Fuel Guage 300D '85
 

How do you adjust the fuel guage settings? The other day it showed below empty but the reserve light did not come on. I immediately pulled into a station thinking I was running out of fuel, but it only took 16 gals. to fill up. (out of 21), so I was not any where near being empty. This is just a minor problem, but I would like the guage to be a little more accurate.

Thanks

Gary

Gary, I just posted to ADIAW83, on a 300 SD. I have had my sending unit out, and apart. It is a slidewire verywell engineered, except it gets dirty. Mine was very dirty and the set of contacts that gives you the light sits at the bottom of this assembly. The holes in mine that allow the unit to fill up and drain were also partially plugged. Giving a false indication. If you are interested, I can tell you how to remove it and take it apart.



PeterG

Would you mind posting how you did it, I'm sure more out there (myself included) would like to know of a possible solution to their funky acting fuel gauge

The previous owner said he had the fuel guage sending unit replaced. They must not have calibrated it correctly. I haven't had time to look to see if it is dirty. The reserve light does come on when I turn the ignition, so the light must not be burned out.

Gary

I apologize for not getting back to you sooner. 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.
[B][U]The first thing I would suggest is disconnecting the battery. [B]

Thanks Peter for all of the good info. Try to get to this soon.