Message text written by "Wages, Jeffrey"
>This is my understanding of the OD solenoid since I have the later J-Type
The later J-type solenoids have an internal plunger type mechanism while
the
older A-types use an external lever, kinda like the lever on the clutch
slave.
<
Correct but not complete. Since the J-type has no accumulator the solenoid
does not have to overcome an existing head pressure, it merely has to close
off the bypass valve and requires much less force to actuate and a simple
single coil solenoid is sufficient.
The A-type has a "direct operating" valve in that the solenoid has to move
the valve plunger which is resisted by the pressure developed by the
hydraulic pump and held in the accumulator. This requires a serious amount
of force and the current required for this is quite large (on the order of
10 amps or so). This level of current will cause enough heating to burn up
the coil. However, due to the characteristics of a typical solenoid (which
has it's greatest strength at full stroke) the heavy current is not needed
once it is fully engaged. The solenoid used in the A-type has a set of
conteacts that disconnect the low resistance coil and leaves energized a
smaller "holding-coil" that draws only about 1 amp.
One common problem with these solenoids is corrosion on the contacts which
will not allow current to flow through the "pull-in" coil. When this
happens the solenoid can not generate enough force to operate the valve and
the overdrive does not engage. If you lift the solenoid manually it will
stay engaged until you disengage it electrically. A little work with a
points file can give you temporary relief. Another problem is caused by
mis-adjusting the solenoid such that the "pull-in" coil is not de-energized
will cause the coil to overheat and let out the magic smoke. Once this
happens it is toast.
Some have advocated a 2 amp slowblow fuse in the solenoid circuit. A
slowblow fuse will carry the 10 amps required to pul in the solenoid for
the brief period that it will be active and if the adjustment is off (or if
a foriegn object blocks operation the 10 amps will cause the fuse to blow
before the solenoid is damaged. It can also save the wiring. Sounds like
a win-win to me.
Cheers
Dave
57 TR3 (A-type)
71 TR6 (J-type)
80 TR8 (Rover 5-speed)
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