SP250 petrol gauge sender replacement

[Martyn Ross]

I've been meaning to get around to this for some time - bought a replacement kit last year.
The gauge is fine when the tank is full....but becomes increasingly erratic (a bit like me) as the level of fuel (or in my case, alcohol) drops below maximum.
Has anyone completed this task before - if so, any handy tips on proceedure?

Martyn Ross

[Kbeal]

Barry Thorne has replacement senders, I think they are about £50

Its a simple straight swap out, but you have to take the tank out to do it. Don't forget to disconnect the battery first - not a good place for a spark when connecting /reconnecting the wires!

Kevin

[simonp]

On a related matter with the sender unit. Mine works OK but doesn't give a very accurate reading as the tank empties. Also when it is full the tank sender unit bangs on the top of the tank. I do not know whether others experience this too.

SimonP

[Ian Slade]

If you remove the gauge and the sender you can adjust the two coils positions in the gauge for various positions of the sender, one should buy a matched pair as originally supplied ideally, but no longer available, however the matched pair was produced by the above method, it is not easy and is time consuming as adjusting one coil affects the other and thus the gauge reading ( sometimes you could be lucky, mostly not ) . Erratic gauge reading indicates the sender wiper is no longer making correct contact with the track, or the track is damaged, if the former the wiper can be adjusted ( bent) to improve contact.

[Martyn Ross]

Thanks all for your sensible responses.

Yesterday I attempted to siphon the fuel out of the tank (yuk) and gave up as it made me feel ill. Then - in a moment of brilliance (rare) remembered the fuel pump would do this for me.
Tank out, noticed that the hole in the floor meant there was a drain plug.
Moment of brilliance now somewhat dimmed, but would never have got the 25L container under the car anyway.
Old sender was an original Smiths (only one coil Ian?)- really solid piece of work. When the replacement unit (sourced from Manners) was connected it read tank level in reverse e.g. tank full read empty and vice versa. Switching the connections made no difference. Moment of brilliance did not extend as far as trying reversing the connections at the gauge end.
Cleaned the original Smiths unit thoroughly and refitted with new washer, refitted tank in the hope that the magic of taking it all apart and looking at it might work but I think not.

Was the Manners unit faulty? Car is positive earth.........

Martyn

[Vortex O'Plinth]

When the replacement unit (sourced from Manners) was connected it read tank level in reverse e.g. tank full read empty and vice versa. Switching the connections made no difference. Moment of brilliance did not extend as far as trying reversing the connections at the gauge end.
Cleaned the original Smiths unit thoroughly and refitted with new washer, refitted tank in the hope that the magic of taking it all apart and looking at it might work but I think not.

Was the Manners unit faulty? Car is positive earth.........

Martyn

A common cause of the fuel gauge only reading extremes (i.e. full or empty) is that the gauge body is not earthed. There should be three electrical connections to the back of the gauge; one to the battery, one to the sender and the third to earth (This is a wire from the gauge case to a convenient chassis earth point). If you have only exchanged the sender and left the gauge untouched, this is unlikely to be your problem. However if during your experimentation you've disconnected the gauge at any point, make sure the case is now soundly earthed.

[Ian Slade]

Martyn,
The coils are in the gauge not the sender, there are two nuts or screws you need to loosen, whether the sender is wrong I'm not sure, but if I remember correctly shorting the sender lead to earth gives a full tank reading, the wiper picks up the resistance so the only way to change it is to move the non earth lead to the high resistance end, or the wiper is going to short circuit at both ends. I suggest you connect up the sender outside the tank and check the movement against the reading, it may have been assembled mechanically upside down. I.E when the float is down it reads full and vice versa

[Vortex O'Plinth]

This diagram might help to clarify the operation of the sender/gauge and emphasise the correct electrical connections.