TB20 fuel tank indicator fault

The circuit diagram of one version of the Socata gauge cluster is

so someone good at electronics should be able to test the meter movement itself, or replace some of the other components. I think the fuel gauges are the two meters on the bottom.

The fault could also be elsewhere. What S/N is your aircraft?

I don’t understand “With security almost certainly the resistance on the level meter is defective at this point”.

The above circuit is from some internal Socata document, not the official IPC/MM, and if anybody can get hold of that secret doc it would be a huge help to all of us. That gauge cluster is now over 3k and is about to become unavailable.

Peter wrote:

I don’t understand “With security almost certainly the resistance on the level meter is defective at this point”.

I take it to mean that he is certain that the potentiometer is defective at the point of its movement where the jump occurs. (And presumably all the way to the empty tank position.)

Right; in that case the sensor needs to be changed.

But that’s a big job; I would still verify the indicator isn’t sticky.

It may not be the sensor. It could be the indicator. That’s why the indicator should be tested, by applying a suitable signal to the instrument. I would guess those meter are perhaps 100 microamps full scale. Somewhere else will be a series resistor…

This is impossible to help with. You need to find an electronics engineer who can read a circuit diagram. Maybe try some local community social media group.

“FL100” means nothing. The indicator is a moving coil meter (also called d’arsonval) made by some obscure French company which probably disappeared years ago. Consequently most Socata part numbers google to nothing. A French person may have luck with French google but very little stuff from 20 years ago remains online (on search engines).

This stuff is trivial to troubleshoot.

Thomas, before going into electronics of the indicator I´d suspect the sensor to be scrap. Most of the typical fuel sensors are the wire resistor type like in cars. So then there is a good chance the wire got a defect, by corrosion or wear. You can test the indicator by hooking a potentiometer on it to simulate the sensor. I guess 100 ohms wire potentiometer will do for testing the indicator for smooth action over the whole range. Only then care for the indicator when it does show erratic readings. On the Yak we got extremely precise readings from both indicators but these are sealed and military standards obviously. We did have defective floats of a foam plastic type when with Super 98 use and 3 decades they shrunk so much for no longer floating in fuel. The stainless type in photo will outlast us by decades I believe. Vic