Various people have been musing over Teriann's question about an
overdrive vacuum switch.
In an earlier post I noted that there was no such thing on an MGB, but
big Healey's (and others) had them to hold the od on at closed throttle.
I was wrong. The Healey's had a mechanical switch in the throttle linkage.
But it was there to hold the od on, not to act as a "'kickdown" switch.
Anyway, the cars are in the trailer ready to head for Charlotte tomorrow,
and I'm home at 10pm rather than my all to normal 2am. The thinking is a
bit clearer, and I know exactly what you're refering to now.
Late MGB's (77->), as part of emissions, have an electricly controlled
valve in the vacuum line to the distributor advance. It allowed the
advance to work only under a certain condition. This condition was the
car be at "high" cruising speed. The easy way to define this is to say
the car must be in 4th gear. So, MG simply pigbacked this valve off of the
overdrive switch in the trans. Therefore, all 77 on MGB's had an overdrive
switch in the transmission to control this valve, whether they had an
od or not.
Randy
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