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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. |
#2
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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. |
#3
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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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1986 300E 5-Speed 240k mi. |
#4
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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
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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 Falls Church VA '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 |
#6
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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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1986 300E 5-Speed 240k mi. |
#7
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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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Ken 87 190E2.6 |
#8
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Have you checked the ground at the sending unit?
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Chuck Taylor Falls Church VA '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 |
#9
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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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1986 300E 5-Speed 240k mi. |
#10
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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 Falls Church VA '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 |
#11
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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: |
#12
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Thanks Chuck.
I will checkout that ground.
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1986 300E 5-Speed 240k mi. |
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