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Author Topic: Murena Voltmeter  (Read 6519 times)
Anders Dinsen
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« on: October 14, 2006, 10:16:44 am »

I have been wondering (as others have too!) what the Murena voltmeter is supposed to display at different voltages - so I took mine out of the dash and connected it to a power supply. But I was really amazed when I saw how it was made: It consists of a bi-metal strip wound with some resistor wire, connected to the car ignition. The resistor wire heats up - and more so at 14V than on 12, which then moves the needle.

So the gauge shows a combination of temperature in the dash and the voltage on the car.... so I wouldn't expect it to show anything really useful!

In other words: Don't worry if the gauge is in the red - it could be that it's just cold in the car! It has nothing to do with the voltage from the generator - chances are that it's probably ok!

Cheers to all,
Anders

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'82 Murena 2.2 prep 142
'01 Grand Espace 24v
'08 Smart Fortwo 0,8 cdi
krede
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« Reply #1 on: October 14, 2006, 03:31:37 pm »

Well... If you come up with some kind of "conversion" to get a correct reading.. please,do, tell  Smiley
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Anders Dinsen
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« Reply #2 on: October 14, 2006, 07:02:27 pm »

Well... If you come up with some kind of "conversion" to get a correct reading.. please,do, tell  Smiley

Adjust the bimetal strip by pulling it either way... but you will probably have to readjust in the summer!

A better solution might be to take one of the other meters (from another set of instruments), move the background over, and control it by some electronics.

I think I'll just leave it as-is and instead make a diagnostic plug somewhere in the passenger compartment where I can connect my multimeter to ignition etc to get proper read outs of RPM etc (rev counter cannot be trusted either).
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'01 Grand Espace 24v
'08 Smart Fortwo 0,8 cdi
Anders Dinsen
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« Reply #3 on: October 18, 2006, 03:52:03 pm »

I think I need to add a word of caution before everyone starts pulling their instruments out the way I did...

Here's the procedure as I have been instructed by Roy Gillard of MECUK:

1. Release the two screws under the heater arrangement.
2. Release the screw on the left side of the heater arrangement (towards the instruments).
3. Slide the trip counter reset dial out so it is haning free under the dash
4. Put your hand out and release speedometer cable. This is difficult and has to be done from the left side of the steering wheel column unless you have a very small hand. A good lamp is needed to show you what you should aim for up there.
5. Push the instruments out from behind until you can release the connector.
6. Release the connector by pushing carefully at the snaps in either side using a screwdriver (be careful not to damage the flexible circuit).
7. Take the instruments out

Refitting is the same, just the other way round... Cheesy

I broke my speedometer cable by doing it according to the workshop manual. Sad Now you are warned!!

- Anders
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'82 Murena 2.2 prep 142
'01 Grand Espace 24v
'08 Smart Fortwo 0,8 cdi
Anders Dinsen
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« Reply #4 on: August 22, 2007, 10:38:40 pm »

After adjusting the voltmeter, I have actually found it quite useful and much more reliable that I expected back when I discovered how it was designed. It may be simple and not very accurate, but it does a better job than one might expect.

Here's a photo of the instrument taken last year when I took the things apart:

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'82 Murena 2.2 prep 142
'01 Grand Espace 24v
'08 Smart Fortwo 0,8 cdi
roy4matra
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« Reply #5 on: August 24, 2007, 08:06:06 pm »

I think I need to add a word of caution before everyone starts pulling their instruments out the way I did...

Here's the procedure as I have been instructed by Roy Gillard of MECUK:

1. Release the two screws under the heater arrangement.
2. Release the screw on the left side of the heater arrangement (towards the instruments).


The reason behind taking the heater panel screws out is that the left hand side of the instrument panel is just tucked slightly behind the main dashboard moulding and you need to move it to the right, otherwise as you push it out (and even more so when you come to push it back) it will damage the soft aluminium instrument surround.  With the heater panel loose, you can move the instrument panel slightly to the right and the screw head on the side will no longer get in the way.

When you push it back in make sure the left hand edge is not fouling the moulding, and once it is down you can move it to the left slightly.

