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  #1  
Old 09-01-2014, 11:40 AM
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Low fuel light on all the time

My low fuel light is always on in my 85 300D when the key is in the ON position. It was working correctly after I removed the sender and performed an internal cleaning. Worked correctly for about 1 year; then started this "on all the time" behavior. I have re-opened the sender and found a lot of gunk in the bottom of the tube; I carefully removed all of that, cleaned things up and reassembled everything. I still have an always on condition. Gauge functions fine.

Is this something that can be caused by an bad ground somewhere? If so, does anyone know where that ground point would be? TIA.
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Old 09-01-2014, 06:01 PM
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The low fuel light contact in the tube switches to ground.

There are 3 wires running the length of the sensor.

The two Ni-Cr wires are the sensing wires, these two get bridged by the slider.

The single copper wire is the low fuel light wire.

When the slider hits bottom it bridges two points on the bottom plate. One is from the copper wire and the other is the ground via the central support rod.

Remove your first aid kit and unplug the sensor connector. Does the light turn off?
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  #3  
Old 09-01-2014, 07:04 PM
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Quote:
Originally Posted by jay_bob View Post
The low fuel light contact in the tube switches to ground.

Remove your first aid kit and unplug the sensor connector. Does the light turn off?
Yes, its stays on with the fuel sender unplugged.
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Old 09-01-2014, 08:06 PM
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Quote:
Originally Posted by mikeyfev1 View Post

Is this something that can be caused by an bad ground somewhere?
Sounds more like a good ground. Problem is, there should be no ground except when the tank is almost empty.
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  #5  
Old 09-04-2014, 05:01 PM
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Any more feedback?

Does anyone else have any help they can provide with this?
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1986 300SDL, 240K+ miles
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  #6  
Old 09-04-2014, 09:26 PM
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How important is the low fuel light to you?
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Old 09-04-2014, 11:05 PM
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When the key is put in glow position, all lights come on to verify their operation. It's possible there is a problem with the cluster board, or the ignition switch itself... Aside from that, you are looking at chasing the wiring to the cluster from the tank...
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  #8  
Old 09-05-2014, 03:22 PM
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If it still comes on with the gauge unplugged then it's either a short in the wiring harness or a blown diode in the bulb check circuit. I'm thinking the latter.

Here's a quick check, with the bulb glowing and the sender harness plug in your hand, short terminals 2 and 3 together. Does the bulb get brighter? If so it's the cluster, if not, it's probably a short in the wiring. The cluster bulb check glows the bulb dimmer than when the light is actually actuated (there is a resistor in series with the diode to cause this).

The diode will be on the circuit board on the F-O-T gauge part of the cluster. If you are really handy with electronics and know how to solder, you could probaby find and replace the bad diode.

If not, go to the junk yard and grab a 124 cluster. Gas or Diesel does not matter. Turn the cluster over and carefully remove the module for the fuel-oil-temperature gauges. If it's a gas leave the other two (speedo and tach) as they are useless on a Diesel (wrong diff ratio on the speedo and wrong pulse multiplier on the tach).

Bonus: If you got a gas model gauge cluster you can search for Sixto's economy to boost gauge mod on this forum and hook up the economy meter as a boost gauge.
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The OM 642/722.9 powered family
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)
2008 ML320 CDI (Older sonís DD)
2008 E320 Bluetec (Younger son's DD)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech
----------
gone but living on
1983 240D (body to greazzer, engine to t walgamuth)
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  #9  
Old 09-05-2014, 03:58 PM
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' low fuel light on all the time'......

The obvious answer is ' TO STOP DRIVING AROUND with only 2 GALLONS of fuel in the tank '...
LOL
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  #10  
Old 09-05-2014, 04:20 PM
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If you have nothing intelligent to offer then offer nothing.

Quote:
Originally Posted by leathermang View Post
' low fuel light on all the time'......

The obvious answer is ' TO STOP DRIVING AROUND with only 2 GALLONS of fuel in the tank '...
LOL
This is not what the OP is asking. The problem is probably in the plug or in the dash if the light does not go off when unplugged. Do what jay suggested.
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  #11  
Old 09-05-2014, 08:40 PM
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Add fuel.

(Just being a smart a$$, sorry......)

Dan
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  #12  
Old 09-05-2014, 08:59 PM
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Pop the plug connector apart that plugs into the sender and see if there is anything bridging the pins together..
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