The throttle switch is included in the OD circuit to prevent the overdrive
from being disengaged whilst the car is in and "overrun" condition where the
car is driving the engine.
The reason is that the "A" type overdrive unit is not designed to withstand
reverse torque and the torque required to accelerate the engine rotating
mass if you change from OD to normal with a closed throttle at high speed
will damage the OD unit.
Later units, used in other vehicles, were redesigned to eliminate this
problem.
Michael Salter
www.precisionsportscar.com
-----Original Message-----
From: owner-healeys@Autox.Team.Net [mailto:owner-healeys@Autox.Team.Net] On
Behalf Of Alan Schultz
Sent: Saturday, November 06, 2004 4:17 PM
To: Bob Spidell
Cc: healeys@Autox.Team.Net
Subject: Re: Overdrive wierdness - I'm not really sure
Bob,
Thanks for the lesson in technique.Hope when I blip the throttle at the
stop light some hot rodder isn't considering it a challenge. I see
little reason to keep the throttle switch. If I plan to pass someone I
could just flip the switch to normal. Seems its the only reason for the
switch?
Bought my 53 BJ8 from a guy living in Madison, WI who had taken it
completely apart years ago. It was a "basketcase" or should I say "boxes
and bags case".
Bob Spidell wrote:
> This should probably be re-subjected to "O/D Technique," I suppose.
>
> Anyway, my O/D tecnhique is a follows:
>
> a) on "upshift" (from straight 3rd or 4th to 3rd-O/D or 4th-O/D) I just
> flip the fascia switch at or near WOT. The result is very similar
> to an automatic transmission upshifting
>
> b) for "downshift" (from 3rd-O/D or 4th-O/D to straight 3rd or 4th) I
> push the clutch in a little (inch or two), rev the engine a little
> and throw the
> switch. If done just right, this results in an almost
> imperceptible downshift;
> the only effect is an increase in engine speed.
>
> I have 140K+ original miles on my O/D, and I've gotten 80K miles--and
> counting--on
> my latest clutch so I think this technique is fairly friendly to the
> machinery.
>
> Alan: yes, that's normal for an O/D. The throttle switch will keep
> the O/D
> engaged until the throttle is opened something like 3/16" measured at the
> stops (probably around 1,500 - 2,000 RPM). BTW, you didn't get your
> "53BJ8" off of eBay by any chance? ;)
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