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MG MGA - Fuel guage occasionaly 'locks' on full

I have just noticed a problem with my fuel guage. When I switch the ignition on with engine off the guage reads steady and accurately. Once I start the engine and drive, the needle fluctuates a little, as would be expected. But when there is a bit more fluctuation from going over a bump, extra acceleration, braking, etc. the needle gets pulled to the "Full" mark and locks in for a little time (maybe 10 seconds or more).

I imagine (and hope) the guage is ok as it is fine with the engine off. Could it be an earth fault at either the guage or sender unit?
Graham M V

Intermittent open circuit in the sender unit or sender wire, or bad ground for the fuel tank.
Barney Gaylord

I have a similar problem with my gauge except it is always locked in the full position and never moves. I installed a wire from the tank to frame to ensure good ground. No improvement.

Always a full tank, talk about a false sense of security....What do you think Barney, faulty sending unit?

Fran
F Valenzano

Thanks for the advice Barney. I will first look at the earth of the fuel tank, primarily because I dont really understand what an open circuit in sender unit is! Nor the same for the sender wire (does that mean a break in wire inside cable?.
Will report back if that's OK.
Fran, presumably from what Barney says, yours is a permanent, rather than intermittent, open circuit.
Graham V

Fran
I have spent some time researching this, using of course Barney's great site and the archives, and now understand things a little more.
I haven't tracked my problem yet, but can recommend some quick checks for you that may help. First I would suggest you test the guage is OK by seeing if you are able to get it to register Empty by connecting the "T" terminal on the guage to earth. Obviously be very careful (disconnect battery) when messing about behind the dash!
When you unscrew the guage, you will also be releasing the earth strap, so make sure you have earth to that centre screw thread too when testing it. Basically the T terminal runs a Green/Black wire to the tank sender unit and the lower the fuel level, the lower the resistance between the sender unit and earth. So with no resistance, your guage should now be showing empty.
You have obviously done the next step which is to check the earth at sender unit.
There is a bullet connector for green/black wire under the bonnet in wing near the starter switch which is worth checking as if this is open, that will obviously make the guage register as Full.
That is as much as I had time for. At least I now know the gauge seems OK.
Tomorrow I will check continuity of Green/Black wire from gauge to sender which hopefully is faulty, as if that fails I imagine I will need a new sender unit. I plan to simply connect the end of the cable/sender terminal to earth, and see what happens to the guage. Hopefully nothing, indicating a continuity problem! That may be hard to find but I dont fancy changing the sender unit as many seem to have had problems sealing the unit properly.
Good luck
Graham M V

Graham

I will check to be sure the gauge is in proper order by following your instructions for connecting the T terminal to ground. I have a suspicion that the fault lies in the sending unit, and like you, I do not relish the thought of replacing it unless absolutely necessary. I understand that most of the aftermarket units are less than dependable...and then there is the task of sealing the unit at the tank.

Thank you for your comments.

Fran
F Valenzano

Graham
I adjusted my guage as per Barneys site and found that the sender unit was faulty I obtained a new sender unit from B & G and fitted it taking care with the seal and so far (2500 miles later) no problems and the guage is pretty accurate
Paul
P D Camp

Thanks Paul. I wanted to avoid fitting a new sender while summer is here and car is running so nicely, as from the archives it seems many had problems getting a good seal. Did you use a cork gasket?
I disconnected the spade terminal at sender unit and shorted to earth, and meter shot to empty so I believe guage and wiring is all more or less OK. But I then took car to petrol station and found it took 9.5 gals (even though it had been measuring quarter of a tank) and so it made me think that maybe both my guage needs adjusting and that the sender gets erratic (showing full) when the tank is almost empty. Could that be right do you think?
Graham M V

Graham
I used the gasket that came with the sender It was rubber and I also used a smear of sealer (petrol & oil proof ) on the gasket and made sure both surfaces were smooth. I also had to put an additional earh wire attached to a sender attachment bolt, So far no leaks
The sender cost around £60 and when I queried the cost B & G said it was to the original spec and not a cheap chinese import ?
I had two attempts at recalibrating the guage as per Barneys instructions only very fine adjustment of the two magnets are required and it takes a little time to get it right.Now my guage seems to work OK shows full when full and half full with five gallons on board( measured with my wooden dipstick which I use as a check)
From what you say I would attempt to recalibrate the guage first that would prove the guage was OK. It is strange it reads full when nearly empty as I would have thought if the guage needed adjustment it would show full all the time
The resistors barney uses can be obtained from Maplins for a few pence I used a spare 12v battery for testing
Paul
P D Camp

Having returned a sender unit back to MGOC - due to poor quality, as P.D Camp reports the Brown and Gammon unit is expensive but appears robust. The picture shows the winding and what appears to be a much stronger connection to the brass connecting stud. Not sure it's entirely as original specification as my measurements were 20 ohms at empty and 100 ohms set at full.

Kevin

K Garner

Thanks Kevin. I am sure you and Paul are right that the more expensive one is worthwhile as I see Moss stocks both the "original for £63 and replacement for £21. Can't see they would stock them both otherwise.
Graham M V

This thread was discussed between 24/05/2010 and 02/06/2010

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