> one been messed with? Note that this unit shows some signs (charred
> wires) of having over- heated the wires from the switch on the gear
> shift. Hopefully, this is not a sign that someone botched a prior repair
> attempt on the O/D unit. Actually, if someone has seen the charred wire
> phenomena before, I would like to know if it means that there is some
> problem like a cooked solenoid coil.
I have no experience with the other internals, but I can answer the bit
about the solenoid based on my experience with the one on my Healey.
The solenoid has two windings, a heavy pull-in winding and a lighter
holding winding. The pull-in winding draws about 60 amps for a fraction
of a second, just long enough to move the operating lever into the
engaged position. Once the internal plunger reaches the end of its
travel, it opens a set of contacts which cut out the pull-in winding, leaving
the lighter winding to hold the operating lever up.
If anything prevents those contacts from opening, the pull-in winding
can draw enough amperage for a long enought time to fry the solenoid and
possibly the overdrive harness. On Healeys, where the circuit is unfused,
this seems to be quite common. I assume that MGs, being more "modern"
have a fuse in this circuit (my MG has no overdrive so I cannot check this).
In any event, the things which could cause the solenoid to fail in the
manner I have just described are:
1. Improper adjustment of the operating lever, such that the solenoid
plunger cannot move up far enough to hit the cut-out contacts for the
pull-in winding.
2. The contacts themselves are improperly adjusted so that they fail to
open when the plunger is pulled in.
3. The operating lever has so much resistance that the solenoid cannot
move it to the fully on position.
You should be able to check the solenoid by removing it from the overdrive
unit and applying power to it. If you put it in series with an ammeter,
you can check that the contacts are functioning properly. The ammeter
should show a high current flow for only a fraction of a second. Then
it should show only an amp or so.
Given the relative high cost of the solenoid vs the low cost of an in-line
fuse, I highly recommend adding a fuse the overdrive circuit if it is not
fused.
Peter Schauss
1963 BJ7
1980 MGB
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