Mercedes SL55 2004 false Fuel Guage reading 3/4

[kalsingh1]

Hi Guys

I have just bought an 2004, SL55 AMG.

The fuel garage keeps reading 3/4 but goes zero when ignition switched off. So I suspected the sending unit. One from eBay on the way. Starting stripping the rear boot carpets in preparation. Looking at it I pulled the connector off the sender unit on the petrol tank. Still reading 3/4. Put the connector back on, still reading 3/4. Very puzzled. So put star system on - no faults.
My sender unit just has one plug connector on with two wires. When I was looking on eBay, some have two seperate plugs. So I am very confused as to what is going on?
I would really appreciate it, if any replies could be practical ones. What could I test next to solve this. I have access to a friends Star System.
I have tried to pull the cluster gauge fuses to reset-did not work

 

[zedmeister]

I don't know specifically on your car, but most senders can be checked with a multimeter if they are a mechanical float type. The resistance will vary according to the position of the float and therefore the level of fuel in the tank. Also, most mechanical gauges also work on voltage so if you ground it, it will read one way and 12v should make it read the other way - the 12v goes through the variable resistor which is the tank sender and results in the variable level on the dash meter. This is my basic understanding of how the fuel gauge works. Hope that helps.

 

[zedmeister]

Forgot to add that the constant 3/4 level indicates a wiring issue maybe.

 

[kalsingh1]

Thanks for the reply

 

[kalsingh1]

PLEASE SEE LINK WHERE THE CONVERSATION IS MOVED TOO

http://forums.mercedesclub.org.uk/showthread.php?t=159967