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MG MGB Technical - Fuel Gauge Testing

How can I check or test the functioning of my fuel gauge or sending unit? Right now, the gauge shows just under 3/4 of a tank of gas after the car is started. It shows this even when the tank is full (really annoyed a pump jockey when I asked him to fill the car and he got in about half a gallon - NJ is 1 of 2 U.S. States that does not allow us to pump our own gas).
I have had the car about 6 months now and driven it about 6 or 8 hours in total, so have no idea what will happen once I use up more of the gas in the tank.
AndyT

Fill the little rascal to the top. Note the odometer and drive about 80 miles.

Refill to the same top and record the gal. Note the odometer. divide miles by gal. 80/4.3 = 18.6mpg.

Refill, note gas guage - that will be full.

Drive 93 miles (18.6x5gal?) thats half tank.

Refill to check mpg and drive about 150 miles. That should leave about 2 gals. Note guage reading.

Of course that's if the mgb has a 10 U.S. gal tank.

Can't pump you own gas?????

Fred Horstmeyer

With the ignition on briefly connect the green/black and the black at the tank sender together. Only keep them connected for as long as it takes for the needle to stop moving.

If the gauge moves smartly up to F the sender is faulty - assuming the tank really is full.

If it only goes up to 3/4 as before, get a good ground from somewhere else and connect it to the green/black again. If the gauge goes up to F now you have a bad ground to the sender, it comes from the number-plate bolt inside the boot/trunk.

If the gauge still only shows 3/4 connect a good ground to the green/black terminal (NOT the light-green/green) on the back of the gauge. If it reads F now there is a bad connection in the green/black back to the tank. If it still shows 3/4 then either the voltage stabiliser is bad or the gauge needs recalibrating - quite possible if the sender has been changed.

To check the stabiliser connect a voltmeter to the light-green/green and you should see it switching between 0v and 12v about once or twice a second. Note that the stabilser also feeds the electric temp gauge if you have this.

To recalibrate the gauge see the above web site - select 'Spanners', 'Electrics' and 'Gauges' and scroll to the bottom of the page.
Paul Hunt

I tried to reply to this last night with no luck!

I had the exact same problem on my car. Right down to the embarassing overfill at the petrol station. A month or so ago I followed the same advice given by Paul here and found the fuel sender was faulty. It was cheap (about $40NZ which is less that $20US) and easy to replace it. I just filled the tank and then reset the trip meter and drove it until I knew the tank was only about 1/4 full. I was then able to lift the right rear corner of the car and replace the sender without spilling any fuel. The old sender had fuel in the float and the resistance of it was way higher than the new one I fitted.

With the new one in the gauge now works fine although I must admit I still set the trip meter when I fill the tank just to keep an eye on how many miles I've done!
Simon Jansen

Andy

I can't offer any ideas that haven't already been mentioned, but I do feel your pain about the gas pumping issue. Oregon is the other state where you can't pump your own gas. We've tryed to change that by vote a couple of times, but there are too many lazy people. Either that, or they feel sorry for all the pump jockeys who will lose their jobs.
Jared Snider

A female of my daughter'a aquaintance bought her first car. First time she went to fill it up she grabbed hold of the pump hamdle and trigger as she pulled it out of the 'holster'. The result? One petrified female spraying petrol all over the pump and forecourt until the till-jockey cut the power.
Paul Hunt

When I first got my car, I had the same problem. Through a similar procedure, I deduced it was the sender as well. At the time, I couldn't imagine replacing it without dropping the tank, so I didn't. Just kept track of the miles - I can also remember disconnecting the filler pipe and looking in with the naked eye! Fortunately for me, the darned thing started working on its own toward the end of the first season I had it on the road. I guess the mechanical bits inside sort of froze but then with all the gas sloshing around, things freed up. Dunno... Point being I guess, maybe let it sit for now while you just watch the miles?

