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MG MGB Technical - Overdrive Vacuum Switch - location & wiring route?

Hi everyone,

Just some quick questions regarding the overdrive on my Mk1 67B roadster.

I have dived into 'the box of parts' I have which need to be put back onto my B. In there is the Overdrive Vacuum Switch. My questions are about this...

Firstly, the part in question is on the Moss parts list as: Part 141-520 (part 56 in the online illustration). The link to it is:

Now down to my questions...

1. Where do you mount it in the B? (beside the heater box?)

2. What is the wiring code colour to both electrical terminals on it?

3. It looks like a pipe/tubing is to be connected to the end of the unit. Where does this connect to on the car and what route does it take?

Its been a while since this part was on the car (in excess of 4 years), and I did not take note of its connection when it was removed from my B (by the company that did the bodywork). So my memory is not too sharp on this... plus a lot of this stuff on the B is new to me. Im a mechanical novice still learning a lot as I go. So bear with me if this is a basic question.

Thanks very much
Safety Fast!


Mark Duggan

Mark - Send me your e-mail address, I have some pictures and information on the circuit that I can send you. Cheers - Dave
David DuBois

Hi Mark

Just got done installing a OD in my 67 GT. Vacuum switch attaches to the carburetor side of the heater box. There are two threaded holes for it close to the shelf. There are also two holes above this. This is where the relay goes. Relay is #55 in the Moss page you listed.

There is a wiring harness for the relay and switch. It is the easest way to do the wiring correctly.

You need to, if not already there, add a nipple to the intake manifold. The hose from the vacuum switch goes there. There is a bolt on the back top of the intake manifold where the nipple goes to. Nipple is no longer available but are not hard to build.

I have a good article on tearing down and putting the vacuum switch back together, including how to set the switch pressure. If you send me your e-mail I can send you a copy.
cunha.bruce at charter dot net

BEC Cunha

The vacuum switch has a yellow wire one side and a yellow/red the other. Both go to the relay, the yellow to the relay W1 terminal (together with one from the manual switch) and the yellow/red to the C1 terminal (together with one from the gearbox switch). The relay C2 terminal has a white from the fusebox and the W2 terminal has a ground.
Paul Hunt 2

You've wriiten an article on tearing down and putting the vacuum switch back together, including how to set the switch pressure? I'd love to read it. Could you send along a copy of it to me? I tried using your Email address that you posted above, but it came back.
Steve S.

Steve S.,
FYI - Bruce took some great pictures at the time of this discussion. The switch is really quite simple in design and you shouldn't have any difficulty in taking apart, cleaning and putting back together.
Posted 19 August 2006 at 00:17:17 UK time
Steve Buchina, Alabama, USA,

Received my OD switch today. Immediately took it to the work bench to check its function. No continuity between the terminals, with or without vacuum (connected 60cc syringe to vacuum port). Had more than enough back suction, so figured diaphragm had to be intact. Had some concern, but grabbed a screwdriver and did a little twisting on the one screw exposed. It's the one at 12 o'clock in the ebay picture (linked above). My concern was whether this screw was an adjustment for vacuum (switching) sensitivity or did it just hold down the cap. It's only for holding down the cap.

With the cap remove, you'll discover an extremely simple switching mechanism. There's a black plastic disk (9mm thick) to which the two electrical connectors are attached (180 degrees apart). Between these two connectors runs an inset copper bridge - attached to one terminal (rivet) and having potential to make contact with the other terminal through bent tension in the bridge. This switching disk is held in place with four screws - easily removed.

With the switching disk removed, you will find a rubber diaphragm that is apparently glued in place. It looks much like what you see when you remove the bottom part of a SU fuel pump. In the middle of this rubber diaphragm protrudes a metal (collared) nipple, 7mm in length. If you push on this nipple with your finger, you will see that its height above the metal base (about 10mm) is maintained by a spring underneath the diaphragm. This metal nipple passes through a hole in the center of the black switching disk. Around this hole is a recess that accommodated the nipple's collar.

With the switching disk unmounted, the copper bridge provides continuity between the two terminals. Mine didn't initially because of oxidation, but a cleaning fixed that problem. So how does this switch work? The switching disk is 9mm thick, but the copper bridge is recessed and lies slightly less than 7mm from the bottom of the disk. So - when mounted, the metal diaphragm nipple passes up through the disk, pushing up on the copper bridge, and breaking contact between the two terminals. However, if under vacuum, the metal nipple is withdrawn from inside the switching disk allowing the copper bridge to fall and making contact between the two terminals.

No Vacuum: Switch On
With Vacuum: Switch Off

David - Did I get this right? If so, I'm still confused. Guess my question is - when one floors the pedal, is the switch/OD on or off? Read what I received from someone else concerning this switch and its function.

