Fuel Gauge Niggle

[almyland]

Having stripped out my old dash and nearly finished a smart (I think) new one, I thought I'd try and solve a niggle that has been with the car since I bought it. That is, the fuel guage works back to front. When the tank is full, the gauge reads empty and vice versa. There are three wires attached to the rear of the gauge (a UBO model cira 1986) all the same attractive shade of purple - not very helpful. One is clearly an earth. Is it simply a case of reversing the other two leads or is it likely to be more complicated?

Thanks

Alan

[martinclan]

Re: Fuel Gauge Niggle

I doubt if reversing the polarity will make any difference. I think the gauge is not compatible with the sensor.

The way fuel gauges basically work is this:

The sensor is a variable resistor activated by the float. In other words the voltage from it depends on the fuel level. The gauge just measures that voltage. What you seem to have is a sensor that works one way (for example empty = min volts, full = max volts) and a gauge that works the other way round.

The best solution is to replace the sensor and gauge with a matched pair - or just carry on reading it back to front!

Robin

[dogoncrazy]

Re: Fuel Gauge Niggle

Hi Alan
I'm not familiar with the make of gauge you have but from information I've come across some gauges do have three terminals, one being 'ground' one from the live ignition and the third being the 'sender' from the tank .Swapping the two feeds wouldn't actually be reversing the polarity so may be worth a try.

[keith_h]

Re: Fuel Gauge Niggle

Take a look at the thread "Matching fuel guage to sender" in the Miscellaneous category. You might end up being enlightened or confused - a lot of it boggled me. Not that I knew but from the content it seems fuel gauges are more likely to be ammeters than voltmeters but thats not to say yours isn't a voltmeter

[almyland]

Re: Fuel Gauge Niggle

Thanks for your thoughts. I've confirmed that the gauge wiring is correct so it does rather look like a mismatch between the gauge and the sender. I'll keep working the issue until I get fed-up then probably accept the status quo. Good news is the new dash panel is in and lookin not too bad.

Cheer Alaan

[macdonald.dm]

Re: Fuel Gauge Niggle

Hi Alan.

Are you up to the job of removing the tank sender and having a look-see?!

I don't know what car your tank/sender has come from, but if it's an 'oldie' then the sender is likely to be a straight-forward coil of resistance wire with a slider running along it connected to a float. I'm guessing one end of this coil goes to 'earth' - ie: the metal body of the sender unit itself. If you could detach this wire and instead connect the OTHER end of the coil to earth, in theory your sender should then work 'in reverse'! You'd also have to check that the area 'covered' by the slider is similar - in reverse. Ie: if the physical stop is, say, one inch from one end and a half inch from t'other, then I guess this should ideally also be reversed to suit.

If you think this is a goer, it might be worth trying to obtain a second sender to modify rather than risk bodging your existing one.

Finally, remember you are working with (a) petrol and (b) electrickery - not a good combination (unless inside a cylinder). I can't help feeling a bit worried about a resistance wire being located inside a tank full of petrol - I mean, what would happen if you shorted the gauge wires - would this heat up the sender? Or am I chust being paranoid...

(Basically, only do this if you think "Yeah, no problem!&quot

[macdonald.dm]

Re: Fuel Gauge Niggle

Oh, any chance of a photie of your dash?! Ta much.

[macdonald.dm]

Re: Fuel Gauge Niggle

Oh, oh, I guess it's possible that the sender coil is connected to the sender terminal instead, so that it's the float/slider that's connected to 'earth'. Same would apply tho' - disconnect one end of the coil and connect t'other.

[almyland]

Re: Fuel Gauge Niggle

Hi Guys and thanks for the continuing helpful advice. However, for once I can report back a result; I've fixed it! Actually, I fixed it with the help of a very nice chap called John Gare who workks for ETB Intruments in Southend on Sea; promised I'd give then a strong recommendation because he was outstandingly helpful. Working on his advice I confirmed that I did indeed have a matched set of VDO fuel gauge and VDO sender. Next I checked the resistance range of the sender which turned out to be 180 - 3 ohms; in my case 180 when empty - should be the other way around. Anyway, it seems that many kit car builders using this set-up build the sender incorrectly as in my case. I can understand how people might get it wrong because it just looks plain right - but it ain't. All I had to do was release the float lever arm and reverse it in its housing, tighten the screw and 'Bob's your aunties husband'. There might be an ongoing minor niggle in that when the float arm is reversed it ends up being about 1.5cm short so the float will read empty slightly earlier than previously - but then it was reading full!!! Dooogh! One problem solved, on to the next.

As to the the dash Donnie, I'll certainly take a picture of it and post it for you.

Cheers Alan

[macdonald.dm]

Re: Fuel Gauge Niggle

Result!

What are the chances, eh?! Doesn't happen often that such an easy solution comes along. Great news.