Errant fuel gauges in all 3 of my MBs

[Zeus]

Ok, I've seen this topic discussed before, but I have noticed something I think goes beyond coincidence.

I've owned three MBs. A 1971 250, a 1977 280E and now my 1989 300E. All three cars have (had) relatively high mileage. On all three cars, the fuel gauge has the same problem. It is accurate at the full level, but as the fuel level drops below 1/2, the gauge is useless. It ping-pongs from empty to 1/2 full. The weird thing is that on all three cars, the RESERVE LIGHT is dead accurate. In fact, that is how I determine how much fuel I have left. I fill the car up, set the tripmeter and then watch my mileage and watch for the reserve light. Sometimes when the reserve light comes on, the gauge actually reads on empty, but other times it reads as half-full or somewhere in-between. It seems to be random in that sense. The reserve light, however, is accurate to within 10-15 KM (I get 525-575 KM on a tank depending on driving conditions).

Now, I have read the other threads here and I have since removed the sender unit and inspected it thoroughly on all my cars. The wires are clean and the float moves freely up and down the wires. I cleaned the wires anyway to be sure. With the unit removed, I have flipped it up and down and watched the fuel gauge. It appears to work just fine, but then again I am shaking it rather erratically compared to the relative motion it must experience within the tank, so it is hard to guess.

All the other gauges on the cluster are fine (temperature, oil pressure, economy gauge). Only the fuel gauge is off. As I said, it reads from full to 1/2 fine, then loses its accuracy completely from 1/2 to empty.

The weird thing is that I have this same problem on all three cars, all with high mileage and all from different years. My mechanic is as perplexed as I am. I am wondering if there is some fault with the gauges themselves. As some people have mentioned, the gauge is not linear. It appears to move from full to 1/2 more quickly than 1/2 to empty, which makes sense. Perhaps the lower half is at fault?

Anyone have any opinions?