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MG MGB Technical - Vacuum switch

Does anyone know the activating pressure of the NLA vacuum switch fitted with the d-type overdrive?
Paul Walbran

Paul - spotted anyway as I always just use the latest posts button...

No idea of a value, but since it is looking to break contact at idle manifold vacuum it should be around 10 to 15 inches of mercury.

Chris at Octarine Services

Found this, hope it helps---7+"

https://sites.google.com/site/cunhab52/mgbvacuumswitchforod
William Revit

Paul,
I saw it too and thought of posting to ask but you beat me to it and I thought it's not a bad idea to have the post in the midget section too (but in Technical) and ask in Triumph (Austin Healey?) places too.

Now I see Willy has answered.
Nigel Atkins

The vacuum switch is designed to prevent OD disengagement on a trailing throttle, and allow it when the throttle is opened. Idle has a vacuum level between those two.

If it doesn't disengage at idle - I can't see it doing that if it needs to drop to 7 in Hg which represents a significantly open throttle, then it seems to me that OD could still be engaged if you move the lever from the 3/4 plane and back again, if you haven't opened the throttle in the meantime. But then I've never driven one.

This results in an unpleasant jolt on the LH-type at least, and is why I made a lockout circuit. This keeps the OD locked out if I've changed down to second and gone back up without turning off the manual switch, which I found myself doing all too frequently, until the manual switch is turned off and back on again.
paulh4

Thanks all.
We have a d-type retrofit, and while not essential for the unit to operate I have always felt the vac switch to be a sensible precaution given the physically smaller clutch of the d-type.
With the switch being difficult to get I'm searching for alternatives.
Paul Walbran

Paul
Don't know if it helps or not but in your search for a switch, a lot of US vehicles, when lock up torque convertors first came out,mostly GM, they had a vac switch for the wiring for their lockup convertors and I can remember years ago there was an adjustable aftermarket one available that had a scale of 0-10 (inches I guess) and it was great for dialing in lockup timing

Here you go, this one does 6-22 and can be wired naturally shut or nat.open

https://www.ebay.com/itm/Adjustable-Vacuum-Switch-Kit-700R4-200-4R-350C-Trans-6-22-Superior-K058-99412/192359828359?

willy
William Revit

Thanks Willy
Paul Walbran

I adapted a vacuum switch similar to the one William recommends to cut the A/C compressor out on acceleration and it works perfectly. It should work fine for locking out the overdrive when a low vacuum condition is present. I usually dip the clutch slightly when switching in and out of overdrive and had not considered using a vacuum switch to avoid engagement under load.

I found the vacuum switch to be exquisitely adjustable.
Glenn Mallory

It's not engagement under load that the vacuum switch was for but to prevent disengagement under a trailing throttle which results in a high torque reversal. It was these torque reversals damaging the V8 OD that led to the removal of OD in 3rd gear on those cars. Which incidentally was nothing to do with why later LHD cars had OD removed from 3rd gear.

The vacuum switch just controls the solenoid so whether you somehow used a vacuum switch or the manual switch to engage OD the effect at the OD would be the same.

I've never declutched when engaging or disengaging OD on either my roadster (30 years and 65k) or the V8 (25 years and over 100k) and not had a problem. The divers handbook specifically says it isn't necessary, but if people choose to do it that's up to them.
paulh4

This thread was discussed between 17/06/2020 and 20/06/2020

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