a month ago my fuel gauge stopped giving reading, found the problem.... a loose wire to the sender, i replaced the spade connector and reconnected the wiring, everything was fine

yesterday the gauge stopped reading again

i checked the wiring at the sender again and even replaced the spade connector just incase even though it looked fine

as i found out.... this didnt fix the problem

all i was doing was letting the car warm up, during warm up the gauge was fine... reading half a tank

i revved it once and looked at the gauge and it had dropped to zero.

filled with 15 litres just incase it was a sticky sender or fuel gauge.. no change

any ideas as most of you will know, knowing how much juice you have is kinda important in a bad mpg daily driver haha

thanks

sam

Have someone observe the gauge when you ground the wire. It should move rather rapidly to FULL. If not fault trace back to the gauge and thence to the voltage stabilizer. Does the temp gauge work?

Jack

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---------------------------------------------------- Services to Interesting British Automobiles-- www.jackscars.net ----------------------------------------------------------------
-------------I like to think that I don't service automobiles so much as I provide entertainment, fulfill dreams and revive wonderful memories --------------

I am in the northwestern North Carolina mountains, right up against the Blue Ridge Parkway. Lawn chairs are in the shop and always time to talk. Drop in any time.

all other gauges work

A couple of things you can try...
(a) Using a hand held Volt/Ohm meter, you can 'measure' the resistance at the sender vs. ground and it *should be* (whilst your tank is 'full') 0-ohms -- meaning the meter is dropping the entire "stabilized"-12 volts across and so *should show* full.

(b) If the sending unit is not being properly grounded, your meter (particularly if its Digital) will move around a bit away from absolute 0-ohms --- this means that the "circuit" is never being completed at the gauge and so will always show "empty."

(c) If you can get a 12-volt battery (or a 3-cell LiPo offers 11.1v) if you put that voltage across the Gauge it should go to FULL (BTW, please please please be careful if you use a LiPo as it *will* dump current like no tomorrow should you short the wires during your experimenting.)

(d) WIth the sender re-connected, you should read a "stabilized"-12 volts between the (disconnected) green-pair wire from the one-side of the gauge and the single green(/black?) disconnected coming to the "other side" of the gauge when the key is turned.

My guess is that ONE of these *must* fail indicating where the problems lies.

Thanks I will have a look into it

Sam

Sam,

Recently went through the same scenario as you. It turned out to be the ballast resistor located on the steel bulkhead behind the dash. A simple small gizzmo that required removing the rev counter to remove. So unless you've got really small fingers, prepare yourself for a not so fun adventure.

Ray

I have a fuel gauge problem with my 74MGB .The gauge never shows more than about 5/8 full. changed the stabiliser-no change.

I never let the gauge reading drop below 1/3 full just in case. Does anyone have a fix for this. Keith in Ontario

Ray what did you have to do with it?

Thanks

Sam,

I had to remove ons small sheet metal screw holding it in place, plus 3 wires attached and let the little bugger drop into my other hand, then replace it with new. The new, instead of mechanical, has tiny electronic parts, less likely to fail in the future - but like so many things was very fiddley to get at. I would suggest a careful observation of which posts the wires are attached too. The new replacement I put in was oriented 180 degrees from the original. The whole bit is no more than 1 1/2" by 3/4". It was nice to see the fuel gauge work properly again. Good luck!

Ray

There issue can arise from (at least) 2 sources...

(a) The SENDING UNIT in the tank needs to be recalibrated --- this involves taking it out of the tank ... Best done after getting it to be as low as you dare, and gas will still come pouring out when you crack off the reatiner ring But here's a trick that I read in this glorious forum : Burn off as much gas as you can -- DRIVING IT SILLY! But then put the passenger's side up on the car-jack which will slosh the gas over the to the far side of the tank! Absolutely Brilliant !!

(b) There is *some* small adjustment on the gauge itself to pull it up to full when you KNOW its full. --- there is a straight-edge screwdriver slot on the back --- but you *do not* "turn it" --- you actually pull to one side or the other whilst the screwdriver blade is in....Thats why the slot looks a bit wide.

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regards,
tony



Edited 1 time(s). Last edit at 07/15/2012 12:22PM by tob1954.

Thanks Tony-it has given me some things to ponder. Incidentally I have recently sent away for "new improved" fuel sender from MGOC spares and will see what it reads before attempting to install. As for gauge itself -will leave as last resort.

Keith -- there is another thread in here somewhere that describes the actual OHM-age-ness you should read at FULL UP and FULL DOWN on the sender's arm.

