Erratic fuel gauge

[andrew97]

Hi all, I have a new issue where my fuel gauge is all over the place, mostly high, mostly way off the scale. Has anyone had this issue? Wondering if it’s a sender or cluster problem.
I did do a search and there is loads of threads about calibration, refuelling strategies etc, but I can’t find anything about this.

 

[T8]

That's a new one on me.

What model is your 996? C2, C4 .............?

Most issues/queries stem from awd cars and the shape of their fuel tank.

 

[andrew97]

It’s a ‘99 C2

 

[The Baron3]

Not seen that before, might be your cluster needle.

On the positive, great oil pressure when cold

 

[Alex]

1st thing I'd do is check the resistance from the tanks sender unit in the frunk.

 

[ndg]

I don't think it will be sender. It's an electronc cluster so will interpret the sender voltage as 0-100% then display that on the gauge. The gauge/cluster would have to be broken to go beyond 100%.

 

[Alex]

It works off resistance, not voltage. I've frigged mine so I can adjust the resistance between sender and gauge and can make the gauge go up and down.

 

[ndg]

The sender will be part of a voltage divider circuit. Microcontrollers can't directly measure resistance, they either measure voltage or current.

It's typical for them to use a voltage divider circuit to create a voltage that is proportional to the sender resistance then use an ADC to measure that voltage (there are specialist chips that can measure current with a fixed voltage across the sender, but this is unlikely in an automotive implementation).

 

[Alex]

Regardless of how you think it's measured, if the resistance from the sender that it outputs is altered, then the gauge follows suit. All I'm suggesting is illiminate the sender by measuring the resistance from it. When I made mine adjustable, from doing some calculations, 1 gallon of fuel in the tank equated to 25 ohms (but not 100% linear over the full range). Half a tank is around 220 ohms and the lower the fuel level, the higher the resistance.

Hope this helps.

 

[andrew97]

Thanks Alex for the resistance figures, gives me an idea of what to look for. Car is pretty much full at the minute, so I should expect to see a low resistance reading, would it be 0 at full tank do you think?

 

[Alex]

I've just read elsewhere that its range is 240 to 33 ohms

Edit: the ohm figures I quoted on my previous post were for the C4 which has a different sender unit due to the shape of the tank.

 

[andrew97]

Thank you for the info, will investigate at some stage.

 

[andrew97]

Just a small update on this, I pulled the sender out and all looks fine, checked the resistance and it checks out too. I was getting a reading of ~300 ohms for empty up towards near zero for a full tank and was linear through the range, so confident enough to say the sender is fine.

I suppose next step would be to see what reading i am getting at the back of the gauge cluster. Looking at the wiring diagram the other side is connected to the analog ground, does anyone know what the significance is of the term 'analog' is, surely a ground is just a ground?