Fuel sender unit

[miniminor]

Hi all,
Bit of a conundrum with my fuel sender unit.
My car is a '63 850. It has what seems to be later gauges including a voltage stabiliser and the fuel sender unit is the lock ring type, not the six bolted version.
Thr car is still positive earth.
My old unit was failing, occasionally showing a level but normally just showing zero.
I bought a replacement unit and fitted it.
Unfortunately it seems to work in reverse, showing empty on the guage when the tank is full and moving to full on the gauge as the petrol is used.
I tried reversing the two wires at the sender unit but when doing so the gauge shows full all the time irrespective of the amount of fuel in the tank.

Any ideas on how to resolve this? Is this a positive/negative earth situation causing the problem?

[Spider]

It sounds like you have the earlier gauge in it.

Does it have a Voltage Stabaliser fitted ?

Does the gauge move fairly swiftly when you turn the key on, or is it slow ?

[miniminor]

It has a voltage stabiliser. The needle moves slowly when the ignition is turned on.

[Peter Laidler]

Exactly as Spider says. You cannot mix the earlier NON stabilized gauges/system with voltage stabilized gauges/system. Never the two should meet. It's one or the other.

So far as the tank sender unit is concerned, we've learned that there seems to have been a cross-over period. So whereas a 6 bolt fixing will indicate a NON stabilized system, a lock-ring unit doesn't necessarily indicate a voltage stabilised unit.

[miniminor]

I have a voltage stabiliser though?
What's the solution?

[Peter Laidler]

Did you try the tank sender before you fitted it? If it only reads part full, it might be a pre-stabilised sender - the conundrum I mentioned.

If your car is a voltage stabilised car, it can only be the B-MVIStabiliser, the tank sender, the green/black wire from the tank to the gauge, the gauge or the light green wire from the gauge to the B-MVIS. The B-MVIS is fed from the fuze box. On that basis, it's difficult to say exactly what the problem is.

But I bet you that within a couple of hours, a forumer who's had the same trouble, will come on board and tell you exactly what!

[miniminor]

Hopefully

[Spider]

At this juncture, I'd firstly short the Green/ Black wire to Earth - briefly and check that the gauge starts to shoot up to full. If that behaves as it should, then remove the sender and check it with a meter that it behaves as it should - empty is circa 220 ohms, full is 33 ohms and 1/2 full is 82 ohms. Sounds like the sender has been assembled back to front, though, I can't see how.

Whether it's Positive or Negative Earth has no bearing on how the gauge behaves, it's just a heating element. The only part you need to be careful with here is the Voltage Stabaliser if it's a modern electronic replacement. It'll have a 10 V output if it has the correct polarity and no output if it's wrong.