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regards,
tony

Here ya go...


http://www.mgexperience.net/article/fuel-sender-adjust.html

http://www.mgexperience.net/article/gas-gauge-adjust.html

Jack

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---------------------------------------------------- Services to Interesting British Automobiles-- www.jackscars.net ----------------------------------------------------------------
-------------I like to think that I don't service automobiles so much as I provide entertainment, fulfill dreams and revive wonderful memories --------------

I am in the northwestern North Carolina mountains, right up against the Blue Ridge Parkway. Lawn chairs are in the shop and always time to talk. Drop in any time.

Dear Jack and everyone that contributed to my fuel gauge question-my thanks to all-now I am a little wiser and able to at least prepare for the task at hand.Keith in Ontario.
ps 95% complete on getting my 1978 MGB GT on the road after importing from blighty Nov 2010.Car was almost untouched and a joy to restore

Ray, I was trying to figure out what you meant by ballast resistor on the bulkhead, since there is no such thing. You were describing the stabilizer that powers the gauges. Since all the other gauges work (in most cases the stabilizer runs the temp and fuel gauges only) it is most likely that the stabilizer is good. If you have an early car only the fuel gauge will be on the stabilizer.

Do the test that Jack posted. If it works then add a ground to the sending unit.

Right... I got my gauge to read full so its not the gauge.. Tried to earth it and no change... Any ideas?

Thanks sam

Okay Sam...

So, you got the gauge to read "full". What did you do to cause that?

The way this works is that battery voltage is passed through a "stabilizer" which changes it to something close to 10 volts and an output in the form of a "square wave", which is nothing more than a rapid on and off signal.

The current then flows through the gauge on its way to the "sender". The term "sender" BTW is a misnomer as it is NOT EVEN CLOSE to sending anything. Instead of being a sender, it is a stone age mechanical rheostat.

The current flows from the gauge into the connection at the outside of the tank unit, then runs through a coil of fine resistance wire inside the tank. This is the coil part of the rheostat.

The flow is interrupted in degrees as the wand end of a lever rod moves across the coil as a result of the other end carrying a float bulb that rises and falls with changes in the fluid level in the tank. As the float rises and falls, the other end, the one I called a wand, moves in an arc as its tip rubs across the windings and effectively shorts the current to ground as the tip contacts one winding of the coil and then the next and then the next. As this happens, the effective resistance changes allowing more or less current to be passed from the gauge to ground. It is this change in current flow that affects the innards of the gauge and moves the gauge needle.

If you can ground the terminal of the current wire coming in from the gauge and the gauge responds to that "no resistance" connection by rising rapidly to "full" then everything will have been ruled out up to and including the terminal at the end of the wire.

That leaves only the possibility of an "open" in the rheostat coil segment of the circuit or the possibility that the wand end of the float rod is not making contact with the coil at all.

In my experience the most common occurrence is the resistance wire being broken between the inboard side of the hot external connection and the beginning of the coil.

The good news is that it is super simple to fix this.

Take the "sender" to the bench and carefully uncoil one or maybe two turns of the coil. Then either twist the end of the uncoiled wire to the end of the wire that is left coming from the power terminal or make a good solder connection between the two. If the wire is broken away flush from its beginning at the terminal, you can simply solder the end of the uncoiled length to the inboard terminal point.

Any theoretical change in gauge operation due to this should be ignored as the gauges are never right to begin with.

If you insist on trying to gain as much accuracy as possible you might want to go through the gyrations that Gary Alpern outlined in one of the Library articles.

Jack

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---------------------------------------------------- Services to Interesting British Automobiles-- www.jackscars.net ----------------------------------------------------------------
-------------I like to think that I don't service automobiles so much as I provide entertainment, fulfill dreams and revive wonderful memories --------------

I am in the northwestern North Carolina mountains, right up against the Blue Ridge Parkway. Lawn chairs are in the shop and always time to talk. Drop in any time.



Edited 1 time(s). Last edit at 07/17/2012 10:33PM by twigworker.

Jack-this is what we need-in depth technical information-thankyou.

I have today recd the new "sender" from MGOC that I talked about-6 days delivery from the UK and for 13.26 pounds sterling.No coils so no repairs possible in the future. All PCB now. Also no instructions.

from the photos there are 3 terminals-the one with the push on is the ground I believe.Would you be kind enough to comment on this new style.

with best regards-Keith in sunny hot Ontario

Keith, I would take a meter and see what the resistance was between the terminals and how it changed when the float lever is moved from empty to full. Two of the terminals should follow a resistance of about 30 to around 260 or 270 ohms. One of them, doesn't matter which one, should go to ground. My guess would be the two exposed terminals.

Muchas Gracias-I'll get back to you.Keith