Curt

Curt

On a similar note. My fuel gage/sending unit does not work either. I keep track by milage, but the car is new to me so I error on the side of caution when deciding when to fill up, and fill up I must or I won't know when to reset the trip for a full tank. Here's the problem, in PA we do pump our own gas. When I fill the tank I find that the geometry of the filler neck must be such that it prevents the "auto shut off thingey" in the pump nozzle from shutting off when the tank is full. Consequently I frequently end up giving my car and my feet a gas bath, quite embarrassing and smelly. Anyone else have this problem or more importantly...a solution. I've tried holding the pump handle in different positions somtimes it will shut off when full other times look out!

Thanks
Brian

Must be something about your pump nozzles, all the ones I have used in the UK on both my cars work just fine (I always fill to full) and are often *too* sensitive.

The solution? Stand to one side! Oh, and get that fuel gauge fixed.
Paul Hunt

Thanks for all of the help. I also have to fix the speedo, can't use that to track my consumption. The speedo problem may be the drive on the transmission.

Not being able to pump my own gas is no loss, as the prices we pay are at or below national levels anyway. Last fill on my daily driver was $1.19 a gallon. And I don't have to get out of the car in bad weather, or end up with the smell on my hand.

The "auto shut off" devices were designed for cars with vapor recycling systems and restrictor plates to hold the smaller nozzle. Having said that, my wife's 2 year old Subaru loves to spit the nozzle out at the end of a fill.
AndyT

I just worked on a car over the weekend that had no fuel gauge reading at all. Rubber Bumper MGB. Checked all "Normal" problems first, then pulled sender unit out of tank. The metal band that carries the current from float to the wire connection had broken for no discernable reason. This isn't a wire, it is a metal band about 1/4 inch wide. Anybody else ever seen this? Soldered heavy gauge wire across break and now works fine.
Wayne N.

I just had similar problem.

1) From my experience, Paul’s advice spot on

2) I found the extensive info on electrics for MGB at following site invaluable in tracing this problem

http://www.mgbexperience.com/electrical/


Question to Simon Jansen, Auckland, New Zealand
$NZ40 – I just paid about $A67 in Sydney which is more like $NZ80 for my fuel sender – does your parts guy have a web site?


Peter
67 Mk1 MGB
Peter Vann

Opps forgot to say that if after doing all Paul says and you suspect the fuel sender, then take it out and test the resistance across the two connectors.

With float up, (ie full tank) it should have very low resistance (mine about 20ohms) so that high current flows to fuel guage causing the needle to move to FULL, and with float down (tank empty) a higher resistance (mine about 250ohms I vaguely recall) so that little current goes to fuel guage resulting in needle in EMPTY position. If you follow the link to the MGB_experience in electricals web site in my previous post, you will see how the resistance range is important for correct fuel guage operation.

My old broken one had a constant resistance of less than 20ohms and my guage was consequently getting a good current and thus showed FULL full-time.

Good Luck
Peter
Peter Vann

"The "auto shut off" devices were designed for cars with vapor recycling systems and restrictor plates to hold the smaller nozzle"

That surprises me, in the UK we have had auto shut-off for as long as I can remember. The restricted entry came much later for cars with a CAT that could only take unleaded.
Paul Hunt

Yes but they took away the little catch you could set to hold the trigger on.
This enabled the pump assistant to leave the pump running whilst the pre set amount of fuel was delivered and he cleaned your windscreen and checked the oil.
Remember those days?
Chris Betson

Gosh, you must be older than me, especially the bit about 'cleaning the windscreen' and 'checking the oil' ...

Roche Bentley got into trouble recently for suggesting one could wedge one's filler cap in the nozzle handle to save one's back while filling one's tank. Had to print a 'retraction' the following month saying that if the nozzle fell out of the filler the cap would be sure to become dislodged when it hit the ground. "Yes Roche."
Paul Hunt

This thread was discussed between 30/01/2002 and 06/02/2002

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