Petrol gauge

[mu0u207b]

i have been having problems with my petrol gauge not registering a full tank for sometime now. If i fill up the tank to brim full it will only register on the gauge as being 3/4 full and now recently it only registers as just over half full. Is this a gauge problem or the sender problem. The petrol tank is definitely full and i have removed it and cleaned it and checked it for a damage and there is none. I very rarely fully fill my tank anyway so its more of a pain not knowing exactly whats left in the tank. Any ideas??

[chickenjohn]

Most likely the sender.

I bought a replacement sender for my saloon and 4/1/2 years later the sender still works.

of course, before oing this, check wiring for continuity, and good earths etc.

[Arfron]

My gauge was a bit odd . Cleaned up the earth between tank and body shell - well one or two screws !! ( self tapping screw / s holding tank to body ) Problem went away . Worth a try?? Best of luck Arfron

[Willie]

I Agree with 'chicken'. The fact that your incorrect reading has
worsened over a period suggests a bad connection which is
gradually worsening. The sender supplies a variable earth to
the gauge, you may find that the petrol tank is not a good earth
to the chassis so you could prove this by observing the reading on
the gauge when you apply an independent earth(chassis) connection
to the tank.

[Cam]

Also, check the voltage stabiliser on the back of the speedo unit as this supplies a regulated voltage supply to the gauge.

I have seen cases of low readings before where the stabiliser has gone faulty.

[Alec]

Hello all,
as most of you probably know, shorting the sender cable to the sender unit should give full scale, i.e. full reading. If not then try from the sender cable to the body, if this gives full scale then the tank earth is poor.

Alec

[mu0u207b]

Thanks all for your comments. Ive removed the sender unit and tried applying the lead to direct earth, i.e. the chassis and get a full reading on the petrol gauge so i assume all is good with the gauge and the voltage stabiliser. I have also reconnected the lead to the sender unit and applied an independent earth from the car chassis to the body of the sender i.e where the screws go through and still get the same low reading as if the sender was in place fitted on the petrol tank. Does this mean that the sender is faulty??

[Willie]

Oviously your 'sender lead direct to chassis' giving a correct reading
proves the wiring is ok all the way to the gauge and the voltage
regulator is also ok. The fault,from your further test seems to
point to a bad connection between the body of the sender unit and
the petrol tank i.e. not a perfect connection. Should be easy to
remove a couple of the screws and clean them up plus the areas
which they will contact and then re-check. I am assuming that you
do not have a decent ohmmeter?

[mu0u207b]

Would it be a bad connection between the body of the sender and that of the tank as i removed the sender with the lead connected and applied the sender unit body to other earthing point such as the chassis and obtained no better results.

[Willie]

Well, that suggests that the fault is inside the sender unit. If you
end up trying another sender unit remember that you MUST obtain
one from the same model. a 948cc cars sender will NOT work
correctly on a 1098cc fuel gauge and vice versa. (a 948 sender will
have a nut on the connector, a 1098,apart from the first two years,
will have a Lucar connector).

[mu0u207b]

Im really confused now. Just bought a new sender unit, connected it up and applied the body of the sender to an earthing point to test it before screwing into situ but im still getting a low reading when i should be getting a full one. I once again applied the lead to direct earth on the tank it self and still get a full reading on the gauge. Could applying the body of the new sender to an earthing point create a sufficient earthing path? Ive changed the connect as well just incase of corrosion and still get a low reading. Am i missing something?

[Cam]

Check the voltage regulator. If memory serves, you can STILL get a full scale deflection if you earth the wire, even if the regulator is a bit duff (low output). Replace it and check again.