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The wiring diagram of the J type fuel tank sender and meter.
So if you re having problems with your fuel gauge, here is how to test it and how it works.
The wiring diagram of the J type fuel tank sender and meter.
So if you re having problems with your fuel gauge, here is how to test it and how it works.With 12 volts on the "B" terminal BUT no connection on the "T" terminal and the gauge earthed the left hand coil (C1) would be fully energised and show full deflection of the fuel gauge needle. The right hand coil (C2) has the resister across it so has less "pull" than the left hand coil but does give a "damping" effect to the needle. (When driving the float would be bobbing about all over the place,). so the the coil (C2) slows and dampens the erratic needle movement.
With a working tank sender now connected the resistance will vary,depending on fuel level.
With a full tank the senders resistance will be very low so now the "T" terminal is nearly earthed, this means the left hand coil (C1) has "earth" on both sides of it so has no current flowing through it and therefore the coil (C1)has no effect on the needle. This now means the right hand coil (C2) has full voltage across it and so has current flowing so it does effect the needle and gives full opposite deflection.
Looking at the circuit its very basic but very clever, the resistors job changes, in certain situations it is a "current" limiting device, when you have a full tank or shorted wire situation it stops the coils burning out, in other situations it balances " in a voltage potential "whetstone" bridge" kind of way, it would take too long for me to explain this.
Also it and both coils balance the gauge reading making the gauge practically independent of normal variation of battery voltage.
The circuit is happy and works just as well on positive or negative earthed vehicles.
A very very clever Mr Lucas.
ROD 525 fuel tank (above) showing it was easy to see the sender but hard to remove. The fixing screws at the rear of the sender were hard to remove as the wooden beam above hampered access and they had been chewed up by a previous attempt to remove it.
OBL 432's Van sender unit sitting under the nice round cover plate by the nearside foot well. Easy to get to and replace if need be.
ROD 525 fuel tank (above) showing it was easy to see the sender but hard to remove. The fixing screws at the rear of the sender were hard to remove as the wooden beam above hampered access and they had been chewed up by a previous attempt to remove it.
OBL 432's Van sender unit sitting under the nice round cover plate by the nearside foot well. Easy to get to and replace if need be.While restoring or conserving ROD 525 one of the job's I needed to do was get the fuel gauge to work. It always showed a quarter of a tank. I drained the tank of water/petrol/dirt and grit and the gauge still said A quarter of a tank. So out with the sender, very easy to get to on the pick-up but due to the timbers of the load deck very hard to remove due to lack of room and chewed up brass screws from somebody else's previous attempt.
Once removed it was easy to see why the gauge said what it did, the float pivot was totally rusted solid in the casting and the coil mount was brittle allowing the coil to move around.
So I had to find a replacement, so I purchased a new one, the sender units are the same as early 1949/1955 Morris Minor which are easily purchased. One word of caution, try to get an original Lucas one as opposed to the re-manufactured look alikes, I have had trouble with both regulator boxes and sender units that look good but have been faulty.
While the sender is out of the tank, switch the ignition on, the gauge should now read a full tank and with the sender unit supply wire earthed it should read empty.
The only thing you have to swap over from the old sender to the new Morris minor one is the float and wire arm which is crimped into the pivot shaft of the sender unit. It is easily done.
