R129 fuel gauge sender

[Reiver]

I have an intermittant fault with the fuel gauge on my 2001 R129 SL.

When the tank is full the gauge doesn't register. When the tank gets down to 3/4 or less the gauge springs to life and works all the way down to however low I dare go.

I've had the sender unit out and cleaned the tracks with electrical cleaner. There is full movement available on the arm.

I used a multimeter to test the resistance at the plug and it went from 35 ohms (iirc) up to 70 ohms.

I connected the sender and tried moving the arm through it's arc expecting the fuel gauge to react at some point but it didn't.

I am convinced it must be a faulty sender unit. Is there anything else I can test ?

 

[LostKiwi]

Try connecting it and earthing the mounting plate. See if that makes it burst into life.

 

[EmilysDad]

In days of old before canbus & electrickery, removing the wire to the sender & earthing it should make the gauge read FULL ....

 

[Reiver]

The body of the sender unit is all plastic, i.e. it doesn't earth to the fuel tank. Instead the unit earths back through the wiring to some remote point.

However, after some more generous spraying of electrical cleaner (or maybe I just pushed the plug back in far enough) I now have the gauge reacting to the position of the float. All the way 'up' reads full, all the way 'down' reads empty (with the low fuel warning light on) and all points in between.

The hinge of the float arm is plastic so I gave that a good spray though it didn't look dirty. Maybe it just needed excercising ?