Lotus Elan

Headlamp vacuum circuit

PostPost by: SADLOTUS » Thu Oct 07, 2010 2:41 pm

I was reading another thread on here about introducing yourself, and Mr Gary was bemoaning the fact that we were going over the same ol'stuff and was thinking about starting a thread re the headlamp vacuum circuit but just couldn't be motivated.

I think one of the best things about this site is the huge resource of knowledge held by its contributers.

Bearing this in mind, I would hate for people to be too disillusioned and would therefore like to start a headlamp vacuum circuit thread.
Here's my understanding of how the system works, drawn for my own reference a long time ago: more pictures please, can the one way valve be down by the crossmember? I used a Volkswagon brake servo valve for years, what does the correct one look like? My pods were VERY holey on the top half but sealed ok on the bottom half, the rods were bronze(?) braizing rods that will never rust and on the original chassis the crossmember was far too rusted through, so was merely bypassed.

OOh the fun innovative days of running the car on a pittance (student) buget?

Probably to a few people's horror, I've now 'gone electric'.
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PostPost by: summerinmaine » Thu Oct 07, 2010 3:58 pm

In both of my S2s, the one-way valve was integral with the "T" junction.
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PostPost by: Allison » Thu Oct 07, 2010 6:19 pm

Just to be different (?) in our two S3s the one way valve was right next to the manifold - well to be accurate they both are (headlights and brake servo).
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PostPost by: frearther » Thu Oct 07, 2010 7:47 pm

Mine was at the intake manifold when I bought the car from the OO (original owner) (via an intermediary, which is another story altogether). It's an S2 roadster. late '65 manufacture.

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PostPost by: RedS4 » Thu Oct 07, 2010 8:52 pm

My OOV's are near the inlet manifold and are HUGE :shock: - about 14cm long and 5cm across. They can be dismantled, cleaned and serviced. They polish up a treat too! 8)
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PostPost by: billwill » Thu Oct 07, 2010 9:38 pm

Re your diagram.

The pull 'switch' must vent to atmosphere when in the closed position. to let the pods down, yet it must also block the pipe to the reservoir in this condition, so as not to let air into the reservoir because it is shared by the brake servo.


I've not taken one apart to see exactly how the plunger works.
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PostPost by: bill308 » Thu Oct 07, 2010 10:35 pm

A one way valve can be inserted in-line anywhere between the manifold and T-fitting. If rigidly mounted to the intake manifold it would be subject to more heat and a lot more vibration. I would rather mount it close to the T-fitting for durability. Anybody find an inexpensive one way valve for in-line fitting?

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PostPost by: oldelanman » Fri Oct 08, 2010 6:45 am

billwill wrote:The pull 'switch' must vent to atmosphere when in the closed position. to let the pods down, yet it must also block the pipe to the reservoir in this condition, so as not to let air into the reservoir because it is shared by the brake servo.


Sorry to disagree with you Bill but I don't think that's right. I think the servo should be connected directly to the inlet manifold with it's own non-return valve. If the two systems were connected together a leak in the headlamp circuit would compromise the servo function.
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PostPost by: paddy » Fri Oct 08, 2010 7:00 am

billwill wrote:I've not taken one apart to see exactly how the plunger works.


There's a good write up by Anna here:

elan-f15/vacuum-switch-t13457.html

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PostPost by: elansprint71 » Fri Oct 08, 2010 7:08 am

Great thread. :wink:
As well as the theories on the vac system I'd be interested in looking at the various way that folks have "gone electric", i.e. which car the motors came from, how the linkages and switch-gear look, etc.
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PostPost by: prezoom » Fri Oct 08, 2010 3:47 pm

It was Anna's write up that encouraged me to take a close look at my vacuum switch. Upon disassembly, I found the bottom surface where the two tubes enter the switch to be less that flat. This prevented a good seal with the 45 year old rubber block. I draw filed that area and then beefed up the flat spring tension with a piece of rubber cut from a large "O" ring. A little rubber grease, and the pods will now stay up for almost 5 days after shutting off the engine. Good enough for me.

