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  #1  
Old 05-11-2002, 03:45 PM
jcd jcd is offline
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Join Date: Jun 2001
Location: Northern New Jersey
Posts: 1,102
Fuel Gauge Going Wacko

My 1977 300D fuel gauge is going nuts,,,,,,,,,reserve light flashing and the needle is fluttering back and forth like I am in the middle of a 6.5 on the richter scale.

I really never use it, fill up based on mileage, so my plan is to disconnect it unless there is an easy fix.

Let me know,

JCD

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  #2  
Old 05-12-2002, 01:45 AM
HGV HGV is offline
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Join Date: Sep 2001
Location: San Francisco
Posts: 424
It sounds like you are having float problems. It is not your gauge but the float located in the tank. If you are lucky, and your fuel tank is below the trunk it is easy to fix. Sometime in the 70's or early 80's they moved the tank from below the trunk to the front of the trunk area right behind the rear seat. If it is in the trunk follow these directions. If not locate fuel tank and then look for the sending unit and follow these idirections.

In the trunk, under ther rubber mat, there is a plastic cap about 3" in diameter with a wires going through it. Remove the plastic cap and remove the plug. You are looking at the fuel sending unit in the tank. remove the bolts, be carefull not to drop any because they are difficult to recover. Slowly remove the sending unit becuase there will be fuel in it and you want it to drain out. When you have it out, remove the small nut on the bottom and carefully remove the aluminum tube. Be carefull because the two resistant wires are very fragile and can break easily. Clean out the tube and the retaining cap and plastic washer with all the small grooves in it. If you open it up and there is only one wire left, you need a new unit. If a wire is broken, you need a new unit. Clean out the contacts on the bottom that turn on the reserve light. After you have it all clean, reconnect the wire and manually move the float and see if the gauge works and the reserve light comes on when you bottom it out. If ieverything looks good, put it back together and reconnect erverything.

Good luck

Henry
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  #3  
Old 05-12-2002, 12:06 PM
Senior Member
 
Join Date: Jul 2001
Location: San Antone
Posts: 408
JCD:

Your fuel tank is located in the trunk behind a metal panel. The fuel sending unit can be R&R'd by removing the metal panel that hides the fuel tank (clean out trunk, 4 - 9 or 10mm head screws hold panel in place), then remove first aid kit and the plastic tray the first aid kit sits in (2 phillips head screws), then remove the electrical connection to the fuel sending unit, then un-screw fuel sending unit using large pair of Channel Locks (or similar tool, tight space), then pull the sending unit out through the space where the first aid kit is located letting it drain before completely removing from fuel tank. Take the sending unit apart and look for crud and build-up interfering with contact for the float to wires and look in the sending unit tube for crud/build-up also - clean everything being careful with delicate wires. Or, you may find something broken/cracked (wires, float, etc.), then replacement is the only option - about $90-$100 for a new one. When installing be careful that the sending unit's sealing o-ring is in good condition (I re-used mine with no leaks). The fuel gauge bouncing around is a common problem, you may also need to check the elec. connection at the gauge in the instrument cluster for corrosion and good contact.

Good Luck!
Tom
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  #4  
Old 05-12-2002, 01:06 PM
jcd jcd is offline
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Join Date: Jun 2001
Location: Northern New Jersey
Posts: 1,102
Thanks a bunch

Mine is behind the seat, thanks for the instructions.

This site is the best.

JCD
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  #5  
Old 05-12-2002, 01:28 PM
mccan
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JCD,
My guage occasionaly does the same. Sounds like another chore on the list. And thanks for the ACC fax. Great stuff.

