Moss Motors, Ltd

The early style MGB overdrive has a vacuum switch which protects the drive drain when disengaging the overdrive. The system uses carburetor (Throttle) vacuum to close a vacuum switch located on the hot side of the driver side firewall. It really doesn't take much vauum to close the switch.  Since the Supercharger  is pulling air through the carburetor all the time, it seems that this vacuum switch will not be able to open.

Does anyone have experience with this set-up? Can the vacuum level on the switch be calibrated to a higher level?  If so, how much?

 

I suppose I could eliminate the switch and install a manual one, but my "better angels" tell me that is probably not the right way to go.

 

Dittybag

"Last days of the install"

Hello Dittybag.

You are right. The pre-'68 cars had a vacuum control switch in them. The switch's job was to keep the O.D. engaged if the driver decided to manually disengage the O.D. while decelerating. This was done using manifold (vacuum from the throttle plate downstream) vacuum. This vacuum is highest during deceleration. When the driver stepped on the gas again, manifold vacuum would fall off and the vacuum switch would open. That would allow the driver's command for the O.D. to disengage to be implemented.

You said it takes very little vacuum to operate the switch. It may be a little tired. I don’t have the specs for what the switch originally called for to close, but if it was calling on manifold vacuum under deceleration to operate, it would have been pretty high.

Can it be calibrated? Not that I’ve heard of.

Even with a supercharger, when you decelerate, manifold vacuum will be high. The presence of the supercharger will have no impact on vacuum on deceleration. If the switch is still operating as it was built to it will continue to work as it did.

Moss Tech 

 

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A properly running overdrive unit is supposed to dis-engage, even if the vacuum switch is "reporting manifold vacumm" whem the two shafts are running at the same speed. BUt on the Supercarged car, It way make sense to eliminate this switch fvrom the circuit, and depend on the smart driver to only disengae when you are "off" the throttle.