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  #1  
Old 11-18-2008, 07:41 PM
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testing the fuel reserve light via FSM?

My fuel reserve light doesn't work and I know it doesn't for a fact. I have installed serval different fuel/oil/temp gauges and none of them worked. I have cleaned the unit and there aren't any broken or damaged wires, I don't think. I have attached the FSM instructions on how to test the reserve light but instruction B doesn't make any sense to me. If someone could help me out it would be greatly appreciated. Thanks
Attached Files
File Type: pdf Fuel%20gauge[1].pdf (53.7 KB, 88 views)
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1985 300TD Turbo Euro-wagon
1979 280CE 225,200 miles
1985 300D Turbo 264,000 miles
1976 240D 190,000 miles
1979 300TD 220,000

GONE but not forgotten
1976 300D 195,300 miles
1983 300D Turbo 175,000 miles

http://i42.photobucket.com/albums/e3...e485-1-2-1.jpg
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  #2  
Old 11-18-2008, 08:23 PM
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I have not worked on the parts and so my answer is suspect but the way I read it the instructions are:

1) Remove the sending unit from the tank.
2) Read the resistance with the float all the way down.
3) Read the resistance with the float all of the way up.

I am assuming that the float rotates a part that turns 180 degrees between empty and full, hence the 180 degrees reference.

BTW maybe we should average our gauges. I have the exact opposite problem. The light does not turn off. Since the fuel gauge works fine I was assuming the problem is in the gauge itself.
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  #3  
Old 11-18-2008, 08:39 PM
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I'll try and test it this weekend. Thanks
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1985 300TD Turbo Euro-wagon
1979 280CE 225,200 miles
1985 300D Turbo 264,000 miles
1976 240D 190,000 miles
1979 300TD 220,000

GONE but not forgotten
1976 300D 195,300 miles
1983 300D Turbo 175,000 miles

http://i42.photobucket.com/albums/e3...e485-1-2-1.jpg
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  #4  
Old 11-18-2008, 09:11 PM
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The 180 degree reference is just a poor translation of "turn it upside down." The float makes contact (0 Ohms) when it is all the way to the bottom (empty tank, light ON) and does not make contact with the float anywhere else (tank not empty, light OFF).

A reserve light that never goes on can be caused by a bad/dirty bulb or its socket, the ground connection at the socket (part of the instrument cluster), dirt on the float contacts or at pins W and/or 31, or a broken wire somewhere.

Pin 31 is the 12 Volt source for the gauge and the light, BTW; "31" is a DIN standard for a switched 12 Volt source. If the fuel gauge works then pin 31 has 12 Volts and the problem is elsewhere.

Jeremy
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"Buster" in the '95

Our all-biodiesel family
1995 E300D (W124) . .239,000 miles My car
1996 E300D (W210) . .313,000 miles Wife's car
Santa Rosa population 170,685 (2012)
Total. . . . . . . . . . . . 722,685
"Oh lord won't you buy me a Mercedes Benz."
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  #5  
Old 11-18-2008, 09:44 PM
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When I turn the key to the run position the fuel light comes on but when the motor is running it goes off and never turns back on,even when the tank is empty.
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1985 300TD Turbo Euro-wagon
1979 280CE 225,200 miles
1985 300D Turbo 264,000 miles
1976 240D 190,000 miles
1979 300TD 220,000

GONE but not forgotten
1976 300D 195,300 miles
1983 300D Turbo 175,000 miles

http://i42.photobucket.com/albums/e3...e485-1-2-1.jpg
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  #6  
Old 11-18-2008, 10:07 PM
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Quote:
Originally Posted by 79Mercy View Post
When I turn the key to the run position the fuel light comes on but when the motor is running it goes off and never turns back on,even when the tank is empty.
Same here.
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  #7  
Old 11-18-2008, 11:25 PM
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OK, that means that the bulb and socket and the wiring (printed circuit and discrete) and such is good. The problem is therefore in the sender or the wiring between the sender and the instrument cluster. My guess would be dirty float contacts.

The light is powered in the "test" position (key ON, engine not yet started) by the ALTernator light. That's why it goes off once the car starts.

