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John McCormack

FOGGING FUEL GAUGE

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Ok, after driving TR2s for 44 years I need to know why the fuel gauge glass mists up. Both my TR2s do it.

It is only the fuel gauge and it demists itself after say 20 minutes driving.

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There's some moisture inside the gauge.

As you drive it, some heat from the engine in front (is your bulkhead a bit hot air leaky?) and the heat from the electricity flowing through it (the guage works off a bimetallic strip that heats and bends as the current flows), evaporates the water.    The glass is still cool, so the vapour condenses on it.      As you continue to drive, the glass warms up, the moisture evaporates and it clears again.

Any cure must remove and keep out any moisture.    Take the gauge out and keep it warm for 24 hours, somewhere any vapour will be absorbed.   A bowl of dry rice is said to be good!   And the airing cupboard, or boiler room for warmth.    An oven might be unwise!      Then, ensure that water can't get back in.   Any open car will see some rain on the dash - new sealing ring to mount the gauge, and attention to leaks behind the dash.

John

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Hi John,

Moisture inside the gauge warms up via the metal gauge body than through the glass.

whereupon it condenses on the cold glass.

Switch the panels lights on to provide more warmth to reduce the fogging time.

Replace the glass with plastic :o

Place your thumb on the glass for a minute to warm it up.

 

Roger

Edited by RogerH

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John,

I have a feeling that the TR2 used a “Moving coil” type movement, not a heated bi-metal strip.

I guess the moving coil would warm up a bit (but not as much as the bi-metal strip version.) and that would have the same drying out effect.

Do you notice if it takes longer to defog if the tank is empty or full?

 

After 44 years have you not got used to it as a "Feature".

You could do as suggested, and remove the gauge.

Of course, when you go to put it back you will discover that the terminals on the end of the wires will have broken off.

As you go to repair the terminals, you will accidentally pull the wires a bit too much and dislodge the connectors further back in the loom, possibly meaning that you will have to remove the heater in order to get to them. This in turn will mean that the demister hoses will turn to dust in your hands.

etc

etc.

 

Charlie

 

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20 minutes ago, RogerH said:

Hi John,

the gauge works by heating a coil to drive the needle

Roger

Oh no it dosn't ! 

Fuel gauges on the sidescreen cars are electromagnetic (moving iron) However yes it is moisture inside the gauge.

When ignition is on the coils of fine wire inside will warm up a little, & that is enough to cause any internal moisture to vaporise, & then condense on the glass.

Mine does it too. Only fix I have thought of is to place some dessicant inside the gauge (not much room though).

Bob.

 

P.S. Charlie beat me too it

Edited by Lebro

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Ah! So!  Moving coil then, although as said same difference, the angry electrons heat up the gauge inside.

There is a limit to the desiccation effect of the crystals, as anyone who has lived in a damp flat will know.   Eventually they "deliquesce", turn into mush  and cease to function, so if there is even a small, but constant entry of water ....

John

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2 hours ago, Charlie D said:

John,

I have a feeling that the TR2 used a “Moving coil” type movement, not a heated bi-metal strip.

I guess the moving coil would warm up a bit (but not as much as the bi-metal strip version.) and that would have the same drying out effect.

Do you notice if it takes longer to defog if the tank is empty or full?

 

After 44 years have you not got used to it as a "Feature".

You could do as suggested, and remove the gauge.

Of course, when you go to put it back you will discover that the terminals on the end of the wires will have broken off.

As you go to repair the terminals, you will accidentally pull the wires a bit too much and dislodge the connectors further back in the loom, possibly meaning that you will have to remove the heater in order to get to them. This in turn will mean that the demister hoses will turn to dust in your hands.

etc

etc.

 

Charlie

 

Haha. How right you are!

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I figured it was a heat issue. In all these years my gauge has worked perfectly, apart from having to fix the earth every 5-10 years, so I have never dismantled it.

Thank you all for confirming it is a feature of these cars. They have lots of features don't they!

 

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It used to annoy me until I took one of the gauges apart, I don't think you will ever cure the problem

The minor gauges do not have internal illumination and are all lit from the outside. To allow the light in, the case of the gauge has slots around the circumference and sandwiched between the case and the dial face there is a strip of plastic which helps diffuse the light and keep foreign bodies out.

However, this strip is not sealed to the case or dial face and, indeed, has ventlation holes in it

So the gauge is open to the atmosphere and when humidity is high, as heat builds up under the dashboard from the engine, gearbox and heater (if you have one) and warms the gauge case up , water vapour will condense against the inside of the glass

Phil

DSC_0025-2 Temperature gauge V2.jpg

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IT'S NOT MOVING COIL, IT'S MOVING IRON  :angry:   :D

There are two coils, & they don't move, the needle is attached to a shaped piece of soft iron, which is pulled towards the core of the coils, one pulls it to full, the other pulls it towards empty, depending on the sender resistance one coil will pull harder than the other, & the needle ends up at some point between F & E.

Bob.

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My TR4 does this too! There's also a well preserved mosquito visible inside the guage that must have got in through one of the illumination slots....  it's been there since 1985 :-)

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2 hours ago, NickKerfoot said:

My TR4 does this too! There's also a well preserved mosquito visible inside the guage that must have got in through one of the illumination slots....  it's been there since 1985 :-)

Haha.

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2 hours ago, NickKerfoot said:

My TR4 does this too! There's also a well preserved mosquito visible inside the guage that must have got in through one of the illumination slots....  it's been there since 1985 :-)

Jurassic park comes to mind

 

Roger

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