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MG MGB Technical - Fuel Sender Jamming?

G'day. I have just replaced the fuel gauge as the old one was no good. Now, when the tank is full the gauge shows about 1/4 full. Checked wiring and voltage at the gauge, pulled wire of tank sender and earthed it and lo and behold the gauge shows full. Pulled the sender out of the tank and measured resistance, all OK. Put wires on sender out of tank and gauge works fine. Put sender back in tank, re-fill and gauge shows about 1/4.

The Moss catalog shows the tank as 456-815 and it is fitted to a '67 roadster.

I guess it is jamming somewhere in the tank but can't see where. Has anyone else come across this problem. Am I on the right track thinking it is jamming or is there another reason?

Cheers

Tony
Tony Oliver

Sorry, forgot to mention. Plastic float is OK with no signs of fuel in it.

Tony
Tony Oliver

You can adjust/calibrate your gauge to the sender.
On the back of the gauge are two oval slots. Sometimes they have cork plugs in them.
The one at the empty end sets the zero and the one at the other end sets how far the needle can move ie the full.

These are not adjustable electrical resistors as you might imagine. The gauge works by having the indicator needle held/balanced between a springy strip of steel and a bimetalic strip with a high resistance wire wound around it. How far the bimetalic strip will bend and how hard it will push against the other strip depends on how much current is being sent by the tank sender.
These two strips are attached to small backing 'plates' by a single rivet each. The oval holes are located a small distance from the rivets and allow you to lever/move the backing plates, rotating them around the rivet, just a little thereby adjusting the tension in the springy strips and the location of the needle. Note the plates don't rotate around the hole, they rotate around the rivet so expect an off center movement.
They can be quite stiff to move and they don't move far, less than a quater of a turn. A very coarse adjustment. Set the zero end first, then set the full range of movement. setting the full end/range of movement will effect the zero setting just a little, but not much.
You'll have to adjust these two or three times if you want to get it perfect.
Peter

Thanks Peter. I did that with the sender out when I fitted the replacement gauge and got full and empty readings as appropriate. I forgot to mention that, sorry.

I have just spent the last hour draining the tank and checking the sender. I even measured the distance from the edge of the sender hole to the baffle I can see to make sure it wasn't a strange sender that was too long. No joy there!

Found that with the fuel just below the level of the sender hole the voltage on the sender is 6.3 volts. Trouble is as I add fuel 5 litres at a time it stays at 6.3 volts. It seems that the sender is not moving as I add fuel. Everything works fine until I put the sender back into the tank.

Tony
Tony Oliver

Take the sender out, Connect it to the sender wire and earth the body of the sender. I'd use an additional length of wire to get it away from the tank and a distance from all those petrol fumes. You don't want any sparks, kaboom!. Move it through it's arc. It should move freely and evenly. The weight of the float is easily enough to move it. My old gauge moves through a vertical distance of 16cm. 8 cm should be the half way mark, level with the sender. Try 0, 4, 8 12 and 16cm full. Give it a minute or two to equilibrate at each. Use the dash gauge itself, not multimeter. You don't really care what current etc, the gauge is what matters.

If the fault is in the sender or gauge setting it will then become apparent. If it works perfectly then you either have an obstruction in the tank or the float is not floating.

Waggle a screw driver or something a little longer than the float wire up and down in the tank to check for obstructions.


Unclip the float and put it in some water to see if it is actually tight. Any bubbles out or water in etc.


By the way, you are actually putting the sender in the right way up? sorry to ask but I've done lots sillier things myself (I'm not saying what!).
peter

That should read " sender float moves....16cm" not gauge moves ..."
p

Peter, thanks again for your input. I had the sender sitting on the boot lid with both wires connected when I tested it. My loom has an earth wire to the sender. It worked fine. Didn't measure the distances but it seemed to be roughly correct. Will look at that tomorrow.

I have had a screw driver inside the tank through the sender hole. There is a baffle or maybe the filler neck to the rear that I could easily detect, there seems to be nothing to the front that would catch the float and there is the baffle that is about 70 mm from the hole towards the centre of the tank. The float is "liquid" tight.

I only used the voltage test to see if the float was moving. The voltage across the float should drop as the tank is filled thus dropping more over the gauge winding and thus increasing the current flow. Everything I have read says the resistance decreases as the float rises.

As before, everything seems normal with the sender out of the tank. The 70mm distance seems longer than the arm plus float and I am sure I have it the correct way up, but I'll check that again tomorrow. I won't ask what silly things you have done if you don't ask me!

Could someone tell me if the internals of the unit, that is the resistance wire, is supposed to be sealed or does it fill with fuel when the tank is full. Mine has no sealant on it so fuel can easily get in. Maybe that is affecting it.

Thanks
Tony
Tony Oliver

Tony, the sender filling with fuel is normal.
John H

You're lucky, you've got one of the early baffled tanks.
The baffle distance.
I'm guessing you meant to type 170mm, not 70.
My old sender needs slightly more than 170mm at it's longest. That would imply that your float is just catching against the baffle prior to the half way mark.
Measure the distance carefully and adjust the float wire so it clears.

Peter

Thanks John, one less thing to worry about.

Yes, it was 170mm Peter. I thought that the float was awful close to the baffle but it seemed to have about 5mm clearance. I'll measure it again. I think I'll just bend the wire a little bit anyway.

Tony
Tony Oliver

Peter and John, thanks for your help and information.

I have just finished bending the arm and now I get past the 1/4 full mark. The tank has an upward dent in it at roughly the point where the baffle sits so I assume that it has bent the baffle towards the sender arm thus causing it to jam. I shortened the arm by about 10mm.

When I replaced the sender I could see fuel just below the hole. With 20 litres added to the tank it reads 1/2 full so it seems that it is fairly accurate but shows empty when there is still a fair amount of fuel in the tank. Just the way I like it!

Don't think I'll bother with a new tank. I can live with a fuel gauge that reads low.

Cheers

Tony
Tony Oliver

Now that you now that its working you can reset the gauge, if you want. Run it to empty, set the empty adjuster on the gauge to zero. Fill it up and set the gauge to full.
Peter

Or if you want a 'reserve' run it empty, put a gallon back in, and *then* adjust it to E. When emptying don't just drive it around carrying a spare gallon until the engine dies as the pump will have been chattering away pumping no fuel for some time while the float chambers run down. Either listen carefully to the pump and stop driving as soon as it starts chattering, or if you can't hear the pump you will have to - with the tank very low - use the pump or a siphon to lower the level in the tank until the pump starts chattering and then switch off. Leaving it chattering for a couple of minutes, which it will do on a fully warm engine, won't do it any good and may well be the cause of problems that people claim to get when they *have* run out, and not sucking up dirt from the low fuel level as is claimed.
Paul Hunt 2

Thanks gents. From my understanding, if I shorten the arm it will hit the lower stop earlier than normal, that is with more fuel in the tank than if the arm was longer. This gives me an automatic "reserve".

As I said, I can live with a gauge that reads low so unless I decide to beat the baffle out of the way and reset the arm I am happy to drive the car and fill up before it hits empty as usual.

Tony
Tony Oliver

This thread was discussed between 27/11/2007 and 01/12/2007

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