I have a 1999 ford fiesta finesse mk4 and the fuel gauge show full when it is empty, I put £10 in it and now shows 3/4 full, please help me?

On a car that is about 22 years old, it could well be the fuel tank sender unit.

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1 hour ago, unofix said:

On a car that is about 22 years old, it could well be the fuel tank sender unit.

Someone said the wires might be crossed or do I need a new one or maybe I have a bad ground wire

Check the wires on the fuel tank. How many wires are there? Do the connectors need cleaning up? Not sure exactly what the wiring set up is for yours. In the old days you had two wires, one of which was earth. If the non-earth wire was disconnected then gauge would show empty, if the two wires were put together it would show full. The float in the tank that moves up and down might have got stiff. I do not know how easy it is to remove the fuel level unit on that car.

10 minutes ago, Arrivedcomic3 said:

Someone said the wires might be crossed

I assume that was a non-technical person. It is not possible to cross the wires, and even it it was the guage would still operate in the correct direction.

So the possibilities are: You have a faulty tank sender unit. You have damaged/corroded wiring. You have an instrument cluster fault.

It gets very expensive if you just guess, so I would get an auto-electrician to take a look unless you have a multimeter of your own and know how to use it.

19 minutes ago, unofix said:

I assume that was a non-technical person. It is not possible to cross the wires, and even it it was the guage would still operate in the correct direction.

So the possibilities are: You have a faulty tank sender unit. You have damaged/corroded wiring. You have an instrument cluster fault.

It gets very expensive if you just guess, so I would get an auto-electrician to take a look unless you have a multimeter of your own and know how to use it.

I have 4 wires coming out of the fuel tank

22 minutes ago, isetta said:

Check the wires on the fuel tank. How many wires are there? Do the connectors need cleaning up? Not sure exactly what the wiring set up is for yours. In the old days you had two wires, one of which was earth. If the non-earth wire was disconnected then gauge would show empty, if the two wires were put together it would show full. The float in the tank that moves up and down might have got stiff. I do not know how easy it is to remove the fuel level unit on that car.

I have 4 wires coming out of the fuel tank

4 hours ago, Arrivedcomic3 said:

have 4 wires coming out of the fuel tank

Yes that will be correct. Two wires for the fuel pump and the others for the fuel gauge 

7 hours ago, unofix said:

Yes that will be correct. Two wires for the fuel pump and the others for the fuel gauge 

Where is the ground wire.

Right. Time to use some logic. Car reads full when empty and with a ten pound fill it reads 3/4 full. I would add another ten pounds worth and get a third reading. Crossed wires cannot happen on their own and the wires go to a polarised connector. Not that then. The gauge moves so it is working. The signal it is receiving is wrong.

Normally the guage sender is a resistive track with a contact that slides over it, driven by a float. The lowest resistance usually sends the meter of the gauge to full scale so a break would normally cause a zero reading and as the float rises the gauge cuts back in but inaccurately. 

If the sender is connected to the input of an electronic module, the sense could be reversed which would give the symptoms you describe. 

I think it is a worn out sender and I would test it by disconnecting the  plug from the tank sender. If the gauge now reads full, then the sender is busted and you need another one.

55 minutes ago, anon said:

Right. Time to use some logic. Car reads full when empty and with a ten pound fill it reads 3/4 full. I would add another ten pounds worth and get a third reading. Crossed wires cannot happen on their own and the wires go to a polarised connector. Not that then. The gauge moves so it is working. The signal it is receiving is wrong.

Normally the guage sender is a resistive track with a contact that slides over it, driven by a float. The lowest resistance usually sends the meter of the gauge to full scale so a break would normally cause a zero reading and as the float rises the gauge cuts back in but inaccurately. 

If the sender is connected to the input of an electronic module, the sense could be reversed which would give the symptoms you describe. 

I think it is a worn out sender and I would test it by disconnecting the  plug from the tank sender. If the gauge now reads full, then the sender is busted and you need another one.

I disconnected the plug from the fuel tank (under rear seat) and it still says I have 3/4.

Then put in another £10, see what the gauge reads and repeat the test in case that is not the right plug because if it makes no difference, at all, I would be surprised. It should make a difference. Cycle the ignition  between tests in case the car's computer memorises the reading and only checks every few minutes.

24 minutes ago, anon said:

Then put in another £10, see what the gauge reads and repeat the test in case that is not the right plug because if it makes no difference, at all, I would be surprised. It should make a difference. Cycle the ignition  between tests in case the car's computer memorises the reading and only checks every few minutes.

That didn't work

15 minutes ago, Arrivedcomic3 said:

That didn't work, I might replace the sender unit.

 

If disconnecting the sender unit made no difference then the fault is NOT the sender unit.

Fault is possibly damage to the wiring or plug, which might be corrosion or you have a faulty instrument cluster.

1 hour ago, unofix said:

If disconnecting the sender unit made no difference then the fault is NOT the sender unit.

Fault is possibly damage to the wiring or plug, which might be corrosion or you have a faulty instrument cluster.

I don't think my instrument cluster is faulty

2 minutes ago, Arrivedcomic3 said:

I don't think my instrument cluster is faulty, will it show on a diagnostic machine?

 

I think you pulled the wrong plug.

29 minutes ago, anon said:

I think you pulled the wrong plug.

I disconnected the 5 pin plug, it had 4 wires coming out of it

Well that is the right plug so if there is no difference with it plugged in or not, it  seems likely that the sensor is open circuit. You could try removing it from the tank, ( cover the hole,) plugging it back in and operating the float to see if the needle twitches at any point. If it does, the gauge is in all probability good.

Unofix: think about it. If the sender is dead, unplugging it will make no difference so it could well be a fault in the sender

10 minutes ago, anon said:

Well that is the right plug so if there is no difference with it plugged in or not, it  seems likely that the sensor is open circuit. You could try removing it from the tank, ( cover the hole,) plugging it back in and operating the float to see if the needle twitches at any point. If it does, the gauge is in all probability good.

I'll try that tomorrow

This came up about a year ago. The suspicion then was that there was a poor earth to the tank module. Seeing that yours is pretty rusty, clean up the top with some Emery cloth and run a piece of wire to the battery earth from the cleaned area. See if it makes a difference.

5 minutes ago, anon said:

This came up about a year ago. The suspicion then was that there was a poor earth to the tank module. Seeing that yours is pretty rusty, clean up the top with some Emery cloth and run a piece of wire to the battery earth from the cleaned area. See if it makes a difference.

Where is the ground wire?

There may not be one. It may be just the metalwork being earthed through the chassis. That is why I would run in an earth to a cleaned area of the tank module. If that fixes it, take the module out and clean up both surfaces, assuming  the tank is metal  not plastic.