Martin:
I don't have a MGB book which covers the 1977 models but I have one
which covers up to 1975. Also I don't have a MGB, but I have an MGA
which is very similar in wiring to the MGB, so FWIW and please bear
with me.
I believe the problem is that the "park switch" is damaged. To better
understand how the system works I will go through the early MGB motor
first.
On page N.15 of the manual I have is illustration N.13 which shows the
internals of the wiper motor for an early MGB. Under the domed cover
is a wiper contact (item 6) which slides over a copper segment inside
the cover. The wiper motor opperates and parks as follows.
1) The wiper operates when one of two grounds wired in parallel is
closed: One ground is the switch on the dash and the other is the
wiper switch shown by item 6 in the diagram.
2) When the dash switch is shut off, the motor continues to operate as
the sliding switch is making contact with the copper segment in side
the cover.
3) When the sliding switch contact moves off the copper segment, both
grounds are now open and the motor stops.
On page N.32 of my manual is a two speed wiper motor for a 1975 MGB.
At the bottom of the gear box is a device called the limit switch
assembly. Most likely this switch senses the position of the large
driving gear and would open and close contacts inside the motor just
like the wiper type of the early motor. My guess is that a stud on
the switch which may ride in a groove on the bottom or over a hump on
the large gear is broken, bent, or out of place.
Blake
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