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Help Please - Fuel Gauge now reads zero ?


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Fuel gauge has always worked OK and over the years never had any issues.  

Today tank had approx. 10 liters of fuel, and a dipstick test indicated about 100mm of petrol from the tank bottom, and fuel gauge indicated circa 1/6 full. Added another 5 liters from petrol can and now gauge doesn't move off the stop position to the zero mark - its dead?   I put a DVM on the tank terminals and it read 12.4V dc (battery voltage), but after 10 seconds the DVM reading switched to  "OL" and the voltage bar on DVM display was "flashing" indicating an overload. 

Any suggestions on how the sensor works, ie voltage or resistance and what should I test for next?

Cheers and thanks, Colin.

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The fuel, and temp, gauges work on a ‘stabilised’ supply mimiking 10v

it does this very crudely , by switching on and off the 12.n volts from the battery using a bi-metallic spring !

so using a dvm at the sender you’ll get the readings you are seeing, ie 12.n volts then nothing, repeating.

the sender is a resistive type, so test by disconnecting the 12.n volt wire and measuring resistance to earth

hth

steve

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The tank sender has gone open circuit Colin. 

The sender is a variable resistance which should be about 20 Ohms with the tank full and about 240 Ohms with it empty.  If the circuit breaks the gauge will show empty.  It could be that the wiper contact on the sender is not making good contact at the position it is at present.  If you add more petrol so the wiper moves, it may come back to life. The other possibility is that the sender body  has somehow become electrically detached from earth. 

Edited by RobH
typo
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8 minutes ago, RobH said:

The tank sender has gone open circuit Colin. 

The sender is a variable resistance which should be about 20 Ohms with the tank full and about 240 Ohms with it empty.  If the circuit breaks the gauge will show empty.  It could be that the wiper contact on the sender is not making good contact at the position it is at present.  If you add more petrol so the wiper moves, it may come back to life. The other possibility is that the sender body  has somehow become electrically detached from earth. 

Rob’s right, i’d be checking the earth first as TRs are full of dodgy earths!

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OK thanks Steve and Rob. 

So Steve, the black wire (small spade terminal) has a zero ohm, good connection to earth. You suggest disconnecting the green/black wire (large spade) from the sender and measure resistance from this terminal to earth - correct?

Rob, when I look down I see nothing, do we have some sort of pictorial on what the tank sensor looks like inside the tank so that I can maybe lift up the float and measure its resistance from where it is to the top? All I've done is add 5 liters of petrol?

Cheers

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8 minutes ago, c.hydes said:

do we have some sort of pictorial on what the tank sensor looks like inside the tank 

Sorry Colin -  can't help with that but one extra test you can do is to take the green/black wire off the sender and earth the wire. The gauge should show full+ if the wire and gauge are both OK and that will confirm it is the sender at fault. 

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51 minutes ago, RobH said:

Sorry Colin -  can't help with that but one extra test you can do is to take the green/black wire off the sender and earth the wire. The gauge should show full+ if the wire and gauge are both OK and that will confirm it is the sender at fault. 

Thanks Rob, green/black wire disconnected from sender and connected to earth, gauge the increases to "slightly over full".

But sender was Ok with 10L, but not at 15 L, so maybe the resistance track on the potentiometer at this point  (15L) is dodgy?  Need to find a way then to lift the float higher need some sort of internal diagram perhaps.

Colin.

 

 

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12 hours ago, c.hydes said:

Thanks Rob, green/black wire disconnected from sender and connected to earth, gauge the increases to "slightly over full".

But sender was Ok with 10L, but not at 15 L, so maybe the resistance track on the potentiometer at this point  (15L) is dodgy?  Need to find a way then to lift the float higher need some sort of internal diagram perhaps.

Colin.

 

 

A wire coat hanger straightened out and a large ish hook formed on the end should be able to pickup the float and then you can move it up an down to see if it reads more or less.

Stuart.

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