As for the accuracy of the battery indicator, as I said in another thread, it is fine IF all the connections are clean and tight, but often you can lose as much as 2 volts between the true reading and the connection at the back of the gauge!  This is why it always reads low.  So you need to check, clean and re-tension each connection, particularly those at the circuit board, and when you have, the gauge will be near to, if not correct, and be more useful.

You should understand that this is only an indicator.  It is not a true voltmeter.  In fact it is a short scale damped meter, meaning that it only reads between about 10 volts and 17 volts and owing to the damping is also slow to react and therefore shows only steady readings properly.  It is fine for normal car use where the charging voltage must be 13.4 to 14.4 volts.  So if it is above 14.4 it is overcharging, and below 13.4 it is not charging sufficiently.  Therefore the gauge (which has no actual figures - only coloured sectors) only needs to show around 10 - 11 volts to show a low battery, around 12 - 13 volts for a good battery, 13 - 15 volts for charging and anything above 15 for overcharging (all figures approx. of course).

It is a normal bi-metallic strip instument as Anders has pointed out, and the strip curves and makes the needle move in proportion to the current going through the windings.  Normal external heat will not affect it so normal interior and dashboard heat is insignificant.  You shoould not be bending or altering anything inside, without first having correct voltages and earth at the terminals.  It was originally calibrated for correct voltages and once you have these, it will give a good indication.

Once I cleaned and corrected all mine, the gauge read fine.  It sits very slightly right of centre when charging, left of centre for battery level (engine not running) and only drops heavily to the left if not charging correctly.

One final thing - the original circuit boards used aluminium strips and circuitry, which is never good for electrics on a car, but later replacements (I have had several now for different cars) are much better quality with a different gold coloured material.  These give better results.

Roy
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hru
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« Reply #6 on: August 25, 2007, 08:19:39 pm »

Quote
It consists of a bi-metal strip wound with some resistor wire, connected to the car ignition

To me it looks like a true voltmeter. The current in the wire creates a magnetic field, causing the needle to move (°rsted, elektromagnetic laws).
Note that the needle move instantly - no delays, which would be if it was a combination of heat and bimetal.

/Henrik
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roy4matra
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« Reply #7 on: August 26, 2007, 08:57:39 am »

Quote
It consists of a bi-metal strip wound with some resistor wire, connected to the car ignition

To me it looks like a true voltmeter. The current in the wire creates a magnetic field, causing the needle to move (°rsted, elektromagnetic laws).
Note that the needle move instantly - no delays, which would be if it was a combination of heat and bimetal.

/Henrik

No, the needle only moves as the bi-metallic strip bends due to the heat from the current passing through the coiled wire.  In a true voltmeter, it would be flickering all the time as it measured every tiny change in the voltage.  If you fed this gauge with 0.5 volt for example, it will not display it.  When you turn the ignition on it should start to move immediately, but it takes a while to settle at the battery voltage if you simply left the ignition on without starting the engine.  A true voltmeter would show the battery voltage instantly.

However, it is a normal modern car instrument, and they nearly all work like this.

Roy
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hru
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« Reply #8 on: August 26, 2007, 07:23:27 pm »

Well  Grin
I have to take a look at it one day - a close look to understand !

/Henrik
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Lennart Sorth
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« Reply #9 on: August 27, 2007, 01:37:36 am »

I have to take a look at it one day - a close look to understand !
Unless your car has had something unoriginal fitted, Anders has already taken that look for you in the beginning of this thread, with pictures and everything.

Actually its a rather ingenious system, implementing both measurement and heavy dampening in the same, quit simple system. Only a shame that the whole shebang is temperature dependent, but no matter how wrong the reading is, any change will always be picked up, and warn the driver.

For years, I have used the voltmeter-drop when the radiator-fan kicks in as an indication of the radiator-fan-switch is still working.  Its of course caused by poor connection, and the need to run current through - which creates a voltage-drop - but I find it quite useful.

/Lennart
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Lennart.Sorth@matrasport.dk
Murena 1983 1.9i silver // Alfa Romeo Giulietta 2.0d 2012 white // Smart 4two cdi 2010 blue //
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