Fuel Gauge Error

[Paul H]

Mk2 Vitesse . Fill to the brim and the needle is just off full . Using the calculation of 6 miles to the litre x 20 litres , the gauge shows slightly lower than half full . Then as soon as the trip show approx 140 miles the needle drops like a stone and bounces on empty . I’ve got used to this now and ignore the gauge and use the trip to refill ie 180 miles go and look for fuel . 
Is there an explanation for this fuel gauge behaviour ? The fuel float is a bayonet fit 
Paul 

[dougbgt6]

Do you have voltage stabiliser? How's your temperature gauge?

Doug

[Paul H]

Just now, dougbgt6 said:

Do you have voltage stabiliser? How's your temperature gauge?

Doug

Yes the stabiliser is an electronic version . The temp gauge is thermal . I think that’s what it’s called . There’s a thick wire from the sender to the gauge and no electric feed other than for the illumination . The gauge displays actual temperature and moves between 80 and 90c when driving 

Paul 

[Pete Lewis]

Dougs in first ,  ha  

the voltage stabiliser has a dramatic effect on gauge reading but this normally gives fuller or hotter readings as the operating voltage is too high

the fact it seems to read ok from full down to 1/2 makes me think this is a sender problem  if it drops rapidly once below 1/2 

this sounds like the resistance wiring on the sweep late in the sender has a fault 

easy to swap if the tanks less than 1/2 full 

you must have a stabilised gauge sender unit not the earlier version which will give very odd results 

being bayonet its not going  to  have that confusion 

Pete

 

[dougbgt6]

Stabiliser voltage should be around 10.5volts, if it's OK then, wot Pete said.

Doug

[Paul H]

1 minute ago, Pete Lewis said:

Dougs in first ,  ha  

the voltage stabiliser has a dramatic effect on gauge reading but this normally gives fuller or hotter readings as the operating voltage is too high

the fact it seems to read ok from full down to 1/2 makes me think this is a sender problem  if it drops rapidly once below 1/2 

this sounds like the resistance wiring on the sweep late in the sender has a fault 

easy to swap if the tanks less than 1/2 full 

you must have a stabilised gauge sender unit not the earlier version which will give very odd results 

being bayonet its not going  to  have that confusion 

Pete

 

Hi Pete , syphoned the fuel yesterday so close to empty 5 litres max . Will I be able to test the sender once out ? 
Paul 

[Pete Lewis]

the readings on a  meter resistance are not linear  its often visible the windings change spacing as the wind down the sweep plate   

it may just need the sweeper given a light bend , but any broken /stray wire is pretty much kaput 

Pete

[Iain T]

2 hours ago, Pete Lewis said:

normally gives fuller or hotter readings as the operating voltage is too high

Similar issue with my gauge it reads past full when the tank is indeed full bt as soon as it gets to below 1/2 I know to start looking for petrol. 

I've changed the sender, I wonder if I just need to reset the float bar as the shape may have been deformed? 

Iain 

Ooops just read your last post, I'll do just that! 

[Pete Lewis]

the stabilsers maintain a steady controlled voltage to run the two gauges  as they are voltage sensitive any changes in the battery state and voltages used while charging if left to the car base voltage would give all sorts of varied readings as the coil in the gauge heats and cools 

with the changes in voltage  so smiths devised a stabiliser to run the gauges on a fixed 10-10.5 volts which may pulse as the thing operates 

electric ones are more stable but cant be a volt dropper they must control the output voltages

when they fail you get no gauge if its burnt out 

you get high fuel and hot water if the voltages are above the steady 10v like upto14.5 v 

pete