Tank gauge

[Thomas Tornblom <Thomas.Tornblom@Hax.SE>]

In my quest to get properly working instruments, I have spent a few hours 
measuring the tank sender resistance and noticing the gauge reading for various 
amounts of fuel.

As I still have the car on jackstands, it was quite easy to drain the tank so 
that I knew the exact amount of fuel for each measurement.

I added 5 liters at a time, noticing the gauge reading and resistance.

Here's what I found:

liters	R (ohm)	gauge
---------------------
empty	~2k 	empty--
5	321	empty-
10	272	empty+		lamp goes out
15	234	1/16
20	208	1/8+
25	180	1/4-
30	160	1/4+
35	135	3/8
40	115	1/2-
45	100	1/2+
50	 88	9/16-
55	 75	3/4-
60	 60	3/4
65	 48	3/4+
70	 27	4/4-
75	  3	4/4+
80	  2.5	4/4+

When I filled the last few deciliters up to 80 liters, the fuel level climbed up 
into the filler tube, but sank slowly as if something was leaking. I believe the 
excess fuel drained into the carbon canister. I have a late aluminum tank by the 
way.

All gauge readings were taken with the ignition on, a battery charger active, no 
lights on.

For those (like me) contemplating replacing the instruments with better working 
VDO:s, VDO North America has a fuel gauge in all common series that probably 
would work fine. They have a gauge with 240 ohm (empty) - 33 ohm (full), and 
adding a ~30 ohm resistor in series with the sender would probably work 
reasonably well.

This type of gauge is not available from VDO Germany. The German gauges are 
either 3 ohms empty and 180 ohms full (opposite from veglia) or 60-90 
(adjustable) empty and 0.5 ohms full.

I will get one of the 240-33 gauges from Summit and try it out with the stock 
sender.

Cheers,
Thomas