Fuel sender replacement

Ever since I've had this car the fuel gauge has had a problem. Fill up the tank and all is well- gauge shows full. Keep driving and as the tank empties towards 1/2 full again, all remains well, but just after the half-full mark suddenly the gauge drops like a stone. It moves from full to empty over a mere 20 or so miles and on the motorway the needle is visibly moving down, eventually to pass right off the gauge scale and rest on its bottom stop. Although the lower half of the tank does appear to be used more rapidly in many cars because of the tank shape, this behaviour isn't right.

At first this worried me greatly, and sent me panicked from the motorway searching for fresh fuel! However, having found that there was no fuel leak and that I could put only 16l of fuel in to a tank reading "empty" I decided this must be an indicator error rather than a dramatic fuel consumption or leakage.

Fuel gauge activity is governed by the sender which has a float that rises or lowers with fule level in the tank. The float is connected to an arm that passes a contact over a variable resistor such that resistance increases as the float lowers and the tank empties, plus a gauge that displays this resistance variation in terms of "tank-fullness". The sender has limited adjustment effected by bending the float arm and the gauge can be calibrated to the sender by means of 2 adjustment sliders on the rear which will set the resistance at which the gauge indicates full or empty.


The later MGBs combined the fuel outlet and float level sender into a single unit. The existing gauge has only a single contact to the variable resistor, the case itself providing an earth current return through the tank and body.

The new (Moss)  gauge is plastic and has three contacts. The centre contact has an earth lead connected so is presumably to replace the metal body-to-chassis contact. Of the other two contacts, one was covered already in a plastic sleeve and the third (top contact) was exposed.

New sender, note filter on the end of the uptake pipe, and variable resistor and moving contact in the top unit in connection with the float arm.

New sender has three electrical contacts, centre appears to be earth, bottom contact is blanked off.

The first task was to check the contacts and the gauge function. I found that there was a variable resistance between the centre contact and the top contact (uncovered) indicating that these provide the sender function. There was effectively no continuity between the centre (earth) contact and the bottom (covered) contact. These I believe are probably for a low fuel warning light and this is not used in the MGB.

In order to investigate whether the gauge will function properly with the sender I disconnected the existing contact from the sender and connected it to the new sender. I could then connect the earth to the chassis and support the sender in a stand so that the float arm could be moved. This showed that the  gauge would indeed read the full spectrum from full to empty in relation to float movement and appeared to give the full range of indication over the full range of the float arm movement. This reassured me that the gauge itself should be fine.

New sender supported in clamps for testing

 I had already run the tank down by driving 250 miles from topped off full, dipping the tank suggested that fuel was now very low and to help matters I raised the rhs of the car on a ramp to tip fuel away from the sender unit.

 The sender had been coated in underseal by Rustbusters so I cleaned this off with paintbrush cleaner and a stiff brush so I could get at the connections and sealing ring.

Disconnecting the sender lead

 The terminal was disconnected before the fuel pipe clip was loosened and the pipe disconnected.

Sender cleaned up removing underseal.

The sealing ring was simply tapped around in an anticlockwise direction until the cutouts in the sealing ring lined up with the lugs in the tank opening and it could then be removed.

Sealing ring removed.

 The sender could then be finagled out

Access to the sender is a little tricky owing to the springs.

 One surprise was that the float had a strange rubber seal on it- I've no idea where this came from or what it was intended to do.

What the f*** is this? A stray sealing ring on the float arm of the old sender.

Moving the old float arm showed that it was very "notchy" compared with the new. I tested the resistance of this one too.  Virtually zero ohms at fully raised, the resistance increased smoothly as the float arm is lowered. It reached about 50 ohm around half way and then shot up to 5 mega ohm almost instantly!  I think this explains the gauges odd behaviour and reinforces the idea that this should solve the problem.  Gosh its satisfying to find something wrong that you can fix!

Old sender- note no filter in position on uptake tube.

I wanted to see what was wrong so I bent back the three tabs and opened the sender body. The problem was immediately apparent as the windings of the variable resistor had broken in the centre.

Inside the old sender! Note the break in the resistance windings in the centre of the loom.

The rubber sealing ring was pulled out of the tank recess

Peeling off the old sealing ring.

 and the recess itself cleaned up.

Clean recess ready for new sender.

Fitting the new sender was very difficult because the design seems to pit the filler pipe against the float when it comes to inserting them both into the tank! In fact it does go if you get the positioning right but its a bugger! Use a new rubber sealing ring gasket. There are two lugs on the sender which locate into the tank recess to set the  sender position. Check that this does position the fuel inlet downwards and allows the float to rise and fall.

New sender in orientation required and about to fit it- you need to turn it on its side to get both float and filter into the tank.

However, it was immediately clear than the sender base plate would not  sit flush down against the tank and so it wasn't possible to fit the clamping ring. It seemed that the  new sender was pressing on the bottom of the inside of the tank and so wouldn't permit the base plate to fit flush so I removed it again. The filter unit on the fuel pickup simply pulls off which allowed me to compare the pickup pipe lengths. It was immediately obvious that the new sender's pick up is some 2.5 cm longer than the old which might account for it fouling on the inside of the tank.

Old and new pick up pipes compared. The old unit's pipe (top) is some 2.5cm shorter than the new

 I used a small pipe cutter to remove  2.5 cms from the new pickup pipe.

 ...and then refitted the filter tube

 This time the sender would down into the tank and allow the base plate to fit flush  against the body of the tank. I used a new sealing gasket between tank and sender unit and clamped the assembly by tapping  the sealing ring round to clamp the sender against the gasket. Make sure that all three lugs on the tank body are engaged with the sealing ring as it turns.

 The fuel outlet on the new sender points upwards, whereas that on the old sender pointed horizontally.

Old sender- fuel pipe outlet horizontal when fitted.

 This meant that the fuel pipe wasn't quite long enough to connect to the new sender. Fortunately I could push some excess pipe length through its retaining clips to provide enough slack to make the connection.

I greased all contacts with dielectric grease and connected the indicator terminal  to the top contact. Owing to the new under seal there was no immediate earthing point for the new sender and the earth wire as supplied was hopelessly short. I made up a fresh lead (green in this case as I had no black wire)...

Fuel pipe connected and clipped in place, sender contact connected to the top contact and the centre connected to a new green earth lead

... and passed this through an existing grommet into the boot.

Earth lead passed through grommet into the boot

 In the boot I was able to crimp a ring terminal onto this lead and connect it to earth using the fuel pump earth point. I took advantage of this step to clean the contact points for the pump as well and refit both using dielectric grease to protect the terminals.

Green earth connected to pump earth inside the boot.

I tested the sender by turning on the ignition and the gauge was reading on ts bottom stop.  Finally I added a spare 4l of fuel to the tank and repeated the test. This time the  needle rose off the bottom stop to indicate empty- which is probably about correct. Signs are therefore good but I will need to refill the tank more completely to test that the gauge reads as appropriate and that the new sealing ring doesn't leak.