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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
bleeding 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 only way I can release it is to rock the car back and forth in
> neutral until it pops out .
> It's probably time to rebuild, I presume.
>
> Any thoughts would be appreciated
>
> DW
>
>
>
> On Fri, Aug 11, 2023 at 11:11â?¯PM Bob Spidell <bspidell@comcast.net> 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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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 class="moz-cite-prefix">On 8/12/2023 2:19 PM, David Wirken
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CALnfbV8X_WZjDAYXZWw5Ldrxv92Ue-hUo9VDCYyihrvtgtAFJA@mail.gmail.com">
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<div dir="ltr">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 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 be appreciated</div>
<div><br>
</div>
<div>DW</div>
<div><br>
<div><br>
</div>
</div>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Fri, Aug 11, 2023 at
11:11â?¯PM Bob Spidell <<a href="mailto:bspidell@comcast.net";
moz-do-not-send="true"
class="moz-txt-link-freetext">bspidell@comcast.net</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex">re:
"... The health of the 8 springs that keep the unit in direct
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. 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. The pressure
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. 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 his
writings <br>
> about the early transmissions "harsh shifting" revolved
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. The only concern is if the
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>
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