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PostPost by: billwill » Fri Oct 08, 2010 4:47 pm

oldelanman wrote:
billwill wrote:The pull 'switch' must vent to atmosphere when in the closed position. to let the pods down, yet it must also block the pipe to the reservoir in this condition, so as not to let air into the reservoir because it is shared by the brake servo.


Sorry to disagree with you Bill but I don't think that's right. I think the servo should be connected directly to the inlet manifold with it's own non-return valve. If the two systems were connected together a leak in the headlamp circuit would compromise the servo function.



I long ago disconnected my servo, so I have forgotten where its vacuum inlet was connected to. My faint recollection is that it shared the reservoir.

The REASON why I disconnected the servo was because you get no braking assistance if the engine is not running so if as I did on two occasions you brake hard you can stall the engine and yet still have forward motion (yeah I know that seems crazy). Your foot reflexes will not then 'believe' the amount of foot pressure you need on the brake pedal (minus the assistance) and you crash into the stopped vehicle in front of you, that was the reason for your abrupt braking. I decided to just use unassisted braking and let my reflexes remember the harder pressure.

The loss of braking like that would be exactly what would happen if the servo was fed straight from the manifold, so perhaps you are right.

OK.. Someone with a functioning brake servo on an ELAN not a +2 tell us where it gets its vacuum feed from. I know the +2 takes it from the manifold of the rear carb.
Bill Williams

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PostPost by: nebogipfel » Fri Oct 08, 2010 4:51 pm

elansprint71 wrote:Great thread. :wink:
As well as the theories on the vac system I'd be interested in looking at the various way that folks have "gone electric", i.e. which car the motors came from, how the linkages and switch-gear look, etc.


Yes I'd be interested in this too :wink:
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PostPost by: bcmc33 » Fri Oct 08, 2010 6:10 pm

nebogipfel wrote:
elansprint71 wrote:Great thread. :wink:
As well as the theories on the vac system I'd be interested in looking at the various way that folks have "gone electric", i.e. which car the motors came from, how the linkages and switch-gear look, etc.


Yes I'd be interested in this too :wink:

From what I remember all the headlamp actuation motors used by Mazda, Toyota and the Elan M100 were common manufacture and varied, obviously, by the bracketry and linkage.
I used Mazda 323 motors and modified the brackets so that they fitted in the vacuum cylinder fixing points. Linkage was easy, but the electrics took a couple of efforts to get right.
Headlamp Actuation -- Installation.jpg and
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PostPost by: oldelanman » Fri Oct 08, 2010 7:00 pm

billwill wrote:The REASON why I disconnected the servo was because you get no braking assistance if the engine is not running so if as I did on two occasions you brake hard you can stall the engine and yet still have forward motion (yeah I know that seems crazy). Your foot reflexes will not then 'believe' the amount of foot pressure you need on the brake pedal (minus the assistance) and you crash into the stopped vehicle in front of you, that was the reason for your abrupt braking. I decided to just use unassisted braking and let my reflexes remember the harder pressure.

The loss of braking like that would be exactly what would happen if the servo was fed straight from the manifold, so perhaps you are right.


Sounds like your servo had no non return valve Bill. You will not lose servo assistance if the engine stalls while the brakes are applied. When operating normally there is a vacuum on both sides of the servo diaphragm, applying the brakes causes the primary piston to move which admits air to one side of the diaphragm - this is the hiss you can hear - so you have atmospheric pressure on one side and a vacuum on the other generating force which is transmitted to the secondary or output piston. If the engine stops the vacuum side is isolated from the manifold by the non return valve so the vacuum is not lost. When the brake pedal is released and the primary piston returns to the rest position the two halves of the servo are reconnected and the vacuum will then be diminished but not lost completely. Subsequent brake applications will have less and less assistance until eventually there is no vacuum left (if that makes sense) and only then will there be no servo assistance at all.

I don't know if it's "standard" or not but my servo has always been fed from a brass "T" piece in the front of the inlet manifold, the other branch of the "T" feeding the headlights. The two non return valves are screwed directly into the "T"

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