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Re: Vacuum OD Switches

To: mgs@autox.team.net
Subject: Re: Vacuum OD Switches
From: jello@dns.ida.net (Phil Bates)
Date: Sat, 2 Nov 1996 21:13:49 -0700
>Were Vacuum Overdrive Switches a success on the early MGBs?
>
>I can't find it as an option in ADCs Original MGB Book..
>
>If you don't know what a Vacuum OD Switch is, I have writen this.  Lucas
>Part #34508 fits 63-67 MGBs with the D-type Overdrive Transmission.  What
>it does is disingage OD for short periods of time without flipping OD off. 
>It would be used to speed up to pass a car, I assume.
>
>The Vacuum switch itself cost about 115 in NOS form.
>
>All help would be appreciated, 
>
>Kai Radicke -- mowogmg@dynanet.com
>1966 MGB -- http://www.dynanet.com/~mowogmg
>
What kind of help do you want??  Did they work?? Yes, I have one on my '67 
MGB.  They were mounted on the left side firewall.  The way they work, 
though is not exactly what you describe.  If you have a vacuum switch hooked 
up, your overdrive will not kick-out of overdrive if the manifold vacuum is 
above about 15 lb/in^2.  This means that to turn overdrive on, all you do is 
flip the switch to on, but to turn it off, you have to turn the switch off, 
and sometime when you accelerate hard enough (and it doesn't take much at 
all to go below 15 lb/in^2) it will kick out.  If you do not have the vacuum 
switch, then the overdrive will always turn off with the toggle.

I have never seen one turn the overdrive off for a brief period of time 
while the switch is on, but if hooked up to port vacuum instead of manifold 
vacuum (manifold is proper) you might get that effect, but I doubt it.  Due 
to the circuit, you have to turn the toggle switch off to turn the overdrive 
off.  The vacuum switch is auxiliary to this.

Phil Bates
58 MGA
67 MGB
75 Jaguar XJ12C
52 MG TD replicar (VW)


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