Dodgy fuel gauge

[andyredtop83]

On my mk1 xe i have GTE clocks which usually behave themselves but sometimes the petrol gauge goes up. It always goes to where it should when i start the car but sometimes after a couple of minutes it always reads about 1/4 tank more than what i actually have. It usually comes back down to normal after a while. Is this likely to be the gauge itself or the sender unit or anything else???

[GGs GTE]

Sounds to me its the sender from the tank take the plug off and sand the spades on it and give the plug a good look and if it looks like its got crap inside it get a small flat head screw driver give it a good clean then blow all the crap out of the plug be for you put it back oh and where some protective goggles for your eyes hope this helps gaz

[andyredtop83]

Thanks for that i'll give it a shot.

[mikey093]

Mine does the same. 9 times out of 10 it shows empty and the only time the needle makes movment is when parked uphill

[Steve]

i'd say its the clocks, my yellow six dials work fine but i put a set of gte clocks in they read fine for a bit but then they kept telling me im had 3/4 of a tank!

[andyredtop83]

i'd say its the clocks, my yellow six dials work fine but i put a set of gte clocks in they read fine for a bit but then they kept telling me im had 3/4 of a tank!

When the clocks were in my GTE they were fine but now there in here they playing up, the clocks are in good condition and the printed circuit board on the back seems to be ok as well

[Chay84]

My Sri did that also changed the clocks and it resolved the problem

[mk3 vinny]

i have the same problem with mine and its the tank sender thats at fault only reads over 1/2 a tank when you get to 1/2 a tank the needle drops off the gauge iv had the tank sender out and it is that at fault is i have also chainged the clocks with the same problem. However you cannot buy tank senders anymore any more well for the mk1 anyway.

[waggygsi]

i bought a brand new sender an mine still dosent work

[mowgli]

the circuit for the gauge is dead simple. its called a potential divider
http://upload.wikimedia.org/wikipedia/en/f/f4/Impedance_Voltage_divider.png


there is a fixed resistor [Z1] in series with the variable resistor[Z2] on the sender. the voltage is measured between Vin & Vout, and this is what appears on the gauge.

when the level is low, the sender is low & its resistance is high, the Vin-Vout voltage is low, so the gauge reads low.
&
when the level is high, the sender is high & its resistance is low, the Vin-Vout voltage is high, and of course the gauge reads high

if the variable resistor wears out, the resistor material will be missing & there can be spots where the level reading will drop off to empty. this indicates a knackered sender.

your best method of testing the sender is to take it out & test its resistance with a cheap digital multimeter set on Ohms.

a quick way to test the gauge is to short the sender wire to the body of the car & if the gauge reads full, its ok.

the sender wire runs from the dash under the carpets to under the back seat, then thru a hole in the floor panel. this wire can often get squashed & short out.