fuel sender - input voltage?

[flaps7]

Having thought that changing the fuel tank was the difficult bit; oh no it wasn't. Fuel gauge now playing up. Before I bore everyone with what I have done and seek even more help is anyone able to tell me what the input voltage to the sender should be. I am measuring 4v.

Rick

[Jonathan Kay]

Disconnect the feed and then measure the voltage (above earth) at the end of the lead.

With the lead disconnected measure the resistance from the sender terminal to earth. And how full is the tank?

Jonathan

[Nigel Fox]

When carrying out the open circuit test described by Jonathan you should see battery voltage.

When the circuit is made the voltage at the sender will change with fuel level if the circuit is healthy. Ie there is no specific answer to the question.

[elie boone]

As you replaced the fuel tank, make sure you have a good earth at the tank

[Nigel Riches]

Hi Rick. 

Four volts is rather low, make sure all the terminals are tight and free of corrosion, if found good check the following. If you have a thermal type voltage stabiliser they are prone to high resistance at the internal contacts, there should be very low resistance between the two terminals of the regulator, this might cause low output voltage. 

The voltage at the sender terminal will be dependent on the fuel sender resistance, if you are measuring at that point. 

The voltage to the fuel and temperature gauges is derived from the instrument voltage regulator, vehicle system voltage is dropped to something between 6 - 8 volts to allow for variation in system voltage. 

So, the voltage comes from the vehicle supply onto the instrument regulator, then to the instrument thermal winding, and on to the fuel sender resistance or temperature sensors resistance. 

As fuel level changes, the resistance changes, and thus the voltage at the fuel sender terminal will change, and the current, which causes the pointer to move across the indicator scale will be inversely proportional, and thus with little fuel there will be max resistance, very little current flow through the instrument coil, very little pointer deflection. 

To test a thermal gauge do the following. 

Disconnect the wire at the sender unit. 

Switch on the ignition, the gauge should read empty. 

Touch the sender wire to a good ground, the instrument pointer should move to the "full"  position.

Don't leave the earth connected to long as this will damage the gauge. 

If the test above failed, either the gauge or wiring is knackered. 

Check out the fuses, (yours are probably ok) check the voltage regulator and the voltage regulator earth, check for volts at both instrument terminals and the disconnected sender  wire, if the forgoing are good, check the sender earth, ie, resistance from sender +ve to the earth terminal. 

And of course yours maybe a magnetic system. A bit different to the above, as it will test in the opposite sense. 

Disconnect sender wire at the tank. 

Switch ignition on, gauge should show full. 

Touch sender wire supply wire to ground, gauge should show empty.

If it passes the above checks test sender earth, sender resistance and wiring. 

Hope this helps, or adds to the confusion, there are also capacitance type indicator circuits. 

Regards, Nigel. 

 

 

 

 

[flaps7]

Thanks for all the advice and the guide to checking. Replaced a couple of dodgy terminals after testing guages as described by Nigel and reading now seems to be OK.

Rick