Steve - Hi and thanks for your inquiry. the vacuum switch on the MGB is not required for the use and installation of an overdrive in the car. The vacuum switch was a extra setup (it was not on all the cars with overdrive, just some years) that allowed the overdrive to be disengaged without throwing the switch. It was used to shift out the overdrive when the throttle was depressed hard and reengage it when the throttle was backed off again. In essence this made the shift like an automatic transmission passing gear for more power when needed on the highway. Austin healeys had a similar switch, but it was mechanically actuated by the throttle likage. Most of these switches no longer work and they don't make them any more because there is no demand and they are too complicated to make for a reasonable price. Let me know if you need any more information. Thanks again! John Esposito

Posted 19 August 2006 at 00:25:27 UK time
Steve Buchina, Alabama, USA,



No Vacuum: Switch Off
With Vacuum: Switch On

Think I just answered my own question - my mind is getting old.

Posted 19 August 2006 at 02:05:25 UK time
Dan Robinson, San Francisco

If you please, where is this unit mounted and what wire does it interrupt?

How does theory apply to O/D on 3rd/4th but later cars only in 4th? (It seems that it was to avoid sudden torque changes.)

Just curious.

Posted 19 August 2006 at 03:12:49 UK time
MWhitt, Tennessee

Dan, no expert on this, but it is the set up on my 67 MGB conv. Without switching out of overdrive and depressing the gas pedal, the vac switch deenergizes the OD solenoid which "kicks it out of od". Much the same way the mechanical switch mounted on the firewall works on the Austin Healeys, except there is a mechanical link to the throttle shaft (on the AH)that deenergizes the solenoid when the switch reaches a certain point and the switch becomes open circuit.

Either way this switch is mounted on the drivers side of the firewall of the LHD MGB's. There are holes in the firewall next to the heater box near the throttle cable exit from the pedal on the vertical wall.

Later MGB's around 77 or 78 (79 for sure) OD only operated on fourth gear, no provision was offered after 1967 for an "automatic" deenergizing of the OD solenoid. It became a fully mechanical move of the hand switch.


Posted 19 August 2006 at 19:03:06 UK time
Steve Postins, Oxfordshire, United Kingdom,

All of this discussion has prompted me to remove the now unused switch from my Roadster. If you'll excuse the shameless plug:

Posted 19 August 2006 at 20:55:02 UK time
Steve Buchina, Alabama, USA,

To those who might have an interest in taking their switch apart.


The switching nipple does not pass through or is it (apparently) fixed to the rubber diaphragm. After a little cleaning, the one on my switch just fell off. Its shape can be described as a fat, rounded thumbtack.

On the bottom side of the switch housing, centered under the diaphragm, is a plugged tube. The plugging material reminds me of hardened plumber's putty. This stuff can be easily removed (i.e., scratched out using a small jeweler's screwdriver) to expose a brass adjustment screw. Backing off on this screw (counterclockwise turns) lowers the vacuum threshold required to close the switch. Additionally, removing the sealant had no apparent effect on the integrity of the unit (i.e., no air leaks under vacuum). I backed mine off two turns and with adjust for proper function once installed.

Except for perhaps a smashed housing, I see nothing in this switching unit that couldn't be easily refurbished or repaired. Even the rubber diaphragm should be easy to fabricate and replace.

Posted 19 August 2006 at 20:57:48 UK time
Bruce Cunha, Wisconsin, USA


Thanks for that information. I may need to clean the one I purchased.

By chance did you take any pictures?

Posted 20 August 2006 at 03:07:28 UK time
Bruce Cunha, Wisconsin, USA


I tested mine tonight.It worked but the vacuum needed to activate it was just above 40 mm (17 inches)of mercury. I am going to clean it as I noted the contacts are a bit green.

Posted 20 August 2006 at 05:33:17 UK time
Steve Buchina, Alabama, USA,

I was going to try taking some pictures, but still haven't figured out all the fancy buttons on my camera. Close-ups are really blurred.

By now, however, you've seen how simple this switch is. For cleaning the black contact disk, I just dropped the whole thing in some diluted glacial acetic acid (about 25%). Undiluted vinegar (5-18% acetic acid) works as well - it's just a bit slower. Thing about acetic acid is it seems to attack the oxides leaving the base metals relatively untouched. You can leave things in for days without causing any noticeable damage.

"It worked but the vacuum needed to activate it was just above 40 mm (17 inches) of mercury."

If you can, would like to know what effect each turn of adjustment screw has on switching vacuum.

Posted 21 August 2006 at 00:43:40 UK time
Bruce Cunha, Wisconsin, USA


Glad I read your e-mail prior to taking mine apart. I will start with turing the adjusting screw. I have a brake bleeder vacuum hand unit so I can check the in/out point rather easy.

Thanks for the glacial acetic acid tip. I can get that at work.

I will also take some pictures.

Your camera should have a setting that looks like a Flower (tulip?) That is the Macro setting. In that mode, it should allow you to take nice close up pictures.

Posted 21 August 2006 at 04:21:26 UK time
Bruce Cunha, Wisconsin, USA


For each full turn of the adjustment switch, the pressure changes by approximately 1.5 in of mercury.

I used bathroom caulk to fill the screw hole.
Steve Buchina

I posted pictures of the switch at

I have done this up as an article, but I am not sure how to reduce the size of it to send by email easily. The picures use a lot of space.

