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  #1  
Old 09-02-2000, 05:54 PM
Robert W. Roe's Avatar
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Join Date: May 1999
Location: Lehigh Valley PA
Posts: 1,322
Driving my SD yesterday with close to 400 miles on the trip odo, I knew I was very low on fuel, but my fuel guage gets "bouncy" below a quarter tank or so. It was showing over a quarter tank yet the amber "Reserve" light came on. How is this possible? I didn't think there was a separate fuel sender for the reserve light.

When I filled up later last evening I took 19.25 gallons, and I think my tank is only 20.4 gallons, so it was very right for the light to come on.

Is my tank over 20.4 gallons perhaps? Any insight appreciated.

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Robert W. Roe
1984 300SD 171K mi
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  #2  
Old 09-02-2000, 07:24 PM
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Location: Gainesville FL
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There are two separate circuits in the tank sending unit. One is a variable resistor. it runs the gauge. The other is a simple switch. It controls the light.

Old sending units usually fail in the way you describe; with the bottom 1/4 tank being either not recorded or the needle bouncing through this area.

------------------
Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician
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  #3  
Old 09-02-2000, 07:24 PM
Moderator
 
Join Date: Mar 2000
Location: Gainesville FL
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There are two separate circuits in the tank sending unit. One is a variable resistor. it runs the gauge. The other is a simple switch. It controls the light.

Old sending units usually fail in the way you describe; with the bottom 1/4 tank being either not recorded or the needle bouncing through this area.

------------------
Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician
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  #4  
Old 09-03-2000, 03:41 AM
Robert W. Roe's Avatar
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Join Date: May 1999
Location: Lehigh Valley PA
Posts: 1,322
Thanks for the reply. I'm happy that my reserve light works in spite of the bouncy fuel guage. It does seem to be coming on a bit later than the three gallon reserve mentioned in the owner's manual. This was the first time I saw it ever come on, at 380 miles, and I refueled at 385, so I had a little over a gallon left. Oh well, at least the light works

------------------
Robert W. Roe
1984 300SD 171K mi
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  #5  
Old 09-03-2000, 11:38 AM
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Join Date: Mar 2000
Location: Gainesville FL
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All MB fuel gauges use similar sending units at least till 1996 when a pressure sensor was added to the tank electronics.

------------------
Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician
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