Fuel Gauge Reading Backwards

[david340]

The fuel gauge seems to read "backwards" on my '75 Sovereign Coupe.

i.e full tand and shows empty then reads along the gauge until showing full equates to an empty tank.

Just acquired the car and I had been told this by the vendor.

Any suggestions as to what the cause might be ? In my mind I see reversed polarity somewhere being the cause, just not sure where to start looking. The symptom is the same for both tanks on changeover.

Cheers.

Dave

[piman]

Hello Dave,

the first check is to disconnect the sender wires and earth them. Then when the ignition is switched on the gauge should read full on both tanks. This will prove the power supply and the gauge. Very early Smiths type fuel gauges worked opposite to that and will read empty, but I would have thought that an early gauge would be obvious as it won't match the rest of your instruments. However I suspect that the fault lies with the senders. I'm not familiar with the XJS as such, but is it possible the senders are handed and they have been put in the wrong tanks so working in reverse? Polarity has no effect, by the way.

Alec

[david340]

Thanks Alec.

Top clarify its a Daimler Sovereign Coupe not XJS.

The gauge looks original. When the switch is operated it switches between tanks ok and there is a reading, but it just reads backwards for both tanks. My gut feeling is that someone may have got the wires mixed up at some stage.

[piman]

Hello David,

sorry, I actually meant to type XJ6 as that is what I had in mind.

I don't believe it can be a wiring fault as the gauge reacts to a changing level, if they were crossed the level would show zero constantly. It can only be the senders, which is why I wondered if they are handed and the L.H is fitted to the R H tank and vice versa? In other words they are installed upside down? Do you have a fuel level warning lamp, because this would illuminate when full then go off after some running? That would be a confirmation of my guess.

As I said the first check is to disconnect and earth the sender cables at the tank end. This will prove the rest of the system .

Alec

[david340]

Thanks Alec, appreciate the advice.

It's a strange one and I follow your theory on the senders.

Will have a look at the wiring first as you suggest - once this weather has warmed up a bit !!

Regards.

Dave

[peterdpps]

On a flippant note; does this mean that as you drive along you 'gain' petrol in the tank?
If so I think we would all like that fault on our cars.

[david340]

Noted the above

Thanks for the pointers and advice all.

My intention is to check the senders top see where the fault lies. Having run the car for a while im not certain that the fuel gauge is reading backwards but just not working at all !

Left hand tank showing empty - right hand tank was showing half full but now registers empty - despite being filled and the needle is not moving.

Couple of points id like to confirm.

1) Whats the best was to gain access to the sender units on a 75 car so I can check connections etc.

2) To test the sender my understanding is that I turn the ignition on, disconnect both wires from the sender and touch them together - result should be a full tank reading. This would show that the gauge is ok and the wiring is ok.

3) To check the earth - I would connect to a positive supply on the car with a test lamp and touch the tank body to see if the earth is ok.

4) Swith to the other tank and repeat the test. If the checks are ok then this would condem the sender units. In which case can they be changed with the tanks in the car and what is the best way to gain access.

Thanks !

Regards.

Dave

[piman]

Hello Dave,

if you have two wires on the sender, and one of them is not black, then one will be the gauge the other the fuel warning lamp. The gauge wire is green\black and the warning light is white\geen. There may be a black earth wire, which goes to a spot riveted tag on the sender.
Touch the green black to earth and the gauge should read full scale.

I am not familiar with your car nor do I have a diagram, but I would expect that the switch that selects which tank you use switches the pump, fuel gauge sender wire and the fuel level warning light if fitted? I would also ensure when doing the earth test that the tank earthed corresponds to the tank selected, if not it's probably that those wires are crossed at the switch.

If the sender(s) prove faulty (you would be very unlucky if both are defective?), access will be the same as for doing the test. Normally, at a guess, removing the boot trim on both sides should give access? Makes sure the fuel level is below the sender then gently tap the outer ring around until it's free, the sender can then be wiggled out.

Alec

[david340]

Thanks Alec thats appreciated.

Looking on the wiring diagram there is a light green /slate colour wire feed for the left tank and light green and red wire for the right tank. Diagram also shows black earths.

Note your comment re it being unlikely that both senders are faulty.

Interestingly , in the paperwork that came with the car the last owner had a "snagging list" of which the fuel gauge fault was noted - alongside he's written "possible earthing fault".

I'll carry out the test by earthing the gauge feed - hopefully resulting in a full tank indication and will also check the sender earth as this may well be where the fault lies. I can then switch tanks over and do the same - cheking that the switch position corresponds with the correct tank i.e. that the wiring hasnt been messed up.

Cheers.

Dave

[david340]

After switching tanks over today , one tank is now registering ok on the gauge. Also noted that the position of the switch does not correspond as it should in relation to the tank in use.