You're right. This site is the best.
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  #6  
Old 05-13-2002, 02:17 PM
PeterG
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JCD:
I posted this a while ago but this is how I went about fixing my Erratic fuel gage in detail. I take pictures of other things I took apart (Heater control valve and fuel injection pump) but unfortunately nothing on the sender.
As I stated before the sending unit is somewhat delicate, so treat it with care. The first thing I would suggest is disconnecting the battery. This is added insurance to make sure we don’t short something out. On my 300 SD, the sending unit was located under the hat shelf (as it is called in my book) under the first aid cavity compartment. I had some trouble getting that compartment out. My rear deck has seen some better days, and it seems the compartment was somewhat brittle. I think there were four screws that held it in. Under that compartment is the sending unit (another reason why MB are so great). Maybe before you go any farther get the pan and rags mentioned below,and a permanent marker, and some tape wide masking or duct tape would be ideal.
The sending unit has a connector on the top that pulls off, straight up. Now place a mark on the sending unit, and the tank. You will use these as your line up marks when reinstalling and tightening the sending unit, unless you have a torque spec and torque wrench and the correct socket in the next paragraph. The top of the sending unit is a huge nut (1.856" or 47 and some MM). Unfortunately I did not have a socket or wrench to fit it, so I used a large pair of channel locks. It is a right hand thread(righty tighty,Lefty loosy). I backed off the nut (which is the sending unit) and withdrew the unit. Have a pan or something because the sending unit will be filled with fuel, depending on how much fuel is in the tank. Like mine the holes were partially blocked, and I had to let it stand over the tank for a while to drain. (I had not planned for this, as I had nothing close by,or rags). The unit is about 3" in diameter and 8" to 12" long. At this point take that tape, and put an FME cover over the hole. (Foreign Material Exclusion) Nuclear plant terminology and a good practice for anything left open.
On the bottom of the unit I believe there was a nut, that holds the assembly together. I loosened the nut and gently tapped on the side to release the can around the sending unit (a coating from the fuel forms locking this assembly together). Once inside look at how it operates. The fuel fills up the can slowly through the holes in the side and moves the float up and down. Now observe the bottom plate. It has almost a circular mouse maze the fuel follows into the sending unit (I think this is used for dampening when the fuel gets down very low). Mine was very dirty. I carefully separated the sections (Make note how they come apart so you don’t mix them up) and cleaned them in mineral spirits. Going back to the sending unit, the float (if I remember correctly) rides up and down on a resistive rail. I cleaned this (the rail) with Q-tips and mineral spirits. At the bottom of the travel (float all the way down) there are two contacts. These activate your reserve light (Contacts on float touch contacts on housing assembly). I used Q-tips and mineral spirits, but I think I also took an eraser from a pencil and rubbed them a little. I had the liberty of checking this out with a ohm meter to check continuity.
I then took the unit back to the car, Plugged it in,(make sure it is not positioned in such a way that the internals can short out on something metal) hooked up the battery, turned the key on and exercised it and observed the gas gauge. Note:I don’t remember if I had to take the unit and ground it by touching it to the tank(it will respond instantly because it is now not dampened. I made sure if I moved it slowly the gauge would follow with no dead spots (Gauge bouncing or going in the opposite direction of the float movement). I then took the float all the way to the bottom, or if you have the sending unit vertical, let gravity take place. The light should come on. If everything works, just turn your key off, disconnect the battery, and assemble the unit the way you took it apart. When installing watch that the o-ring is centered in the bottom of the sending unit. You don't want to pinch it, or cut it. I would tighten to the same mark you place on in the beginning. Connect connector reinstall first aid compartment, and enjoy now knowing how many miles to the gallon these soot blowers get. If you have any questions feel free send me a thread. I will answer any questions you have before you attempting it.
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  #7  
Old 05-13-2002, 02:23 PM
jcd jcd is offline
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Join Date: Jun 2001
Location: Northern New Jersey
Posts: 1,102
Peter G...thanks

I appreciate the detailed instructions.

At this point, I'm going to give it a try on Memorial Day weekend.

Or, it looks like I should wait for the "How to get diesel smell out of clothes" post to come up with a difinative solution...LOL.

JCD

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