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Fuel Sender Resistance Values - (Read 2,198 Times)
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ribbit
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Post Icon Posted: Nov 29th, 2009 at 04:19 pm

Am doing some work on my fuel gauge on my frog at the moment. Does anyone know the approximate resistance of the fuel sender when the tank is full and when it is empty.

Cheers R.
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Post Icon Posted: Nov 29th, 2009 at 04:28 pm

If it's like the standard BMC one it will be about 20 ohms full and 220 empty
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Post Icon Posted: Nov 30th, 2009 at 04:15 am

I think I must have my wires crossed as I am reading 1.3k ohms with the tank fairly full.

It is wired through the ignition and I am trying to measure the resistance with ignition off - maybe I should be trying with it on as I think the off position actually grounds the supply rather than just being N/C.

I've just done the maths and at 20 ohms, with a 60 ohm gauge in series would only draw 0.15A - what I am reading would draw 9mA, less than a 10th of the current.

I'm really puzzled now! Smiley Back to the circuit diagram and the vain hope that the previous owner stuck to it!

Cheers R.
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Post Icon Posted: Nov 30th, 2009 at 06:21 am

Quote:
I'm really puzzled now!


Use the Force, young Jedi, use the Force Smiley
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PAUL R
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Post Icon Posted: Nov 30th, 2009 at 07:40 am

This might help

http://www.mgaguru.com/mgtech/electric/fg104.htm

Then again !

The bottom section of the read, refers to the resistance on the clock.

But surely there should be no resistance in the cable, so the readings would be the same at the clock as the sender unit,

If you see my logic.

« Last Edited by PAUL R Nov 30th, 2009 at 07:45 am »
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Post Icon Posted: Nov 30th, 2009 at 07:47 am

Quote:
This might help


...and this http://www.mgb-stuff.org.uk/fuel.htm

... or not!
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Post Icon Posted: Nov 30th, 2009 at 08:37 am

Thanks both. Paul's MGA link describes a gauge testing setup similar to what I tried to emulate, only without the soldering and by using a potentiometer in conjunction with a digital multimeter. Smiley

The MGA fuel gauge looks to be completely different operation though - it has three coils versus the one in mine. Mine looks like it can be calibrated by altering the angle of the spring inside as the two backplates can be rotated through a small arc.

Ian, I'm afraid mgb-stuff is currently offline and not even Google cache is helping me see it either - might have to try the timemachine portal later and see if I can see the site from a month or two back.

I will have another look at the resistance values of the sender (I am measuring from the gauge as getting the sender out requires dropping the tank) and see if I can trace the wiring as it wasn't clear which of the two wires at the gauge went to the sender - I assumed it would be the one that went to ground (-ve earth) - but considering the way the horn is wired it could well be the other way round! At least I now know I am looking for something in the 0-200 ohm range.

Cheers R.
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Post Icon Posted: Nov 30th, 2009 at 10:33 am

Hi R

Is it true to say that your frog has been wired up total as a negitive earth car, using all the later componants,
Like the Alternator and all the later clocks

Or

Has the car has been conveted to negitive earth, just for the use of an alternator, and still using frog period componants.
Which originaly were +ve earth but just been run negitvely.

The reason i ask is because the way you describe the fuel gauge, it appears to be the later one that was used in the negitively earthed cars.

Paul



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Post Icon Posted: Nov 30th, 2009 at 12:12 pm

Thanks, Paul. I think they are mainly later components as it also had an engine swap and seems to have been rebuilt with the alternator and electronic ignition in mind, but difficult to tell whether some of the choices were deliberate, incompetent or just what the guy that restored it could get at the price he was willing to pay.

The rev counter had been modified internally from positive to negative earth - but it was from a later car anyway - I've now done it properly with a true negative earth type suitable for electronic ignition (I am sure you saw the thread).

The fuel gauge is not the original frog part number (it doesn't tie up with the ones listed in the Moss catalogue anyway). Difficult to tell what it is off, but it doesn't seem to matter which way round you connect it, you get the same reading, so I guess it isn't polarity sensitive. This is what it looks like:



Sorry the part number is obscured, but I can't find another photo with it on - I will go and check it tomorrow. Wish I'd taken some of the insides as well now.

Some other things still don't seem quite right either, like the horn, which is simply being run the wrong way round (the switch is on the wrong side of the horn if you were doing it properly as it leaves a permanent live under the bonnet - it is at least fused). I have a suspicion that the wiper motor is slightly later too, as again the numbers don't quite tie up, but it operates in the same way as the original. Headlights are also much later (and brighter!) and have MGB clip on chrome rings on them.

The more I look, the lower I seem to get in the originality stakes. Smiley Smiley Smiley

Cheers R.
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Post Icon Posted: Nov 30th, 2009 at 12:50 pm

Hi R

It may sound trivial but now i understand what has hapeened,it makes it a little bit more easier to look for things that are more relivent.

Like you said regarding the fuel gauge,
The one I was showning worked electro magneticly, and the one your showning works off resitance (that is inside the gauge),
Thats why there are no erratic movements in the fuel needdle when going round corners, sloshing the fuel around in the tank,which means they are totaly different, as you are no doubt your aware.

Yes the fuel sender arm will be the same and the way it reads should be the same.

This seems along way off from the original question Smiley
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Post Icon Posted: Nov 30th, 2009 at 12:52 pm

Quick update - mgb-stuff is back up and running again - the internals of my gauge are almost identical to this one (thanks Ian Smiley ):

http://www.mgb-stuff.org.uk/fuel.htm

So it is a bimetal strip and uses heat to create the rotational movement - explains why it doesn't care which way round it is connected - you get the same heating regardless of which way the current flows. Smiley

Just seen your post, Paul. Yes, you are quite right. The resistance change in the sender will control the heating in the gauge and thus the moment. It is nice and stable, if only it read right!

Cheers R.

« Last Edited by ribbit Nov 30th, 2009 at 12:55 pm »
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Post Icon Posted: Nov 30th, 2009 at 01:53 pm

Could this be the link that does it ?

http://www.mgexperience.net/article/fuel-sender-adjust.html
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Post Icon Posted: Dec 1st, 2009 at 03:27 am

Thanks Paul. I think I have enough to go on now. It will probably be the weekend before I can get another look at the car - pesky work getting in the way of cars again!

Thanks R.
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Post Icon Posted: Dec 14th, 2009 at 01:38 pm

Quick update. I managed to use some fuel out of the car and recheck the resistance values weekend before last and have then been too busy running around all over the place to post up the results.

First reading with tank fairly full: 36.5 ohms
Second reading after about 50 miles: 51.3 ohms

The gauge has dropped approx 2 mm between the two, but is still resolutely reading almost full. I need to drive it more.

My previous reading of 1.135 kilo-ohms was the ignition switch side which apparently grounds out via a resistance when in the off position. When I remove the fuse, this goes to open circuit.

My fuel gauge is marked Smiths BF 2300/02. I'll do a bit of detective work when I have time and find out what it is off.

Next step is to see if I can drain the tank right down and see what reading I get when the tank is empty and again when the tank is full. I can then hopefully have a go at recalibrating it or possibly I might buy another off ebay and then I can play to my hearts content.

Thanks again, R.
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Post Icon Posted: Dec 14th, 2009 at 03:55 pm

Just a thought,
If you have the original tank fitted with the drain plug,

Drain the fuel out of the tank, take a reading,and as it is a 6 gallon tank,fill acordingly and take the resistance readings from the sender unit.

Just another take on things if you have the original tank fitted. Smiley
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