SLK fuel gauge - life of it's own

[Miss Aturning]

Hi all
I'm new to these forums, having taken delivery of a 1998 S reg SLK 230 three days ago. I've just emerged from my mesmerised state to notice a slight problem with the fuel gauge. It seems to spend some of the time flopped right over towards the engine temp guage, and then jerks over to 'empty'. To check, I filled the tank today and it took around £20 of fuel. I can't believe that the tank is that small! After filling up, however, the needle seemed to regain its sanity and is sitting just a tad below full, where it should be. As I'm not an experienced owner yet I don't really know how many miles to expect from a full tank, and am a bit worried about running out of fuel owing to a faulty readout. Has anyone else come across this? And is it easy/cheap to fix? Any advice would be welcome.
TIA
PS: always had a fantasy that one day I would own one of these - so far the reality is far better

 

[scotty2hotty81]

sender unit in the tank, easy to replace, unscrew and remove your boot liner (roof tray) pull the carpet out the way behind it and in the centre of the fuel tank you will see the sender unit, 3 screw 1 in each of the holes in the sender then use the third to unscrew it.

 

[Miss Aturning]

Many thanks for the info. I don't think I'll try changing it myself, even though it's easy. I'll see if I can source a unit and get in touch with my friendly mechanic down the road Meanwhile I assume it isn't anything more than an irritation, as long as I keep the tank reasonably full. Do you know what I can expect to get from a full tank? (just so i don't run out of fuel half way up the M42!)

 

[Blobcat]

~400 miles if the tanks is around 65ltrs, depending upon how you drive it of course

 

[Miss Aturning]

Thanks for that - I worked out from the specs that it should cost around £55-60 for a full tank, so between 350-400 miles sounds about right. I will be contacting the seller tomorrow to see if he can sort me out with a new sensor unit. I was going to do it myself until I saw the price of one new (£240 from a local MB dealer). I've seen quite a few refurbished parts on the net, but am not sure what the risk might be with a component like this of buying used.

 

[tonythepony]

Hi I too have this problem, 1998 slk 230 replaced the tank sensor with second hand one off e bay ...they assured me it worked fine !
my part no is ..1705421217...they sent ..1705420917...supposed to be the same. looks identical.
Any how I still have a gauge not working . Can anyone tell me how to test the gauge without connecting the sender ? can I put a voltage on the wires that go to the gauge.The two wires at the sender ..is one earth and the other 12 volt ? What is the resistance range of the sensor. ?
Any help apreceiated.
Tony

 

[Ken_R]

Have you got a multimeter, and know how to use it?

 

[tonythepony]

Hi Ken ,yes I have a meter, just done a few tests, with meter on 200 ohm setting ,i tested the sender . It goes from 2 to 198.
then I checked the voltage on the live to the sender it is 5 volts, the other wire goes to earth.
gauge drops to zero when disconnected,then judders and goes way past the full when connected.
Am I doing it right ?
Tony

 

[Ken_R]

Mmmm. Have you checked both Sender units? The old one and the one you bought as a replacement, and do they read similar?

Do the Sender units work on a Linear (as opposed to a Logarithmic) scale? In other words, at approx half way, do they read about 100 ohms?

I suspect, from your description, that the nominal +12v feed goes to the dashboard gauge and its path to 0v is then governed by the resistance of the Sender unit.

Sender units and dashboard gauges have been about for aeons but with the introduction of CANbus systems, and systems that compute fuel range, etc, it is entirely possible that the +5v signal is 'interrogating' the availability of fuel. However, I'd think such most unlikely. And I can't really think of a logical reason why such would be. Are you getting +5v both with the ignition on, but with the engine both running and not running?

I'm suspecting that the fault lies within the fuel gauge itself, rather than the Sender unit.

I would have considered suggesting buying a cheap variable resistor (now that you know the Range) from Maplins/RS Components and connecting the feed wire through that, to Earth but, it would be no different to having a Sender unit connected and manipulating the setting, by physical movement, of the Sender unit.

 

[tonythepony]

HI Ken, thanks for that informative reply, i,ve been doing some more investigating ..it would appear the wiring to the sender
has been modified / replaced at some time. the original connector still exists to the sender with Blue/ blk wire and the earth
brown /blu...these are about 4 inches long, then wires have been soldered to different wires one red one brown these then dissappear under the petrol tank insulation. dont know why ..but could the difference in resistance of the wires make a difference to the reading at the gauge.
what happens if the wires are switch over at the sender .
By the way where is the diagnostic socket ?