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Petrol gauge misreading
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rob needs



Joined: 14 Jun 2008
Posts: 22
Location: Bristol

PostPosted: Sun Mar 11, 2012 6:03 pm    Post subject: Petrol gauge misreading Reply with quote

Humber Sceptre Mk2 1966

Ages back i tried to fix this problem of not having the correct reading on the dial. I put another sender in the tank and also changed the dial.
I'm sure i even changed that small metal voltage regulator. Can the regulator be checked using a multi meter?

It says in the book that that mean voltage between the "I" and earth should be 10 volts. Do i set the meter at ten volts and put the multi meter wires on the regulator and see if it reads 10 volts.
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Phil - Nottingham



Joined: 01 Jan 2008
Posts: 1252
Location: Nottingham

PostPosted: Sun Mar 11, 2012 6:56 pm    Post subject: Reply with quote

Yes but you set Voltmeter at next highest range say 20v

Are you sure stabiliser is connected correctly and is earthed properly.

Tank unit relies on earth via tank fixings - if corroded then may be high resistance.

If gauge has been run at 12v because of faulty stabiliser or wrong connections than it may be damaged now,

Does it over or under read
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pigtin



Joined: 23 Nov 2007
Posts: 1879
Location: Herne Bay

PostPosted: Sun Mar 11, 2012 8:32 pm    Post subject: Reply with quote

Simples! I just change the position of the notch on the stick with my Austin 10/4.
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Julian



Joined: 19 Apr 2010
Posts: 278
Location: Warrington

PostPosted: Sun Mar 11, 2012 9:03 pm    Post subject: Re: Petrol gauge misreading Reply with quote

rob needs wrote:
Humber Sceptre Mk2 1966

Ages back i tried to fix this problem of not having the correct reading on the dial. I put another sender in the tank and also changed the dial.
I'm sure i even changed that small metal voltage regulator. Can the regulator be checked using a multi meter?

It says in the book that that mean voltage between the "I" and earth should be 10 volts. Do i set the meter at ten volts and put the multi meter wires on the regulator and see if it reads 10 volts.


You can't easily check out the old instrument voltage regulators with a multimeter - they work like an indicator flasher unit, ie a heater and a bi-metal strip. Thus as you correctly point out they put out 12v-0v-12v-0v etc with the aim being 10v as average. A heavily damped gauge prevents the needle dancing around!

9 times out of 10 an over-reading gauge is because the regulator has lost its earth due to corrosion.

These days I believe that 'solid-state' regulators are available.

Julian.
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Jim.Walker



Joined: 27 Dec 2008
Posts: 1229
Location: Chesterfield

PostPosted: Mon Mar 12, 2012 10:02 am    Post subject: Re: Petrol gauge misreading Reply with quote

Julian wrote:
rob needs wrote:
Humber Sceptre Mk2 1966

Ages back i tried to fix this problem of not having the correct reading on the dial. I put another sender in the tank and also changed the dial.
I'm sure i even changed that small metal voltage regulator. Can the regulator be checked using a multi meter?

It says in the book that that mean voltage between the "I" and earth should be 10 volts. Do i set the meter at ten volts and put the multi meter wires on the regulator and see if it reads 10 volts.


You can't easily check out the old instrument voltage regulators with a multimeter - they work like an indicator flasher unit, ie a heater and a bi-metal strip. Thus as you correctly point out they put out 12v-0v-12v-0v etc with the aim being 10v as average. A heavily damped gauge prevents the needle dancing around!

9 times out of 10 an over-reading gauge is because the regulator has lost its earth due to corrosion.

These days I believe that 'solid-state' regulators are available.

Julian.

I agree with this, except that the "thermo-static" gauge is not heavily damped, but is motivated by a bi-metallic strip which moves the needle slowly in response to the regulator average output.
But we can rule out about half of the possibilities if we know whether the gauge is over or under reading.
Jim.
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Julian



Joined: 19 Apr 2010
Posts: 278
Location: Warrington

PostPosted: Mon Mar 12, 2012 12:35 pm    Post subject: Re: Petrol gauge misreading Reply with quote

Jim.Walker wrote:
[I agree with this, except that the "thermo-static" gauge is not heavily damped, but is motivated by a bi-metallic strip which moves the needle slowly in response to the regulator average output.
.


Ah yes, that makes sense, I wasn't thinking clearly and assumed the gauge would be moving coil.

Julian.
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Jim.Walker



Joined: 27 Dec 2008
Posts: 1229
Location: Chesterfield

PostPosted: Mon Mar 12, 2012 1:57 pm    Post subject: Re: Petrol gauge misreading Reply with quote

Julian wrote:
[

Ah yes, that makes sense, I wasn't thinking clearly and assumed the gauge would be moving coil.

Julian.

The difference between the use of moving coil and thermo-static gauges is that a moving coil gauge will read "full", whereas the thermo-static gauge will read "empty" if disconnected from the sender unit. Problems of continuity or resistance of the circuit between gauge and tank will have opposite effects. Hence my query about over or under reading.

Another of my little tales here!
About 1960 we had a Sheerline Hearse. It is not commonly known that a hearse is (or was) a goods vehicle and was subjected to taxation by weight. The difference in road tax was quite significant between up to 2 tons and over 2 tons. Periodically the Ministry of Transport people used to demand it was presented at a weighbridge of their choice to be checked.
It actually weighed 2 tons and about 50lbs. kerb weight. But as a reduction of the weighbridge figure was allowed for the quantity of fuel shown on the gauge we always presented it with the gauge showing full. A tank-full of fuel weighed about 100lbs. so we always beat the 2ton barrier.
In fact the tank was almost empty, but the wire from the gauge to tank unit was deliberately disconnected so the gauge showed "full"
Jim.
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Quote from my late Dad:- You only need a woman and a car and you have all the problems you
are ever likely to want". Computers had not been invented then!
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rob needs



Joined: 14 Jun 2008
Posts: 22
Location: Bristol

PostPosted: Tue Mar 13, 2012 7:55 pm    Post subject: Reply with quote

I think my gauge shows more than it should.
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Jim.Walker



Joined: 27 Dec 2008
Posts: 1229
Location: Chesterfield

PostPosted: Wed Mar 14, 2012 10:24 pm    Post subject: Reply with quote

rob needs wrote:
I think my gauge shows more than it should.


Then, if it is a thermo-static gauge (slow moving). The possibilities would seem to be (not in the most likely order):-
Too high a supply voltage. Check the voltage stabilizer. Usually attached to the back of the speedo. Difficult to check except by substitution.
A partially sunk float in the tank.
Or coils shorting together on the tank rheostat, thereby reducing the effective resistence.

I suppose the gauge itself could have a tired bi-metalic strip, but I do not know how likely it is. I have never come across a faulty thermo-static gauge unit.
Jim.
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Quote from my late Dad:- You only need a woman and a car and you have all the problems you
are ever likely to want". Computers had not been invented then!
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rob needs



Joined: 14 Jun 2008
Posts: 22
Location: Bristol

PostPosted: Thu Mar 15, 2012 9:10 pm    Post subject: Reply with quote

I checked out the earth at the tank, one wire goes to earth and one back to the dash and cleaned it all up.
In fact with the black earth wire removed it still showed the same on the clock,but there is a strap around the tank that must act as an earth.

Of course the the small voltage regulator is well tucked away. I put 10 Ltr's of fuel in on a near empty tank and it read near 20 Ltr's.

Have to decide whether i start taking things apart,seeing as i did all this a couple of years back and never solved it.

The trick with the dipping stick will work as i have already marked one off at 10 Ltr's Very Happy
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