BEC Cunha

Mark , I have some pics of my 67 MK I car with its factory fit o/d. I sent them to BEC(I think it was u I did this 4 BEC) with some circuit info but did not see it in his article. If you want to send me an e-mail stanandjanebest(at) I will reply with the pics I took for BEC
Stan Best

A point from the above, the OD was never 'automatically de-energising' (apart from when deselecting an OD gear which was always the case). The vacuum switch and relay keep the OD *engaged* under conditions of high inlet manifold vacuum i.e. the overrun even when the driver has turned the manual switch off. This prevented high reverse torque which could damage the D-type unit. Selecting a non-OD gear would turn it off instantly, but since this usually meant the clutch had also been operated there was no reverse torque to worry about. The later LH-type is stronger and did not need this feature.
Paul Hunt 2

Hi guys,

Thanks for all the great info. Is very much appreciated.

If anyone wishes to send me additional photos etc (which will be gratefully received), my email address is:

duggans (at) clear (dot) net (dot) nz

And thanks to Dave who already has sent some... awesome!
Bruce... tried to send you an email to your email address with no luck. Can you send the info to my email address? Thanks

Mark Duggan

Hi there everyone,

Firstly, I would like to thank all who have sent me photos and data on the correct setup.
Makes sense to me now (more or less). However, I am bamboozled over the setup my B has.

I had a dive into the big box of photos at home and found one of my B just after I purchased it (about 18 years ago).

Anyway... I have scanned the photo and placed it on webshots... Check out this link:

Sorry about the quality of the photo... A very bad photo. It was taken in a time before digital cameras etc. In it I can definately see the lack of a pipe to the manifold.

My assumption is that the vacuum switch (labelled in yellow) is essentially redundant as it was not connected to the manifold.
Also... the metal tubing which usually connects the Vacuum Unit to the carb has been replaced with a plastic hose (labelled in cyan). So this doesn't have the 'junction' point in the metal hose it should have.

So this is where I am confused... Can any of you knowledgeable bods enlighten me as to:

1. Any thoughts as to why it is not connected to the manifold (yet the overdrive still worked)?
2. What is the 'junction point' on the vacuum pipe for and is it really necessary as my B has a flexi hose instead?

My plan is to return it to its stock condition... unless it is unnecessary. Your input would be most appreciated.

Mark Duggan

The vacuum pipe and switch are not used to make the OD work. They prevent excessive stress on the unit if it is manually switched off under conditions of high manifold vacuum i.e. the overrun at high rpms by *keeping the OD engaged* until the vacuum reduces.

If not connected it is not 'redundant' (which means 'not needed'), just 'not connected' which is a different thing. You could drive it for many years and tens of thousands of miles and never have a problem with it being disconnected if you drive your car very gently, but if you drive it with more gusto (which is how they were intended to be driven) problems could develop over time.

Presumably the T-junction in the original metal vacuum pipe from carb to distributor *was* for the vacuum switch! To restore the vacuum switch to working condition and function it must have a vacuum supply from the inlet manifold. Depending on how far you intend to go to return it to stock you could either fit a Tee in the current plastic pipe, or replace it with the original metal pipe with Tee.
Paul Hunt 2

Mine picks up directly on the manifold, I e-mailed Mark and Paul a pic.
Stan Best

If the vacuum connection on a MKI model would be at the carburetor, and not at the intake manifold, what would be the difference if it was taken directly from the intake manifold?
John Barber

True, true. I was going by the reference to 'junction point' made by Mark and forgetting the distributor vacuum came off the carb. Maybe by 'junction point' he meant the bulge in the pipe, which is a fuel/vapour separator?

OD vacuum *has* to be from the inlet manifold and not the carb otherwise it wouldn't do what it is supposed to i.e. keep OD engaged on the overrun, as there is no vacuum in the carb/distributor pipe when the throttle is closed.

If, John, you mean connecting *distributor* vacuum to the inlet manifold you would end up with what North American cars had from Aug 71 and UK from Sep 76 i.e. maximum vacuum advance at idle, which basically just reduces emissions a little at idle.
Paul Hunt 2


Can't see it in your picture but there are two bolts in the manifold. One to the front and one to the back of the PCV valve. Both of these go directly into the intake manifold. Vacuum for the switch was provided by a port that screws into the bolt hole towards the firewall on the manifold.

The port is NA through all the US providers. The bolt is a standard thread in the US and I just bought a longer bolt, cut the head off and drilled it. I then screwed a nut onto the threads and threaded it into that hole. The nut can then be tightened to secure the port.

Part 14 on MOSS

BEC Cunha

Thanks for the info and a way around 'bolt connection' on the inlet manifold. I have contacted the parts supplier I normally use to see if he can supply me with one (new or second hand). If that doesnt pan out, I will use your suggestion.

Thanks for the info also. Yes, the 'junction' point I was referring to is the 'fuel/vapour separator'. I just didnt know the name/purpose of it. Thanks for that.

I'm awaiting some parts to arrive (hopefully this week). When they do, I will tackle this.

Mark Duggan

This thread was discussed between 16/02/2007 and 21/02/2007

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