damping fuel gauge response

qwertytam · #1 (**permalink**) 08-20-2007, 08:35 PM

Hi Guys,

Well as much fun as it is watching the fuel gauge on my '75 2000 GTV bounce around like a pogo stick in a washing machine it would be nice if the response was a bit (maybe a lot!) slower. So I was thinking the easiest way to do this was to install a capacitor in line in the fuel gauge circut. Not sure what size cap to go for.

Anyone got ideas/comments on this??

Cheers,
Tom

**alfaparticle** · #2 (**permalink**) 08-20-2007, 08:48 PM

If you put a cap in series it will not work at all. It is a DC meter and capacitors only pass AC. You could try connecting various size caps in parallel with the meter. You could also try filling the gauge with oil. No guarantees.
Ed Prytherch

qwertytam · #3 (**permalink**) 08-21-2007, 12:20 AM

How quickly I've forgotten my first year electrical engineering from uni! Of course the good old low pass filter is a resistor in series with a capacitor in parallel across the load (follow wikipedia link: http://en.wikipedia.org/wiki/Low-pass_filter).

The cut off frequency is given by f=1/(2*pi*R*C). So if I go for a cut off frequency of 0.5Hz (frequency response will half for every doubling of the frequency) and say a resistor of 1k ohms, then I have a capcitor of approx 320 micro farads.

So since my practical electrical circuit experience is limited (working as a mechanical engineer) what practical design/safety concerns are there with introducing the capcitor into the circuit? Of course keeping the additions to the fuel gauge circuit in the cabin well away from the fuel tank for starters, but do I risk any sparking or overloading of gauges anywhere?? Should I go for a smaller capacitor (hence a bigger resistor) to reduce the discharge from the capacitor?

Any feedback much appreciated.

**alfaparticle** · #4 (**permalink**) 08-21-2007, 04:55 AM

The sender is a variable resistor. If you wire in a series resistor it will introduce (or create a bigger!) zero error. The gauge will read lower throughout the range. I suppose that I should pull a Toyota sender/gauge sometime to see how they accomplish the damping. It might be in the sender.
Ed

Alfacliff · #5 (**permalink**) 08-21-2007, 06:10 AM

theere have been several threads about cleaning the resistance element in the tank. that should clear up a lot of the bounce. a cap across the fuel sender could store enough energy for a spark.
cliff

**papajam** · #6 (**permalink**) 08-21-2007, 06:35 AM

Alfas obviously use a straight 12VDC system. So as the fuel sloshes in the tank, the fuel gauge sloshes with it. Many other manufactures use a pulsing, low voltage (3-5VDC) system with a dampened gauge resulting in the fuel gauge being very slow to react to changes in the tank fuel level.

I never bothered to do anything about it because, in my opinion, why fix it if it ain't broke and it is just one of the idiosycracies (sp) that make Alfa so endearing to me. Besides, I refuel on tripmeter mileage, not the fuel gauge reading.