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
• 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.”
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.