Message text written by "J.H. Roulleaux"
>I have no experience with overdrives yet ( I just fitted a J type on
my 6 and only drove around 300 miles) but I heard from people
stopping with 2nd gear engaged in overdrive and than reversing,not
good for an overdrive.For that reason I did not mount the 2nd gear
switch .Understanding that the overdrive pump is operated by the
propshaft I don't see how it could disengage with the propshaft not
rotating,but can understand that it will disengage as soon as you
back up and the pump is resuming pressure. Is that soon enough
to avoid damage. Big question.
Jean
<
Jean,
The J-type overdrive is a bit different than the A-type and some of the
potential hazards associated with the A-type are not present in the J-type.
The A-type has an accumulator which holds a volume of fluid under pressure
for instant activation of the overdrive. The J-type does not and only
engages as fast as the pump can produce fluid volume. Consequently the
J-type takes longer to engage. And the slower the speed is the longer it
takes. When moving off from a stop the O/D can take quite long, indeed.
It is for this reason mostly that the J-type is not normally fitted with an
interlock switch for second gear. This ensure that the road speed is
sufficient (third gear engagement) when it is activated.
The A type overdrive has an accumulator that will allow activation of the
overdrive even at rest (unless the pressure has leaked down but as others
have noted here recently the accumulator can maintain pressure overnight
quite well).
In either case, the interlock switches are intended to prevent overdrive
engagement in gears where it is not desired regardless of the position of
the selector switch. So in either case selecting reverse with the
overdrive switch in the ON position will not endanger the overdrive.
Unless someone has bypassed the interlock switches.
As an experiment I engaged the overdrive on my TR6 (J-type) in third gear
at about 700 RPM (10 MPH approx) and it engaged in about 50 feet. It is
conceivable that if overdrive was engaged in reverse it would start to
engage before you reach the end of the driveway. The A-type could engage
immediately, however, and cause problems much more quickly.
Either way, the moral of the storey is: Don't bypass the interlock
switches! Please!
Felicitations.
Dave Massey
71 TR6 (with J-type) (with a 73 frame)
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