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Author Topic: How much BAR vacuum?  (Read 3087 times)
Grapes
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« on: June 20, 2023, 11:01:59 am »

Might have missed it in the manual but how much BAR vacuum should the vacuum system have? And/or should the engine generate? I'm thinking about buying a vacuum meter but don't know if I should get one that goes beyond -1 BAR.


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TELBOY
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« Reply #1 on: June 20, 2023, 07:15:44 pm »

Haha, how funny, spent last week pondering the same. Have wired up the vacuum light on the dash! Originally purchased relay and in line fuse then realised that the wiring is already there! The light receives power from the same circuit that provides power to the fuel brake and charge lights and is just an earth from terminal 6 behind the dash! I connected it to a vacuum brake switch from a landrover series 2 and mounted it near the fuse box. Choose this one as it was only £25. Was gonna see about getting a t piece and connecting it tomorrow. At present the light works when ignition turned on but that's as far as I have got. I know at present I don't even have enough vacuum to lift the lights! Will keep you posted.
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TELBOY
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« Reply #2 on: June 20, 2023, 08:06:28 pm »

"frog eye" Vacuum warning light
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Grapes
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« Reply #3 on: June 21, 2023, 07:58:58 am »

Ah that's interesting! My lights wouldn't lift for a while but they also creaked a lot when I lifted them manually so I sprayed wd40 on the joints and now they lift again. However I'm not happy with the vacuum yet since it doesn't hold it for long after engine shut down so I figured I'd investigate the system and see if I can find out at what point it is leaking. I ordered a manual brake bleeding kit with a gauge on it so I might be able to use that I'm thinking.
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TELBOY
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« Reply #4 on: June 21, 2023, 05:20:33 pm »

SUCCESS!!!! Of sorts. Connected all vacuum lines (replacing the section with a small split) started her up and after a couple of seconds the "frog" warning light flickered and went out. Left her with engine off ignition on and after repeated checking the vacuum light came back on after 1 1/2 hrs. I have suspected I have a leak at the solenoid so have disconnected and blanked off that part and started her again. Again light flickered and went out, let her run for about a min then turned her off and ignition back on. Now been 6hrs and no vacuum light! Looks like I may have to get that new head light replacement for the solenoid assembly. WELL HAPPY!!!!!!
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Grapes
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« Reply #5 on: June 22, 2023, 11:21:13 pm »

I created my own with two very cheap 3 way switches from Aliexpress, some tubing and two T splitters 😅 cost me less than 10€ in total.
They run in tandem and the tubing makes it so the pressure normalises on one line when the other is pulled vacuum. It's not original, nor looking clean but it works like a charm.
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Murena1400
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« Reply #6 on: June 23, 2023, 07:26:36 am »

That vacuum light only exists in very early Murena 1600 dashboard panels and was replaced with a cold start warning light that is wired in a 2.2 but remained unused in a 1600.

The vacuum light is a leftover for a Bagheera that used to have a vacuum switch in a German car.
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TELBOY
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« Reply #7 on: June 23, 2023, 01:04:43 pm »

Yes I am aware of all that, but I'm a bit of a @@@ If I have a warning light doing nothing I prefer it to do something! and it wasnt too difficult once I thought about it.
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roy4matra
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« Reply #8 on: June 23, 2023, 08:29:09 pm »

That vacuum light only exists in very early Murena 1600 dashboard panels and was replaced with a cold start warning light that is wired in a 2.2 but remained unused in a 1600.

The vacuum light is a leftover for a Bagheera that used to have a vacuum switch in a German car.

Not just German cars as I have seen the sensor fitted to other Bagheera too.

As I have pointed out many times, keeping the head light vacuum system air tight is very important, because any leak into it, means the engine will run weak and weak mixtures can lead to stalling as well as engine damage.

So as I pointed out once before on the forum here, it would be useful if that vacuum warning was still set up, as it could alert you to a fault with the vacuum system and possibly prevent a cracked head.  Since my 2.2 Murena had the choke warning light instead, I had considered getting a vacuum sensor and as it is just an earthing contact to bring on a warning light, I would wire mine to the existing warning light system, as I have already done with the header tank low level float sensor.  So the dash warning light becomes ever more useful as it would then alert me for low brake fluid, or low coolant, or a leak in the vacuum system.

Originally it was also the low pad material warning light, but since only one or two pads out of the eight were connected and they didn't work that well, now that the pads have the aural warning button, that function is better served by that mechanical system which is far more reliable and covers all eight pads.

Roy
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TELBOY
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« Reply #9 on: June 23, 2023, 09:00:23 pm »

Thanks for the input Roy. Couple of questions if you don't mind?
How long should I be aiming for for the system to maintain a vacuum (if I don't use the lights when engine off)?
Would it be prudent to wire it to the brake light test warning button?

Thanks in advance
Terry
« Last Edit: June 23, 2023, 09:09:11 pm by TELBOY » Logged
roy4matra
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« Reply #10 on: June 24, 2023, 02:10:51 pm »

Thanks for the input Roy. Couple of questions if you don't mind?
How long should I be aiming for for the system to maintain a vacuum (if I don't use the lights when engine off)?

