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  #1  
Old 06-26-2002, 01:09 PM
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Question W124-Intermittent Fuel Gauge Problem

W124-Intermittent Fuel Gauge Problem

I have searched past threads to no avail.
Model: 1986 300E

Symptom:
Fuel gauge sometimes reads max full, even when tank is not.
Sometimes it reads accurately. Seems to be no pattern to failure.

Attempts:
Since these symptoms have typically indicated a sending unit failure in the past (other cars), I 'pre-emptively' replaced the sending unit. No improvement. Wires and connectors on tank end are in perfect order.

Questions:
Is this a possible failure mode for the gauge, or is a short in the wiring more likely?
If it is a gauge failure, does anyone have a 'Radio Shack' repair procedure?
I'm good with a soldering iron...
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1986 300E 5-Speed 240k mi.
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  #2  
Old 06-26-2002, 01:34 PM
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Smile

are any other dash gauges or instuments playing up?

if so could be the voltage regulator on the back of the dash display.

or possible lose connection in the wiring.
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Old 06-26-2002, 02:16 PM
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Thanks for the response.

Everything else in that cluster is working, including the Low fuel warning light.

Could it still be this voltage regulator you speak of?
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Old 06-26-2002, 03:14 PM
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i can't be sure on the location of the voltage regulator,but this certainly was faulty on a previous audi 5 cylinder car that i owned and on that car it was behind the instument cluster -it was a small electronic fuse like device solder onto the electronic wiring of the board on the audi-do not know if mercs have a similar set up.
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  #5  
Old 06-26-2002, 04:15 PM
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Check the battery and alternator. I know this sounds nuts but see my previous posts on this topic.
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Chuck Taylor
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'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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Old 06-26-2002, 04:37 PM
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Thanks Chuck,

I did see your posts with that extraordinary charging system 'failure prediction behavior', but I have replaced my alternator and battery recently, and they test up to spec.
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  #7  
Old 06-26-2002, 05:33 PM
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i use to get the fuel gauge not going to the full position after i refueled. i replaced the fuse that goes to that set of intruments and its working fine now. not sure if its related as the fuel gauge problem was intermittent.
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  #8  
Old 06-26-2002, 05:54 PM
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Have you checked the ground at the sending unit?
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Chuck Taylor
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'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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  #9  
Old 06-26-2002, 06:44 PM
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Chuck,

Perhaps I have the 'fuel gauge wiring concept' wrong in my head.

I would envision ground being at the gauge end of this circuit.

I picture the fuel gauge as being essentially a 'resistance meter'.

I would expect current to flow from the gauge, then through the sending unit, where the sending unit applies a certain amount of resistance (depending upon fuel level), then back to the gauge which measures this resistance, then to ground.

Am I on the wrong track?
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Old 06-27-2002, 09:30 AM
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You have the right idea, but the wiring diagram shows the ground off the sending unit. The guage gets positive and then I think measures voltage from it through the sending unit to ground. At the sending unit, the blue /black comes from the guage and the black/green from the low fuel warning.

The ground is common to both.

I could fax the diagram to you later today. It is all in the Haynes manual.
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Chuck Taylor
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'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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  #11  
Old 06-29-2002, 05:33 AM
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sound like the fuel gauge problem is contagious!!

i filled the gas tank on my merc c200 -uk spec-and the fuel gauge stopped working after that.

it worked temporarily once the failed completely,

anyone have an ideas on a similar problem?

need help soon:
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  #12  
Old 07-01-2002, 10:01 AM
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Thanks Chuck.
I will checkout that ground.
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