Fuel gauge. How does it work and why doesn't it go bang?

[ColinM50]

I bought a kit car last year and have done about 2,000 miles in it but the fuel gauge has never worked, it's always been one of those roundtuit jobs that got left and left. So over the weekend I finally got round to it and tried to fix it.

Tested the gauge and that's fine, but the wiring from the gauge to the tank sender wasn't connected. Tried connecting it but gauge still didn't work so took the tank off the car to access the sender which is on top of the tank.

Got it out and checked it with a meter and I reckon it's duff. No problem, went to a breakers yard yesterday and got another one off an old Sierra and I'm pretty sure that when I fit it tonight it'll work fine since the multi meter readings now make sense.

But it got me thinking. The sender unit, the thing that sits inside the fuel tank, is under petrol when the tank's full and is permanently exposed to fuel vapours. And this sender has open uninsulated 12v connections and wiring to both sides of the rheostat and a little PCB. So there's electricity flowing through the gauge at all times.

So how does that work then?

And more to the point why doesn't the petrol short it out? Or even more worrying, why doesn't the voltage cause the petrol to go whooomph?

[HellDiver]

ColinM50 said:

Or even more worrying, why doesn't the voltage cause the petrol to go whooomph?

Why would voltage make a liquid explode?

[jr6yam]

You need a spark to create a bang.
This is unlikely to happen with one side of the sender grounded

[Steve_D]

If you really want to worry then consider your daily driver which most likely has the pump in the tank. The petrol fills the pump workings and to an extent cools it. In some cases the fuel is actually pumped through the motor.

Steve

[Jack_and_MLE]

To ignite, the air(o2)/petrol mixture needs to be correct.
In a tank there is more fuel fume than 02 so it doesn't ignite.

Jack

[ColinM50]

Thanks for the info chaps and you've sort of put my mind at rest. So would it be fair to say the most risky time is when I've drained the fuel out of the tank and got the sender out and there's all sorts of airflow through the tank, that that'd be when I could have a problem? i.e. when there's a higher percentage of air in the tank?

Cor this DIY malarkey's all a bit worrying don't you think?

But thanks for the explanation chaps. If you hear a large kerboom from southern France, you'll know that it didn't all go according to plan.

[Steve_D]

ColinM50 said:

Thanks for the info chaps and you've sort of put my mind at rest. So would it be fair to say the most risky time is when I've drained the fuel out of the tank and got the sender out and there's all sorts of airflow through the tank, that that'd be when I could have a problem? i.e. when there's a higher percentage of air in the tank?

Cor this DIY malarkey's all a bit worrying don't you think?

But thanks for the explanation chaps. If you hear a large kerboom from southern France, you'll know that it didn't all go according to plan.

You won't have a problem when you take the sender out of the tank because you have already disconnected the battery.....you have disconnected the battery haven't you?

Steve