Yep, sender or tank unit - same thing. I have it out & I measured the resistance from full to empty. Empty was 71 ohms & full is 283 ohms so I can replicate this with a trim pot.
The sender has 2 wires - one from the dash panel which has the square wave on it & one which is the return. This return wire is connected to ground somewhere. The trim pot makes gauge go up & down & works equally well connected to the ground frame or the return wire.
There is a bit of damping but there would be less than half a second in sender-gauge response.
Only the peak voltage of the square wave changes going from full to empty.
Average voltage measured with DMM is in a previous post.
I have found something curious though. If I disconnect the trim pot mid stream & reconnect it again, the gauge will stay where ever it was & won't move again until the ignition is turned off & back on again.
These 2 scope shots show the wave at full then at empty. The probe is connected before the trim pot.
The peak voltage at full is 3V & 1V at empty. Going straight across the sender & ground (no trim pot connected) gives the same wave but with a peak of 4.5V.
The spikes are 12.2mS apart. The frequency (80Hz if I have calculated right) does not change, only the peak voltage does.
