Craig Allardyce wrote:59wagon wrote: Tue Aug 28, 2018 10:36 am
Hi Sarge, I have one here that goes to full when you tip it upside down.
A better test is to put 12V across the gauge. Connect positive wire from your battery to the positive of your gauge (left hand side terminal looking from the front) and negative to the chassis of your gauge or dash cluster, NOT the sender wire terminal. The gauge should go to above full.
Cheers,
John
Gauge should go to empty with it powered up and earthed, not full. Powered up with high resistance or open to earth will always read full. power it up with it not earthed and give it a few taps. Sometimes they will stick in the empty position.
Hi Craig, I agree with your statements above if the sender unit is fully installed and the tank is empty, or the sender unit is removed with the sender wire still connected and the body of the sender unit is earthed.
Sarge said his gauge was out of the dash, so I assumed the sender wasn’t connected. With the sender disconnected, if you put +12v on the +ve terminal of the gauge and negative to the body/chassis of the gauge as I said initially (NOT the sender terminal), it’ll read full if everything’s ok. If you put 12v across the +ve terminal and sender terminal, it’ll read empty.
For anyone interested, here’s a brief bit on how the fuel gauge works. First, a CAD schematic
Inside the gauge there are two resistance coils. In the CAD above, the top coil between “P” and “T” measures about 54 ohms. The lower one between “T” and the gauge body (which is to earth when installed in the dash cluster which is screwed to the car body), measures about 30 ohms, which is the same as the sender unit when in the full position.
Here’s a pic of the gauge coils
There’s a moving magnet between the coils that’s attached to the gauge needle. As the current running through the coils changes, the magnetic field changes and affects the movement of the magnet/needle unit.
So we have two circuits - one from P to earth via the two gauge coils, and one from P to earth via the top gauge coil and the sender unit resistor. Electricity takes the easiest path, or path of least resistance. The resistance across the gauge’s two coils is about 84 ohms. When the sender is in the full position, the resistance through P, T, B to earth in the CAD above is also about 84 ohms and the gauge will show full. In this instance, the current is the same through both circuits. Anything less than a full signal from the sender will have less resistance and that’s the main flow path of the current, so the gauge will show less than full. There’s still some flow (current) through the lower coil, but less than through the sender resistor. When the tank is empty, the circuit resistance through P, T, B to earth is at 54 ohms, which is when the gauge shows empty.
Here’s a couple of pic’s of the sender unit guts, showing the resistor and moving arm that changes the resistance as it moves.
With power to the gauge, if you disconnect the sender wire from the gauge (“T”) or the sender unit itself (“A”), or remove the sender unit from the vehicle so that it’s not earthed (“B”), the current flow can only go through the 84 ohms of the two coils in the gauge, therefore reading full on the gauge.
Another type of gauge used in cars uses a resistance wire wrapped around a bimetallic strip connected to the gauge needle. As resistance decreases at the sender unit, the gauge’s resistance wire heats up and the bimetallic strip starts to bend and move the needle.
Sorry if I’ve hijacked your thread Sarge, but thought this relevant and someone might find it interesting.
Cheers,
John
Sent from my iPhone using Tapatalk
Next day I get some spare time I will operate on the old one. If only for learning where the failure was. Great CAD by the way. Sarge