fuel gauge jumps back and forth. 1980 300d 123
 

Is the caused by the gauge in the instrument cluster or at the diesel tank. got to fix today please help so i can start on it.

It's usually caused by the accumulation of gunk on the sending unit. I had the same issue with the '87. Removal and cleaning of the sending unit cured most of it. Powerservice cured the remaining problem after about 2K miles of use.

is there anything that can be added to the diesel tank that can clean it

In the diagram, which one of these would be the sending unit. and about where is it.

Found it in one of the diy's thanks http://boostnbenz.1baddsm.com/DIY/

I removed my fuel tank from the car and the sending unit from the tank and washed and flushed both out . Since then I have run BD in a variety of blends for the past 5k miles so I know it is clean in and out and my; fuel guage is stll erratic most of the time although today before I filled up, the guage read 1/4 full and it took 13.75 gals which is pretty accurate considering.
So I don't know what the issue might actually be caused by but the guage seems to relatively inaccurate at the best of times.

cleaned the sending unit and it still flickers. any thoughts

the instrument cluster gets a lil squirrelly after 30 years
kinda like me after 55 years

weird electrical stuff usually turns out to be bad grounds. Take out your cluster and look at the solder traces to see if any are starting to crack. I used to have a pic of this, but I'm not at home til next week.

This is the cause, a weak ground to the gauge. Sometimes the traces burn up because they carry more current than they are designed to due to weak grounds. This usually happens on the dash lighting because it uses the most current of anything in the cluster.

You can always run a ground jumper from the chassis behind the dash directly to the ground on the gauge to fix this or you have to take the cluster apart and clean the ground connections. Mercedes doesn't have the best electrical design because ground is distributed between the cluster segments through bolted connections. They develop oxidation or the nuts get loose over the years causing a weak ground. If you take the cluster out and take it apart you will see what I mean. Just clean up the connections between the cluster segments at the bolted connections. I use a product called "de-oxit" when I put them back together to help prevent them from oxidizing again.

Ray, I just changed my sending unit from the parts car to mine and no more jumpy guage. I had removed and perfectly cleaned my sending unit but it still jumped. I replaced the sender and the guage is rock steady. Paul

I have the large socket you need to remove the sender if you need to borrow it.

I trust that you took the sending unit apart and removed the wire assembly from the can. If so, you probably found a pile of gunk at the bottom. The entire assembly needs to be thoroughly cleaned with brake cleaner.

After reassembly, the flickering should be lessened. If it has lessened, the problem is with residual gunk at the bottom of the tank. I solved this with the use of Power Service conditioner for about four tanks. The '87 has improved to the point where it barely flickers below 1/4 tank.

Sure, it could also be bad grounds...........but the cleaning of the sending unit would have to produce zero benefits for me to go in that direction.

I cleaned the sending unit I guess now , i'll get one off of my parts car and try it. i've got the tools.


I just fixed the dash lights. I needed to jump th connections because the dimmer switch was bad and My daughter had no lights. I'll have to pull it again if the senders don't work. they are the easier thing to check.

when in the dash i useg 3 or 4 gifferent clusters to repair hers and I'll have to check again because the odomoter is still not working. Thanks

Ray, I'm in Mobile, where are you at? If you're close, I'd like to meet. I fly out Saturday morning. Nothing like meeting forum members on a business trip.