MG TD fuel tank sensor leak

The TD fuel “gauge” is in fact a straight forward float device that operates a “switch” to break (open) the dashboard warning light circuit when the fuel level is sufficiently high. When the fuel level drops to around 2.5 gallons the float activates the switch and the “fuel” warning light illuminates indicating a low level of fuel has been reached.

This simple device requires no maintenance but problems can occur should the fuel seep past one of the float sender unit gaskets.

This “seepage” occurred on my TD fuel tank when I noticed a stain beneath the sender unit, although no fuel had actually been seen dripping out at this stage.

Before commencing any remedial work the first thing to do is to completely drain the fuel tank. Near the centre bottom of the tank is the drain plug that will require a 13/16 AF spanner or socket. At the very bottom right of the tank is the fuel supply take-off fitting, as viewed from the rear of the car.

The fuel integrity at this point is maintained by a fibre washer on the drain plug, this is worth replacing, when refitting the plug, to guard against leaking when the tank is refilled.

With the tank now completely empty, unscrew and disconnect the single wire lamp warning connection that is held in place with a large bakelite screw fitting. Next, undo and remove the outer ring of six slot headed screws and gently pull the complete unit away from the tank. Bear in mind that the float cylinder is some way inside the tank and the original sealant will have hardened. A large circular cork washer (2) fits between the gauge and the tank itself. If this has been untouched for many years it will probably disintegrate, so acquiring a new washer is a good plan as they are readily available.

Tank access for the fuel float and level sensor fittings

The vertical hole, towards the lower bottom of the back of the tank, where the cork washer fits, needs to be cleaned ready for the new replacement.

Take care to guard against foreign bodies entering through this hole otherwise the tank might need to be flushed out.

Fuel level Sender Unit

Sender unit components:

1. Paper washer 6 hole.
1a. Paper washer 3 hole
2. Cork gasket for sender unit
3. Cover fitting plate / Sender unit
4. Six screws for sender unit to fuel tank.

The low fuel tank sender unit (1950) had a 3 screw cover fitting plate (3), whilst later cars seem to have moved to a 6 screw configuration (as in this drawing). The early cover plate, apart from having only 3 screws, also has a pressed indent in its cover that provides some additional rigidity. It is quite possible that the early three screw cover was considered not as “fuel tight” as a six screw option, and hence the move to the improved version at some time after 1950.

Although our regular MG parts suppliers all seem to catalogue the sender unit and the large round cork gasket (2), I did not find reference to the paper gasket (1 or 1a) located behind the 3 or 6 hole fitting plates.

As the small paper gasket was the item that had failed it was necessary to fabricate a suitable replacement from heavy paper. After carefully cutting out a replacement paper gasket I coated both sides of the paper edges in a suitable “fuel proof” non setting gasket compound prior to fitting.

Metal cover plate gasket

The fuel sender gasket is made of heavy weight drawing or cartridge paper. The size of the gasket is given in approximate inch dimensions but you can always take your metal cover plate to use as an exact former.

Note. As the early cover plates were just a 3 “hole” design and later ones had six holes, the drawing may be used as a dimensional guide to either of these two configurations.

View of dismantled fuel sender unit

Components include:

• Float cylinder and wire lever activating arm.
• Metal 3 screw cover plate (dark grey) & paper (light brown) 3 screw gasket.
• 3 screws for fixing plate
• Body of sender unit showing “dry” inner switch contacts.
• Bakelite threaded top thumbscrew (on left) for connecting power to low fuel switch contacts. This attaches to the top of cast alloy body.
• Sender unit gasket with 6 screws.

The cast alloy body, housing the “low fuel” warning contacts, is air tight and completely sealed to keep fuel away from the live contacts. The switch is made up of a simple static contact strip that is “shorted” out by the action of the float arm when the low fuel level is reached. This shorting out completes the live circuit to the fuel warning lamp on the dashboard causing it to illuminate. It is advisable to test the switching action of these contacts, by the movement of the float arm, before re-assembly. This simple contact “shorting circuit” switch looks simple but is a very well thought out design, considering it operates in a hostile environment with no real chance of maintenance.

The large circular cork gasket is coated on both sides with the fuel proof gasket compound, and then positioned on the tank. The complete sensor body assembly is now offered up to the gasket encircling the tank hole with the float entering the tank clear of all interference. With the assembly in position the 6 screws are passed through the cast alloy body and the cork washer into the tapped holes in the tank. These tank screws should be tightened up firmly so that the cork gasket is snugly sandwiched between tank and sender casting.

When everything is back together satisfactorily I added a small quantity of fuel to the tank, to the point where the “empty” light just goes out. I now waited a couple of hours to be certain the fitting was fuel tight, after which I felt it was safe to top up further.
According to the TD handbook…”The green low fuel warning light comes on, on the instrument panel, when the fuel in the tank falls to approximately 21⁄2 to 3 gallons, thus giving warning that the fuel supply is getting low and in need of replenishment at the first opportunity.”