The system was designed with a vaccum storage tank and a one way valve, so that the vacuum created in the manifold when the engine is running creates a vacuum in the tank, and providing the system is air tight, it should retain that vacuum for some days without the engine running.  So the first thing is that the one way valve must be in good condition to retain the vacuum side air tight.  I have seen or known of some one way valves that are not sealing and therefore once the engine is not running, the air will simply come back in from the inlet manifold.  In one case, a piece of debris had got stuck in the valve and prevented it from closing.  So first check that the one way valve will open under suction, but close as soon as the suction stops.

As long as the one way valve is good, then the next most likely places for vacuum leaks are the rubber joins in the pipework, and the white shuttle valve at the front.  The rubber joints in the pipework get hard with age, and crack or split, but you can replace these joints with new soft, tight fitting rubber hoses.  The white shuttle valve at the front that controls which side of the double acting servo the vacuum is directed to, is known to leak after some years either because dirt gets in or the valves become gummed up and won't move or seal easily.  Usually I have found a strip and clean should cure these leaking valves, but I know some have had difficulties getting them to work correctly, but now there is another option as a Murena owner has designed a more modern system, which is available through Matra Magic if you don't wish to do it yourself.

Finally, the double acting servo under the front can leak after years of use and normal wear and tear.  It may have a split in the diaphragm or it could be the seal around the lift/lower activating rod.  This should also tell you that the seal for the double acting rod should be a service point that you check and clean occasionally and lubricate to prevent that seal wearing!

Quote
Would it be prudent to wire it to the brake light test warning button?

Thanks in advance
Terry

Strictly you don't wire it to the test button, you wire a sensor to the warning system, and the test button is simply part of that system.  If you are fitting the vacuum sensor in the front compartment, then you already have a suitable wire to the brake fluid cap, so simply link it there, or use the now unused left front pad warning wire.

If anyone has a Murena with the low vacuum warning light in the instrument panel, which is the early 1.6 cars (and possibly some of the very first 1981 2.2 models, although I'm not sure if they ever had one?) then all those cars need is a low vacuum sensor inserting into the vacuum sysem and connecting to the provided orange warning dash light, to use that already fitted system; but if you have a 2.2 or later 1.6 where that light has been used for the choke warning light instead, then the ideal is to use the red warning light provided for the low brake fluid and low pad material warning, which saves having to fit any additional wiring and an additional warning light somewhere else.

Now, the low brake fluid/pad warning light is also provided with a test button which is to check the bulb is still working, as normally you might never see that bulb lit if the car is used frequently and the system does not leak, certainly not often anyway if the routine maintenance is done and you note visually when the fluid is getting low, alerting you to check for pads nearly worn out, or a potential fluid leak.  However, in the case of a vacuum sensor linked to a warning light, it is probable that the light will illuminate when you first turn the ignition on, if the vacuum has dropped, so that in itself will check the bulb is working.  As you say, when you start your engine, the light comes on, then flickers and goes out, as the vaccum builds.

So if you connect the low vacuum sensor to the low fluid system, (the black & red wiring is for this system) you can use the test button to check the bulb any time as normal, but if you use the provided low vacuum warning light already fitted in the early cars, then there is a simple way to test if that bulb works, assuming the system is air tight and still has vacuum, so the warning light is out when the ignition is turned on.  Simply operate the head lights a couple of times without the engine running to deplete the vacuum in the system!

Roy
« Last Edit: August 20, 2023, 06:42:44 pm by roy4matra » Logged

TELBOY
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« Reply #11 on: June 24, 2023, 05:54:57 pm »

Thanks Roy, I do like the dea of linking it to the test button and hadn't thought of using the brake reservoir circuit! It's almost next to it so will go this route.
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roy4matra
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« Reply #12 on: June 25, 2023, 10:09:37 am »

Thanks Roy, I do like the dea of linking it to the test button and hadn't thought of using the brake reservoir circuit! It's almost next to it so will go this route.


I should point out (for all owners) that the brake reservoir cap has two wires to it, one that goes to earth, and the other goes to the warning light on the dash.  So if the fluid level drops below a certain point, the float contacts should join, to earth the system, and bring on the warning light.  Similarly, there is a two wire loom down to the left front caliper and originally you would connect one wire to the pad and the other to earth (there is a tag on the bracket behind the hub).  Now if you are no longer using these wires to the pad, then you could bring that bit of loom back into the front compartment (where it originally comes from) and attach it to the vacuum sensor.  So you don't need to make any wiring alterations at all!

As I stated before, if the pads have the aural warning buttons fitted, then they provide a better more reliable low pad warning, and for all pads not just one or two, simply making a noise as they touch the disc.  They are soft material and won't damage the disc, unlike the steel backing of the pad, which, if you wore the pad down completely and had metal to metal contact, will damage the disc.

Roy
« Last Edit: June 28, 2023, 10:22:42 pm by roy4matra » Logged

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