Thread: Fuel Gauge says full at 30% and up.

If this happened when you installed the LED lights, I'm guessing that you disturbed the earth to the gauge, and it is no longer properly earthed. It is possible that the earth was already non-functional, but the gauge was "sort of" earthed via the instrument light bulbs and parking light bulbs - but this circuit is inoperative with LED lights.

John

so i'll pull it open again and look for the earth wire on the fuel gauge.
whats that about not being operative with LED's?
wouldn't everyone who put in LED's be having the same problem?

Originally Posted by gmeddy

so i'll pull it open again and look for the earth wire on the fuel gauge.
whats that about not being operative with LED's?
wouldn't everyone who put in LED's be having the same problem?

It's probably not the LED's themselves, but rather you physically disturbed *something* when you were fiddling around behind the dash. The fuel gauge has three connections, positive 12V, negative, and a third connection from the fuel sender in the tank. The sender line has a resistance between it and ground, depending on the amount of fuel in the tank, and this is compared against the 12V and negative coming into the gauge. Check for obvious things first, such as a connection that was pulled loose during the LED install, a short between terminals, etc. If there is nothing obvious, then check the actual lines into the gauge - all three are easy to check with a multimeter.

Positive: take a voltage reading between the 12v line into the back of the gauge, and the negative terminal of the battery. Should read in and around 12v.

Negative: Get a resistance reading (or continuity) between the negative into the back of the gauge, and the negative terminal of the battery. Should read 0 Ohms, or continuity.

Sender: Get a resistance reading between the sender line into the back of the gauge, and the negative terminal of the battery. Should read somewhere between 10 Ohms and 360 Ohms, depending on the level of fuel in the tank.

I get a good 12v at +
I get 9ohms resistance between the battery and the gauge's negative.
From sender to Neg I get 7.5 ohms resistance and I've done 300kms out of the tank - So it's probably about 30ltrs down from full.

I've checked the connections as far as I can see behind there, and they are all well attached. I dont know where the Neg wires go once they join up the big loom, so i can't go to their source of earth.

Any suggestions on a way to get a better earth if thats the problem?

Thanks for the help mike_ie

The gauge needs to be earthed - it actually has three connections, one to the battery, one to the sender, and, via its mounting bolt, to the earth. If the earth connection is poor, the gauge may have earthed through the bulbs of the gauges and the parking lights. The LED lamps offer a much higher resistance, hence the changed reading.

The most likely problems with the earth are either at the actual gauge or between the chassis and the bulkhead (which is where the instruments are earthed to). Measure the voltage between the mounting bolt of the gauge and the battery negative (with ignition on) - should be zero.

Hope this helps.

John

John is correct, the third connection (earth) is through the metal body of the gauge. If in doubt, you can run a ground wire up to the mounting bolt for the gauge.

Originally Posted by gmeddy

From sender to Neg I get 7.5 ohms resistance and I've done 300kms out of the tank - So it's probably about 30ltrs down from full.

This is interesting - I would have expected a much greater resistance than that for a tank that is about half full.....

To test Jon's theory, take out the LED bulb completely and see what readings the gauge gives you.

Originally Posted by mike_ie

John is correct, the third connection (earth) is through the metal body of the gauge. If in doubt, you can run a ground wire up to the mounting bolt for the gauge.




This is interesting - I would have expected a much greater resistance than that for a tank that is about half full.....

To test Jon's theory, take out the LED bulb completely and see what readings the gauge gives you.

Should make little difference - replace with the original bulb would do it. But much simpler just to measure the voltage as indicated.

John

With a 7.5 Ohm resistance between sender wire and ground (I'm assuming that you actually pulled the wires off the back of the gauge before taking your readings ) I'm beginning to suspect that the problem may be a short in the sender wire - a 7.5 Ohm resistance wold make the gauge read full.

What model defender do you have? If it's a tdi then go underneath the rear passenger side wheel arch and find the sender on the side of the tank. Follow the two wires until you come to a plug in the line, and unplug it. Take a resistance reading between both terminals on the plug (i.e. directly from the sender) and see what it reads...