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MG MGB Technical - Fuel Gauge Sending Unit/Float

Good morning,

I have a 1979 mgb roadster. The Fuel Gauge has never worked since I've owned it (9 years). Now that most of the "other" repaires needing attention are complete, I have decided to fix the fuel gauge.

Step 1. Per direction in the "Moss" book, I pulled the wire off and conected a test lamp, the guage slowly went to full, and the light lit. So, I suppose the Sending Unit and Float need to be replaced.


Step 2. Looking at the "parts list" (Moss pg. 28) it looks as though I need to order the Sending Unit... do I need to order the unit that includes integral fuel pick-up tube?

Plus, a new float ring locking and ring seal.

Questions: any tips/suggestions? Does the "seal" need any extra sealant to prevent leaks? In the Haynes manual it says to use tool 18G 1001 to remove the locking ring.... hmmmm, what else can I use using regular tools?

Does the Gas Tank have a drain plug on this year? The book says "as fitted"... if so where abouts is it?? If I can find it, I will also replace the drain plug washer to insure a good seal.

Looking forward to all of your replies.

Don 1979 MGB Carmine Red
Don

Don,
You'll need the later sender with the integral fuel pickup. The earlier tanks had a seperate fuel pickup, but the later tanks did not.

The seal should be installed dry - no extra sealant is required. Just double check that the mating surface is clean of rust and adhered rubber from the old seal.

I use an old blunt chisel to remove it. A dull screwdriver will work as well. There are 3 tabs to work against. You'll want to alternate between them as they get chewed up pretty quickly.

You won't have a drain plug in yours, again the earlier cars did but it was removed for cost savings.

Now for tips, this is a SAFETY CRITICAL task. Do NOT smoke or have any form of open flame or spark genearation anywhere in the area! Do this OUTSIDE! Many years ago a friend burned down his garage, tools and 2 Jaguars (one a 66 E-Type roadster) because he drained the gas into a bucket with a trouble light next to the bucket. The hot bulb ignited the fumes. He was lucky to escape before being engulfed - really!

Wait to do this until you have the tank mostly empty. Having 2-3 gallongs left is ok. Jack up the right rear corner to force the gas to the opposite side of the tank. Pinch the fuel tubing with a pair of vise-grips to keep the gas from draining back from the carb. Remove the locking ring, hose clamp and sender wire (not in that order!). As they say replacement is reverse of disassembly.

Do check out your fuel tubing underneath. Now is the time to replace it if it is at all suspect.

Cheers,
Tom
Tom Sotomayor

Also check the grounding of the tank, that will prevent the gauge working as well. If your test-lamp conencted to the *body* of the sender also lights, and the gauge moves, then it is the grounding. If not, it is the sender.

The locking ring has three tabs that stick up, as well as three tapered sections that wedge under three flanges that are part of the tank. Make sure you tap on the locking ring tabs and not the flanges. As you tap on each one you should see the ring move in that direction, which is why you have to progressively work round at least two of them if not all three as Tom advises.

Make sure you use a new sealing ring, and install it between the sender and the tank.

Personally I wouldn't clamp the fuel hoses, it can split them, the non-return valve in the pump should stop any back flow, and it is only going to be a pipe's worth anyway.
Paul Hunt 2

Hi Tom and Paul. thanks for the replies.... it looks like I had better "re-test" with my light. I used the frame for the bubber as my ground source. I'll try again using the "tank" as the ground to see if this works ok too.

Tom, it's great to have someone remind us of the safety required... I'll make sure once I start this project that it is done outside!

Is the LOCKING RING threaded? So that is why I need to tap on each tab to remove? (counterclockwise??)

Looking forward to hearing from you.

Don
Don

Update: I just "re-checked" the light test using the fuel tank as the ground and the test result was the same (light works, gauge moves to full).

Now it's time to order parts!

Safety fast,
Don
Don

Don, The lock ring isn't threaded.

Clifton
Clifton Gordon

Hi Clifton, I guess I'll have to see the new part to better understand how it works (tabs). When I get ready to take the sending unit out, I'll sketch how it looks so I can replace properly.

Thanks for giving me a heads up about the locking ring. Enjoy your evening.

Don
Don

Don,

Yep, turn the locking ring counter-clockwise to remove. As Clifton mentioned, it is not threaded. The tank has a welded on ring that utilizes a 'cam lock' arrangement. It functions like a coarse thread but it isn't one. I know, clear as mud - but you'll see how it works when you take it apart.

The three tabs are nothing more than bent sheet metal. When hitting them do it as close to the ring itself as possible. Otherwise the tabs will bend and eventually break off. If it's been on there for any length of time they'll get pretty mangled during removal. Don't worry since you're planning on replacing it anyway.

Paul mentioned the fuel hose may split by clamping it off - absolutely true! I consider that as a good sign the hose needs to be replaced. ;o) If it is anything but a good quality reinforced hose that is still pliable it should be changed out. The factory used un-reinforced hoses during later production. Those can fail spectacularly - literally falling to pieces and spilling fuel everywhere. If you still have these hoses it's best to replace them now.

