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-   -   How does it know? (http://www.peachparts.com/shopforum/tech-help/9248-how-does-know.html)

Robert W. Roe 09-02-2000 04:54 PM

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

stevebfl 09-02-2000 06:24 PM

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

stevebfl 09-02-2000 06:24 PM

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

Robert W. Roe 09-03-2000 02:41 AM

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 :)

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Robert W. Roe
1984 300SD 171K mi

stevebfl 09-03-2000 10:38 AM

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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