>
> release when overdrive is not called for. A clean unit,
with proper <br>
> 30W oil, will allow that to happen. All this some old
theory and <br>
> history to ponder. Hank<br>
><br>
<br>
_______________________________________________<br>
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