no fuel!

Just don't follow the AA's advice to 'carry a fuel can in case you have to go to get fuel' (or words to that effect). What's the point in an empty can?

Howdy Bristol LR owner :)

The sensor works on resistance, so if you can work out what the empty resistance is (it could be 0, I forget which way things go) then you could add extra resistance to the loop. Of course if it's decreasing resistance that's measured, this would cause you to think you had more fuel than you actually had, so would be bad :)

IIRC there's an ESSO at the top of blackboy hill, near the top by the downs, but nowt that comes to mind until you get out towards the cumberland basin (another esso iirc) or you're looking at the station at the bottom of the m32, asda in bedminster, or a texaco up the glos. road.

On or around Tue, 27 Jan 2009 22:19:04 GMT, SpamTrapSeeSig enlightened us thusly:

they work by luck. The gauge will show full for a given current reading and empty for another current reading, and the sender unit has a variable resistance. tolerances are such that mostly, full is full and empty is empty, but you never want to rely on 'em absolutely.

for example, the LDV does about 60 miles or more before the needle shifts off full, where the Range Rover shows half-tank when there's only about 35 litres gone from it, and IIRC the tank holds 90

The point about an empty fuel can is that it can't leak explosive petrol all over your car...

In article , Alex Threlfall writes

Quite. That's why I thought bending the gauge (to be sure) was a good idea, and a simple one. It's possible it was way off and I didn't bend it enough - I wonder, idly, if there's some sort of calibration procedure in the official manual when fitting replacement sensors.

Indeed there is. I was almost on fumes at that point, and couldn't turn across the traffic flow (it's a steep hill and all the precious drops were either in the pipes or at the back of the tank at that point).

I'm very lucky I didn't airlock it.

In article , Austin Shackles writes

I ought to calibrate it. It's evidently over-reading. I put 70 litres in, and it was way over the end stop (full), but the tank wasn't close to full to the brim.

Alex's idea may be the way forward. I've got to replace the plastics round the instruments soon, and that's an opportunity to have a play around to see how the reading might be "tuned" without molesting the tank.

IIRC zero (or very close to) resistance is full so adding resistance to the circuit should make it under-read.

It could be reduced by adding a resistor in parallel but that could make the linearity either worse or better, as the resistance of two resistors in parallel is calculated by the following;

.. so you can reduce the resistance by putting in a lower value resistor in parallel to the resistor you want to reduce the resistance of.

Hmm, not sure it would change the linearity at all really so might be worth trying. I'd suggest a variable pot of the same range as that in the tank, connected in parallel, then tweak it to suit.

It doesn't have to be a lower value resistor as even a higher value resistor connected in parallel will lower the resistance of the circuit by providing an additional path for the current to pass through.

Probably, trying to remember my electronics. IIRC two identical resistors in parallel would end up at half resistance. Either way it'd be a case of plotting it out on a spreadsheet or something to make sure that one resistor that doesn't change and the other that does (the one in the tank) would produce values that wouldn't be too outlandish. Might sound like a lot of work but you can do it while drinking beer inside rather than faffing about replacing the tank sender!

Yes I understand, but If EMB is right, I need to increase the resistance (so that it looks more empty on the gauge than it really is). So a low value series resistor would probably suffice, or a network, to give better control. If it's only driving the meter it's low current, and 1/2 watt ought to be fine.

The fact it can go 'beyond' full indicates the meter is faulty though. I assume it's a simple, moving-iron type (for cheapness). I have had those cans apart in the past to make repairs - not easy but do-able. It may be possible to adjust the hairsprings. I feel just replacing the meter isn't in the spirit of the thing!

Just found these pages:

(MG gauge, but may be similar to a Landy one).

and this was good:

(several variations on a theme).

If resistance is too low then yes, but if resistance is too high then parallel resistors will work, it was someone saying that you can't reduce resistance that lead us off on this parallel resistance side-track, potentially needlessly as usual ;-)

I don't know what vehicle you've got, but most of the ones I've worked on have a voltage regulator feeding the gauges. When this fails, all the gauges read higher than they should, with the fuel gauge reading well above full when the tank's full. On my Series 2, it's a small corroded looking metal box with 2 wires connecting to it, sitting behind the dash, but I've seen them in the engine bay on other vehicles.

Borrowed from the North American MG register site:-

"VOLTAGE STABILISER: The voltage stabiliser powers two instruments at the most: Petrol gauge and Temperature gauge. Test this unit by inserting your 12v test light into the GREEN/BLUE temperature transmitter wire at the front of the engine. With the key ON and the test light earthed, the light will glow for about 20 seconds then begin winking ON OFF ON OFF like a slow turn signal. The stabiliser rarely fails but sometimes comes loose, which causes these instruments to read high."

Liquid petrol isn't explosive but the vapours in an "empty" can are!!

I've never managed to get liquid petrol without fumes.

Perhaps you'll tell me how to manage that...

In article , John Williamson writes

Good thinking that man!

I'm morally certain there is such a beast behind the dash somewhere. I've seen an odd box there. It might also explain bouncy volts indication when the flashers are on (despite a suitable qty. of revs.

Will check when weather improves sufficiently.

On Wed, 28 Jan 2009 23:02:31 +0530, "William Black" wrote: .

You should always fill a petrol can to the top, or leave the can empty with the cap off or loose so any expanding vapour can escape. A half-full can is the most dangerous of all.

Diesel - doesn't matter a damn

Alex

So that when you have to walk to the nearest petrol station, you don't have to *buy* a bloody can as well. There was a time when a kindly soul would give you an old gallon oil can, or even lend you a petrol can on a promise of returning it (it's happened to me) but I think those days are long gone.

Check for a dodgy earth connection behind the dash, in that case. Loose fixing screw, dirt behind the connector or similar are common problems. The voltmeter and flashers are quite possibly earthed to the same point as the stabiliser.