Fuel Gauge and Odometer Stop Working Intermittently

1990 240 fuel & odometer gauge stop working intermittently. Where do I
start to investigate?

 

I had odometre working sporadically on my 1990 240. The reason was
cracked soldering in odometre step motor.

This might come handy if you intend to test the odometre outside the
vehicle:

<URL:http://cleanflametrap.com/speedo.html>

 

this might also just be a bad fuse..
try to extract the fuse from the fusebox, check if it has corroded at the ends.

you can spray some CRC in the fusebox and check if the fuses are too loose, if
so, you can bend the fusebox metal thingies.
I think it's related to power, as those two instruments should not affect each
other.