>My OD unit is a new addition to my 3 synchro box. I have had the unit in for
>about 2 months with no vacuum switch. In fact, I didn't know it was a part of
>this unit until reading about it on this list.
>
>I engage my OD with the toggle switch in 4th without using the clutch. I wait
>until about 2700+ RPM prior to engaging. The unit engages and the rpm's drop
>immediately, much as Will describes " like an automatic" . I read somewhere
>that this unit won't engage under a certain RPM? Anyway,I'm timid about my
>unit and could definitely use some advice.
>When disengaging, I'm told to flip the switch and depress the clutch. I
>agree, it takes a very short period of time to disengage this OD and run in a
>"normal" 4th gear. I'm not sure that this procedure is correct; however it
>seems to be working OK.
>
>My questions are :
>
>1. What does this vacuum switch do?
>2. Where can you find one of these units?
>3. I'm told that declutching after disengaging releases pressure on the OD?
>4. I've found no difference in 3rd gear OD as in normal 3rd gear. Possible
>reasons?
>
>I agree with Will, there is nothing like cruising at 70mph and 3000 rpm...
>
>Bruce Durgin
>'64B ( 3 synchro, 3 main, + earth , blah , blah...)
An MGB with overdrive is something great!! I've had mine for 4 years, 3 of
which with overdrive and a vacuum switch.
Question 1: What does the vacuum switch do?
1. Most of the time, nothing.
2. OK, enough with the snide comment, really it makes it so that your
overdrive can only be turned off when you are accelerating. This means that
if you have your foot off the gas, and you turn off the switch, your
overdrive stays on. When you step on the gas (having recently turned off
the switch), overdrive turns off. If you are driving normally, foot on the
gas, not accelerating hard, and you switch off the toggle, overdrive will
probably turn off, but it will depend on where the switch is set, and how
how much gas you are giving the car. (I inserted a manifold vacuum gauge
just to watch this stuff happen!)
3. More specifically, as long as there is manifold vacuum (not port
vacuum), the vacuum switch holds a connection that the toggle switch had
previously. This does not do anything while the toggle is off, because of
the relay system. When you stop drawing a manifold vacuum i.e. accelerate
and draw port side vacuum, the contact is released, an it lets the overdrive
kick out. NOTE TO WILL - Do you have the fitting for your manifold, this is
also hard to find.
4. Bruce, you have done this correctly by leaving the two connectors
un-attached, had you attached them, you could not turn off your overdrive.
If you get the wrong vacuum switch (one for a Healey?) it will work in
reverse, and only disengage the overdrive on decelleration (high manifold
vacuum, low port side vacuum). Been there, done that.
Question 2: Where can one find a vacuum switch?
For selfish reasons, I really don't want to tell the whole list. I know a
supplier with 7 of them. It took me a year and a half to get any switch and
it was the wrong one. Then, as luck would have it the same place got some
of the right switches (a BL supplier buyout), and exchanged mine. These are
rare, and hard to get. I want to keep my supply, so I'll post to Bruce
later. Most people don't want or need these anyhow.
Question 3: Declutching after releasing the overdrive releases pressure on
the overdrive.
I don't really know about this one, but it is logical. My former mechanic
told me to depress the clutch when either engaging or disengaging the
overdrive. However, from my MGB Driver's Handbook (a B.M.C. Publication), I
quote:
"The overdrive can be engaged at any speed within the third or
fourth gear range at any throttle opening between light and full throttle.
DO NOT depress the clutch pedal to select 'overdrive` or 'normal`.
To prevent overspeeding the engine DO NOT select 'normal` top from
'overdrive` top at speeds in excess of the direct drive maximum or change
from 'overdrive` third to 'normal` third at speeds in excess of 'normal`
third speed maximum.
If increased acceleration is required when in 'overdrive` top, the
overdrive can be switched out to give normal top gear."
When I showed this to my former mechanic, he said "job security." (Note I
moved, that is why he is a former, no issues with his work). When you look
at how this unit works, there is stress on the gears, and the clutch may
help reduce it, I really am not sure.
Question 4: No difference in 3rd overdrive and normal 3rd
These should be different! Maybe your 3-4 lockout switch isn't operating
properly in 3rd. 3rd with overdrive should be similar to 4th without
overdrive though, so, you probably are not missing much.
Phil Bates
67 MGB
75 Jaguar XJ12C
52 MG TD replicar (VW)
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