Fuel gauge not registering properly

[Barry]

The fuel gauge on my V12 struggles to exceed a reading between half and three quarters. This morning the gauge measured zero, then slowly picked up above half a tank, when clearly it should have been reading full. Is this the float in the tank, or the gauge itself?, or something else?

[SRK]

My bet would be the sender has some corrosion on it. I'd pull it from the tank and check it out. Just by cleaning it a little you should get a few more years.

Steve

[Barry]

Thanks Steve, I'll try that when I get back home.

[Barry]

Mystery! After the gauge was reading as per my first post, I take the float and sender unit out of the tank. Put new unit in. Gauge then reads full and some with probably 3/4 tank full. Take out new unit, put old unit back in the tank but with a new float. Same again, the unit reads "over" full. Wiring fine, and as it should be connected to the correct terminals. Any ideas what is going on? Thanks.

[Series1 Stu]

Try filling your tank up to rated capacity (should still be below the sender mounting flange) and then tweek the float arm to give the correct full reading on the gauge.

While you're playing with it, check the gauge reading when the tank is nearly empty. It might keep you off the hard shoulder at some point.

Regards

[Barry]

Thanks Stuart, I'll give it a try!

[lundabo]

The fuel gauge on my V12 struggles to exceed a reading between half and three quarters. This morning the gauge measured zero, then slowly picked up above half a tank, when clearly it should have been reading full. Is this the float in the tank, or the gauge itself?, or something else?

This is total speculation on my part (I'm not an electrics guy), but perhaps this is an instrument voltage regulator issue? The fuel and temperature gauges on my V12 occasionally show less than what they actually should be, and I'm told the culprit is a balky IVR. Just a thought, but feel free to discard or verify with others more knowledgeable. You'll see that multiple providers sell "upgraded" IVRs due to the regularity of problems with this part.

[Barry]

Well, the conclusion has been most bizarre. I took the sender unit out of the tank, connected it up, and it worked perfectly! Put the sender back in the tank, but as soon as I secured the first screw, the gauge changed to reading full as far as it can go! Take the screw out, and it went back to reading half. I ended up putting a gasket not just between the sender and the tank but also on top of the sender before screwing in the six retaining screws. Perfect, so somewhere there is an earthing problem......anyway, if it works ok with the double gasket solution, I'll leave it......

[jagwit]

but as soon as I secured the first screw, the gauge changed to reading full as far as it can go!

It seems to me that you have the two wires being connected to the sender the wrong way round. The one wire should earth the "chassis" of the sender. The other wire goes to the gauge. With what you are describing this then results in the wire going to the gauge being earthed as soon as you put the first screw in and hence the gauge speeds to the top.

[Barry]

Thanks, but I have tried different wiring configurations. I took note of all the colours before I disconnected. Green is obviously earth, and there is only one earth terminal. The other two wires, it does not matter which way round they are connected.......I appreciate everyone's help in this.

[mark10337]

are you sure about that? According to the wiring diagram, White-Green will go to the low fuel lamp, Green-Black will go to the fuel gauge and usually a black wire goes to ground.

[Barry]

One way of finding out.........!

[MarekH]

So what you are saying is that you have randomly connected up all of the wiring because it is obvious and then reported that something that you haven't touched..... doesn't work properly.

The correct way to proceed is to make a truth table.

You have described three wires and a chassis connection, so make a 4x4 table out of this and put an ohmmeter across each of the sixteen possible connections. Now operate the float arm from full to empty each time and record the results.

You will now easily deduce whether green is indeed connected to the chassis and also see what the other circuits are and how they operate.

One of the combinations will operate over a large range of about 40-240ohms and be insulated from earth; the other will operate more like a switch, i.e. over a smaller ohm range with earth/chasis as one of its connections.

You can then look at the circuit diagram and see how that correlates to the expected routing of the wiring colours.

kind regards
Marek

[Barry]

Seems like a plan. Thanks Marek

[cunpr]

Dear All,
Resurrecting this thread rather than starting afresh.
How accurate should the fuel gauge be? (S3)
My fuel gauge registers incorrectly, never reads full when it should and within 20 mins will be down to 3/4 empty. The reserve tank light does work. Having looked at the unit I believe both gauge and sender to be the original 1973 units.
Having read all the above apart from checking the float, earth (which is correct), the sender for corrosion and the 40 -240 ohm range is there anything else I can do? All teh other gauges appear to work normally so i don't think its a voltage supply issue.

[lowact]

How is yr tank vented, could there be air trapped and the gauge reading correctly?

[cunpr]

Dear Lowact,
Its vented correctly, judging by the petrol fumes I get in and around the car when she's full!

[lowact]

Sounds like not, you wouldn't get those fumes if it was properly vented. Do you still have your expansion tank and charcoal canister? With the filler cap removed, can you blow back through the vent pipe into the tank? With the tank fully drained, how much fuel can you put in before it thinks it's full?

[MarekH]

could there be air trapped and the gauge reading correctly?

No.

If your temperature gauge appears to read correctly in normal usage, swap the feed from the fuel sender with the feed to the temperature gauge.

If the problem persists with the fuel gauge, then the fuel gauge at the dash is where the problem is. If the problem transfers to the temperature gauge, then the problem is in the electrical circuit starting with that fuel gauge wiring, through the sender and it's earth back to the chassis.

Almost certainly the problem is 240-30 ohm range reading incorrectly inside the sender. This is because you've told us that the fuel low light works. That means the float was working and it also means the earth return connection was good.

I expect your fuel sender is corroded inside and either has a very poor connection due to sediment stuck to the insides of the sender OR the wires inside that part of the sender are so corroded that they have disintegrated. Either way, I'd be pulling the float out and opening up the tabs on the metal box to see why the resistance doesn't vary from 240-30 ohms as the float arm is moved over the full to empty sweep.

kind regards
Marek

[cunpr]

Dear All,
Thanks for responses, hints and tips, weekend's work clearly identified. Colin, mine is a cinamon brown as well! Expansion tank, didn't know about that, i'll investigate.