Thread: Any 12v guru's

Have you checked the gauge is earthed properly. Was it working before ?

Ean

Do NOT apply power to the fed wire from the tank sender,or you will damage the guage.

You need to think of the fuel guage as an OHM meter,as it is just indicating resistance,and the variable resistance is built into the sender unit.The wire is only transmitting that resistance to the guage.To check things properly you really need a variable resistor unit and connect it in place of the sender unit ,and see if the guage moves.But this being an older vehicle,with older style guages,you could do the "not really recommended" thing,and ground the wire to a good chassis earth,and see if the guage moves.If the guage still doesn't move,then you will need an ohm meter and need to check that the wire is not broken between the connection for the sender and the rear of the guage.

Originally Posted by Debacle

Have you checked the gauge is earthed properly. Was it working before ?

No it did not work before, so im starting from scratch.

The old sender was in a bad way so I needed to get a new pick-up anyway.

Assume it would be earthed under the dash?



Cheers Ean

Originally Posted by LowRanger

Ean

Do NOT apply power to the fed wire from the tank sender,or you will damage the guage.

You need to think of the fuel guage as an OHM meter,as it is just indicating resistance,and the variable resistance is built into the sender unit.The wire is only transmitting that resistance to the guage.To check things properly you really need a variable resistor unit and connect it in place of the sender unit ,and see if the guage moves.But this being an older vehicle,with older style guages,you could do the "not really recommended" thing,and ground the wire to a good chassis earth,and see if the guage moves.If the guage still doesn't move,then you will need an ohm meter and need to check that the wire is not broken between the connection for the sender and the rear of the guage.

Thanks for the info, if I ground the wire and the guage moves, then is it and earth issue or ???

Cheers Ean

Originally Posted by Ean Austral

Thanks for the info, if I ground the wire and the guage moves, then is it and earth issue or ???

Cheers Ean

If it moves,then the sender is at fault

I am not an auto elec but I know something about electronics.

The basics:

Analog gauges are all based on a current meter. They are effectively a motor and require current to get the needle to move. Depending on how these things are configured with resistance in series or parallel, they can be used as a current or volt meter. In order for them to be used as an ohm meter (to measure resistance) you need a power source and a return earth and the resistance you want to measure to be placed in the circuit to get a current to flow through the meter. If you by pass the sender (a variable resistor) you will get the meter to go full scale and bend the needle sort of speak. That is why you have been warned not to just earth the point on the meter that connects to the wire from the sender.

To test your sender use a multimeter and check what resistance values it provides. As long as it is not an open circuit or a short, there is a chance your sender is working. (multimeters have their own internal battery to generate a current to be able to measure resistance)

You have to think about the circuit, where is the power coming from? What is the path the current takes to provide a circuit to generate a current to get that needle on the gauge to move. Therefore if the gauge does not move when you connect everything up, it is either, no voltage source is present in your circuit, there is no earth to provide a return path in your circuit or there is a break in the circuit somewhere. The break is hopefully not in your sender or the fine wires in the gauge itself.

Good luck.

In a nutshell, the sender is easy to test. Connect a multimeter (resistance range, 0-200 ohms or similar) between the sender terminal and the sender base, manipulate the arm and see what resistance reading is obtained at the two extremes of full and empty. In between the resistance reading should vary smoothly without a break.

Then install the sender, test the resistance between the chassis and the sender base. If it is open circuit your petrol tank is not properly earthing to the chassis.

Then test the resistance between the gauge sender connector and the chassis, it should reflect your fuel level.

Then test the resistance of the sender wire at the gauge, compared to chassis. It should be nearly the same as at the tank end.

Once you have determined the sender is working you can test for 12 volts and earth at the gauge. If they are present and it still doesn't work, your gauge is likely stuffed.

Ford fuel gauges from about that time are 78 to 10 ohm - ie 78 ohms empty and 10 ohms full.

This setup I'm going to describe is from 10 years later (XC), but should still be the same:

The gauge itself is a bi-metallic strip with wire wound around it. The more current that flows thru it heats it up and it bends from E to F. It has 5V power supplied to it by a nearby resistive wire wound around another copper strip. After a while, the wire corrodes and may have broken - so no power to the gauge. ( Unlike a RRC etc, the instrument power reg is not a module but inside the dash cluster)

If the 5V supply is cactus, visit jaycar and look for a 5v regulator that can handle 3amp - I've had success with a USB charger for the cig lighter.

The rest you can check with a multimeter.