Tom
Tom Sotomayor

Check and see that the wire is properly attached to the sender unit on the tank. Sometimes these can come loose. Wriggle it a bit, might be corroded.
Right hand side of the tank about two thirds of the way back. Tab in the middle of the three inches (roughly) round sender, next to the fuel line. You will be able to see it without jacking up the car.
The check to see that the wire from the sender is continious to the wire on the back of the gauge.
Also check out Pauls excellent, how to adjust and calibrate your guage. www.mgb-stuff.org.uk
I have just used it to calibrate my temperature guage to a Discovery V8 sensor, (thank you very much Paul!)
Peter Sherman

Good morning Tom and Peter. Thanks for your responses. The "hose" is black, looks like an original piece of the car. I'll change out at the same time I install the sending unit. Should be able to get this gas hose at a "Auto Zone" or NAPA to the length / size needed to replace.

The Sending unit currently is covered with black paint, this was done soon after I purchased the MGB... kind of a "duh" thing I did... repainted the gas tank to make it look better. While I am waiting for parts I am going to strip paint off to better see the sending unit, and make replacement better.

Keep in touch,
Don
Don

The new part won't help with understanding, unless you replace the locking ring as well. This *does* sometimes need replacing if heavily corroded, but not usually.

Tom - it all depends on what you use to clamp a hose. Use something with sharp edges and over-clamp it and you will damage a good hose. OTOH a purpose-made hose clamp with rounded edges, or a Mole wrench with a couple of sockets slipped over the jaws, won't, unless the hose *is* time expired. The point is that the non-return valves on the pump will stop any back-flow from the carbs anyway. You need to be prepared for some fuel spillage as there aren't any ball-valves on the end of the hoses to prevent it, so you will always get some from the sender feed pipe, even if the hose leading off it is clamped.
Paul Hunt 2

Good afternoon Paul,

I am going to replace the locking ring, add a new seal when installing the new sending unit. Kind of figure I might as well use new parts instead of trying to use something old that I might damage when taking it apart.

Kind of curious, what happens to the sending unit that makes it not work? MG sitting around too long without running? Bad float? Just poorly made? "Or??"

Just wanted to say "Thanks" to you for your help on this and previous projects I've requested help on in the past.

Don 1979 MGB
Don

Paul,
Probably should have mentioned to not clamp the vise grips on the hose, but to slowly close them using the adjusting screw. It doesn't take a ton of force, more like a gentle pinch.

Maybe I'm too anal about spilled fuel, but I don't like any going anywhere it's not supposed to. Especially down my arm soaking my shirt! I've also dealt with a number of cars where the non-return valves, well - didn't. All the gas in the line drained out pretty quickly. Despite the hazard it's also pretty pricey now-a-days to let it run on the ground. Must be some of my Scottish ancestry showing!

Tom
Tom Sotomayor

If the float in your existing sending unit is not leaking, that is no fuel inside, you may just have a frozen pivot point due to rust and/or gum. The lever/arm should move freely and be able to drop under its own weight. If it doesn't try a little WD40 or Liquid Wrench, allow to penetrate for an hour or two, and then gently try to free it up. Once free, if you have an ohm meter check to see if you have variable resistance while moving the lever. Sorry, don't know the correct values. I had the same problem on my 65's original sending unit and after a little WD40, it's still working fine after seven years. Good luck. Always fun.

Bond
Bond Germain

Good morning,

Thanks BOND for explaining about the float/sending unit.

I am going to replace the sending unit... however I will check out the old unit to see if the float has a hole in it, and also if the swing arm is frozen, and the ohm readings too.

Then it will wind-up in the pile of parts I have collected for future ??? Don't we all have one of these boxes??? Why do we keep these parts??

Enjoy your day,

Don
Don

With the new ring you will see where to place your drift, then, which should be on the narrow end of the tapered sections.

Normally the fuel gauge will move a little way from its 'ignition off' position towards the E even when the tank is completely empty. This is because the maximum resistance of the sender is about 250-300 ohms and not infinity, which sends a small amount of current through the gauge which moves it a little way. If yours *does* move this little bit when you turn on the ignition, then likely the float is punctured. If it were stuck it would always indicate the same level, and it is unlikely that would be empty, unless the tank had been left empty for long enough for the sender to stick in the fully down position. It's possible for condensation corrosion to do this.

But if the needle doesn't move at all, then from your tests the sender is open-circuit. This could be because the wiper has lifted off the resistance track for some reason, or the connection between the resistance wire and the sender body has broken, or the resistance wire has broken. But in this last case it *usually* breaks part way along, which means it will work normally from F own to some level, then suddenly drop off the scale altogether as if the ignition has been turned off.

Paul Hunt 2

This thread was discussed between 30/08/2007 and 01/09/2007

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