Wacky fuel gauge - PeachParts Mercedes-Benz Forum

Habanero · #1 09-09-2003, 09:53 AM

My fuel gauge is not working correctly, and I am having a little trouble hunting down the problem. While driving down the road, it fluctuates between 3/4 and full all the time, no matter what the fuel level is in the tank. The previous owner had a new fuel sender put in, but that did not fix the problem. I have pulled the plug off the sender, and the gauge goes to empty. When I jumpered the wires on the gauge plug, the gauge goes clear to the full peg. It would seem like the gauge is doing what it should, so I am thinking maybe the new fuel sender is bad. Anybody have any other ideas before I go spend $100 on a new sender? By the way it is an 87 SDL.

JimSmith · #2 09-09-2003, 09:57 AM

Habanero,

Try a search on the subject. This is a common problem and one solution is a new sender. Another solution is to fix the sender you have, which is described in a number of older threads. Good luck, Jim

#3 09-09-2003, 02:02 PM

I had the same problem and did the search - then still took it to my mechanic because I didn't have the 32mm wrench to remove the sender.

Good news - my mechanic just took the sender out and rotated it back and forth a few times and it worked. I cancelled my order for a new sender and will now buy the requisite wrench or socket to do it myself next time. This was several weeks ago and it still works.

#4 09-09-2003, 05:12 PM

When the tank is full the sender should read about 3 ohms. When it's empty it should read about 80 ohms. If you substitute resistors of those values for the sender, you should get stable readings of full or empty at the gauge.

A fatigued fuse can cause the gauge to jump. The fuses (when used at 80+% of their rating) will fatigue over time and stop provideing a reliable low resistance path. If you change fuses every ten years or so, such failures will seldom occur and the cost of a set of fuses is VERY modest considering the number of intermittant electrical glitches it prevents.

Dirty wires can cause problems too.

Marshall

Habanero · #5 09-09-2003, 05:31 PM

Thank you all for the help. I will try cleaning up the sender tonight to see if that helps. I am not sure if that is truly the problem, since it had a new sender put in which did not help the situation. I had not thought of the fuse being a possiblity, so I will try that as well. Anyway, thanks again for pointing me in the right direction, and I apologize for not searching first.

Thomaspin · #6 09-09-2003, 06:17 PM

It's not always the sender or fuse.

In my case, after replacing both, I had to pull the gauge and drop a small amount of very thin oil on the needle shaft. That cured the binding which was causing the jumpy needle - similar symptoms to your.

See my web site on how to pull the gauges.

Maxwell · #7 09-10-2003, 02:49 AM

I had a jumpy gauge caused by a dirty sender. For those who haven't taken apart a sender, it is a float on a rod that makes contact with two fine copper wires. the gauge reads the resistance, which increases as the float goes down, and the amount of wire the current is traveling through increases. If there are small dirty spots on the wire the resistance jumps and the needle falls, or if the whole wire is dirty, a clean spot would make the needle jumps.

So, I took mine apart and cleaned it. you definitely don't need a special tool to get it out in a 123, just some channellocks. once it's out there is a weird little nut on the bottom of the tube, the assembly comes apart, and you can clean the wires and the contacts on the bottom that make the reserve light go on.