Gauges Bouncing, Pics of electrical connections - PeachParts Mercedes-Benz Forum

DeltaFox · #1 09-27-2008, 07:20 PM

I have read that bouncing gauges were usually caused by a bad ground. The two gauges I am having issues with are the fuel and the temp. I read one post where a guy mentioned that the two shared the same ground connection. I have heard lots of guys comment on fixing the ground and having great luck so I figured I would give it a try.

My problem is which one is the ground? I looked at the traces and didn't have much success at finding the "ground". I saw tons of brown wires grounded to the chassis behind the instrument cluster but can't seem to find out where the ground wire connects to the instrument cluster. All the grounds were securely connected too.

I also noticed what I think is rust forming around the soldering connections. How do you get rid of that stuff without damaging the rest of the board? The traces I saw all look good and aren't broken so I'm stumped.

I don't want to keep this out of the car long since I have three young kids running around the house.

Bens lover · #2 09-28-2008, 01:13 AM

MMMM Ground you say?
Well let me tell you that Ive solved the temperature gauge issue,it was the sending unit (on the engine) itself that was faulty.As for the fuel gauge I would probably look at the sending unit in the tank first before replacing it.There could be a wiring issue there.
Keep us posted.
Good luck
Dan

DeltaFox · #3 09-28-2008, 02:28 PM

I forgot to mention that I have already replaced the sending unit near the thermostat and have had the same results.

babymog · #4 09-28-2008, 03:44 PM

The "rust" in that photo is the flux from the soldering process, and should be harmless. If you want to clean it, isopropyl alcohol / rubbing alcohol will work but is IMO unnecessary.

pmckechnie · #5 09-28-2008, 04:58 PM

If you look closely at pin #4. it looks like the solder is cracked. I don't know right off what pin 4 is for but could cause a problem like you describe. I fix problems like this almost on a daily basis. The crack will usually be on pins that have a heavy unsupported cable (like the one for the cluster) attached or on joints which hold heavy parts (relays an such) to the board.
Paul

cheapguy · #6 09-28-2008, 06:05 PM

I had that problem, though just with the temperature gauge and found it was a (chassis?) ground cable to the transmission/engine interface that was causing the problem. I re-worked it and the gauge works fine. Whoever who had done work on the transmission hooked the ground cable up to the wrong size bolt and it was functioning intermittently because it was loose. The car had been starting only part of the time, and investigation (sparks)revealed the problem. The gauge now reports without waving at me. In other words, the ground in question may be somewhere else besides the dashboard.