Jonathan Goddard

Ed’s Note: Jonathan’s book Practical MGTD: Maintenance, Update and Innovation, which has sold hundreds of copies worldwide, is available from the T-Shop at 6.99 GBP plus postage.

Also available are the MGTD Operation Manual and MG Midget (Series TF & TF1500) Operation Manual at 6.25 GBP plus postage. These are offered at little more than cost price as a service to our readers.

8 thoughts on “MG TD fuel tank sensor leak

  1. Lee Jacobsen says:

    Nice article and process, I would mention that the key component with regards to a leak free installation is the gas proof sealant.

    I would reccomend Hylomar or its equilvalent, Permatex Permashield, item #85420. A thin coating on each side will do the job.

  2. Ken Aiken says:

    Hi,
    A good article though from experience I would add a few points:

    1. Depending on the state of your tank/sender unit flange plate, the cork gasket may not be the best solution. I have heard of cases where the cork gasket supplied with the replacement sender units is not sufficient to seal and may leak after the replacement is undertaken (and the tank recommissioned or refilled). There are double sealed neoprene gaskets which may be a better solution. I have used one and it is a better fit for my car.

    2. My new (replacement) sender unit from (Moss) leaks through the electrical terminal post. I have not found a solution and now that the tank is fitted it is not very accessible. If anyone has any ideas for solutions I would appreciate hearing them.

    3. The Moss replacement sender unit has a neoprene (hopefully petrol proof) backplate template gasket which (from experience) distorts on reassembly and is a pain to re-site and get flat. A flat one piece paper gasket is a much better idea and should be undertaken prior to fitting and installing the tank.

    These are hard learned lessons from an unsatisfied customer.
    If anyone has an original tank float and sender unit or original spare that I can overhaul please send me an email.
    Thanks.
    Ken

    • John Powell says:

      I am also having trouble with my Sender Unit despite using a good quality fuel-proof sealant on the Gaskets.
      The Moss neoprene gaskets lasted 2 weeks after I accidently put some Ethanol fuel in. The Gasket deformed and almost melted away. This was replaced with a cork gasket and sealant.
      The most interesting thing was that the square electrical cover plate was still leaking. After a thorough examination, it was discovered that the 4 brass screws were too long (or the threaded holes not deep enough) thus preventing the cover plate compressing the gasket.
      I suggest you look at that.

    • Ian Theobald says:

      Hi have just fitted my new sender unit that has been on the shelf for over 12 mths .
      I removed the old one and smeared locktite sealant around both sides of new neoprene gasket only to discover that the holes in the new sender were of a smaller diameter. This meant the old one had to be refitted or my garage would be fumed out.
      Once holes were drilled out discovered that one of my screws was shorter and with the thicker gasket would not take up. Eventually was able to swap around and carefully tightened all screws up evenly worried that threads would be found to be stripped but luck was on my side.
      Every thing is nipped up tight but yes the neoprene gasket did start to distort but noticed in time and nothing is leaking so hope it will stay like that for the next 60 yrs untouched.
      Never got to drain the tank but sometimesthink is best to leave undisturbed.My copper pipe has been cut and some flexible fuel line joins,I guess it has a fuel filter fitted at some stage but why would some one do that when there is one on the fuel pump and on carburettors?

  3. Jyrki Hamalainen says:

    The float arm is very tight when I dismantled my TD. I barely moves at all. Is there a way to dismantle the float arm axle from the body?

  4. Peter says:

    Reference the fuel leak through the electrical terminal on the tank sender unit. I had this problem on my MGB replacement sender unit. I spoke to the (very well known and regarded supplier) who was aware of the problem across the whole manufactured batch ( from a long way from the UK). Their solution was to apply a layer of quality super glue around the terminal on the inside of the unit and of course let it completely harden befor refitting. It worked perfectly with no reacurring problems for the next two years or so of my ownership. Thought this might be useful information. PW

  5. Tom Kirkland says:

    Just had a faulty MG YT 1950 to service. Fuel Gauge showed no reaction at all. By testing it, it worked OK. But at the tank end, I got no reaction when earthing it. Emptied Tank, and removed Unit. Found Mud “Gumf” all around unit. Cleaned that off, then opened the outer cover, found the inside was solid with the same silt. Once all was cleaned, I then tested that unit to find once the float was lifted it showed a gradual increase in voltage which proves that, once reinstalled it should operate correctly. Now where did all that silt come from? I will now remove the whole tank for cleaning. Before attempting to get it correctly sealed as noted in your interesting solutions to that problem. Thanks for your help! Tom Kirkland — JHB South Africa.

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