Jeremy
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"Buster" in the '95

Our all-biodiesel family
1995 E300D (W124) . .239,000 miles My car
1996 E300D (W210) . .313,000 miles Wife's car
Santa Rosa population 170,685 (2012)
Total. . . . . . . . . . . . 722,685
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

Last edited by Jeremy5848; 11-19-2008 at 11:59 AM.
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  #8  
Old 11-19-2008, 01:25 AM
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If the light comes on during the initial test, when the key turns on then the problem is probably in the fuel quantity sending tube. the tube has 2 wires on either side that a float goes up and down on changing the impedances and the gauge position. When the float gets to the bottom, 2 contacts are made at the bottom of the tube and the reserve light comes on. I would pull the quantity tube in the fuel tank and clean it. Make sure the contacts are good. You may have to clean them up with some sandpaper.

Dave
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1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
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1965 Mustang Fastback Mileage Unknown(My sons)
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  #9  
Old 11-19-2008, 07:40 AM
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Where is the sending unit on a 1987 300 TD. It would be nice if the light came on when the tank is low.
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  #10  
Old 11-19-2008, 12:16 PM
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Quote:
Originally Posted by jfikentscher View Post
Where is the sending unit on a 1987 300 TD. It would be nice if the light came on when the tank is low.
If you really have a 'TD' (station wagon), I don't know. If you have a sedan, it's accessible from inside the trunk. Make sure the tank is no more than 3/4 full (less is better). Remove the fabric trim that covers the fuel tank and you will see the sender high in the middle of the tank. A three-wire plug connects the contacts inside with the wiring harness. The nut that holds the sender in place can be unscrewed (normal right-hand thread) with a large pair of pipe pliers. With the nut removed, the sender can be wiggled out of the hole, tipped back to clear, and gently removed, with a rag underneath to catch the drips (there will be fuel inside that will slowly drip out). If you are not in a hurry, get the sender part way out and leave it there until all of the fuel has dripped out (say, 10 minutes).

Once the sender is out, you can carefully remove the tiny nut from the bottom plastic piece and pull the sender mechanism up out of the top of the aluminum tube. Now you can check and clean the contacts. It's pretty obvious once you get inside.

See this thread for a DIY w/pix.

Jeremy
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"Buster" in the '95

Our all-biodiesel family
1995 E300D (W124) . .239,000 miles My car
1996 E300D (W210) . .313,000 miles Wife's car
Santa Rosa population 170,685 (2012)
Total. . . . . . . . . . . . 722,685
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970
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  #11  
Old 11-20-2008, 09:50 AM
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Yes I have the wagon.
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1987 300TD 440K - My car
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2000 - CLK320 100K - Deb's car
1994 C230 150K - Josh's Car
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  #12  
Old 11-20-2008, 11:24 AM
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Quote:
Originally Posted by jfikentscher View Post
Yes I have the wagon.
If the 79 300TD is anything like a 1980 300TD, then the fuel sending unit is accessed via a plastic disc cover under the wagon black insert. It's about 4 inches in diameter and is clear\white-ish plastic disc. Take off the disc and the sending unit has an electrical connector on the top.

I have a spare 300TD fuel sending unit if you need one.
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  #13  
Old 05-19-2009, 11:44 PM
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im having a similar problem where my reserve light doesnt come at the gp postion.

i followed the fsm procedure and the gauge reades fine. bulb at instrument cluster is goood, harness is good. the 180 degrees makes sense if you think of a circle
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  #14  
Old 05-20-2009, 12:54 AM
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Quote:
Originally Posted by Oracle12345 View Post
im having a similar problem where my reserve light doesnt come at the gp postion.

i followed the fsm procedure and the gauge reades fine. bulb at instrument cluster is goood, harness is good. the 180 degrees makes sense if you think of a circle
Does the reserve light ever come on? Problem could be a bad bulb or poor contact of the bulb's socket to the pc board that makes up the back of the instrument cluster. Slight possibility of a cracked conductor in the pc board itself.

Jeremy
__________________

"Buster" in the '95

Our all-biodiesel family
1995 E300D (W124) . .239,000 miles My car
1996 E300D (W210) . .313,000 miles Wife's car
Santa Rosa population 170,685 (2012)
Total. . . . . . . . . . . . 722,685
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970
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