Tach Fluctuates - PeachParts Mercedes-Benz Forum

Darrell Woods · #1 12-25-2004, 07:49 PM

The tach on my SL never quite stays in one spot. I don't feel a miss, and know that I need to hook up an external tach to know for sure what's going on. Just curious, are these things known to "float" at highway speeds? For those of you who may be having a miss problem, I spent a long time running mine down. Items replaced without helping were the fuel pump and filter, all eight injectors, ignition points and plugs, trigger points, plug wires and distributer cap, and vacuum hoses. Rather expensive trial and error, but I figured these items could use replacing anyhow (except the fuel pump...that was a must when I went in the garage and found gasoline all over the floor). Still, after all this, the miss was as bad as ever. Finally checked the pressure to the injectors. It was all over the place. Replaced the pressure regulator and it purrs now. Still wondering about the tach though. That little float of about 300 RPMs bothers me.

bwheitman · #2 01-11-2005, 10:16 AM

Darrell,
I have a 96 SL320 and the tach does move just a hair up and down at idle. Before the motor mounts were replaced you could feel it, but the dealership says it is normal, the ETA is actually controlling the idle and it moves it up and down for various reasons, it does it when I have my air on even more. Keep Benzing!
Brian

Steve Gutman · #3 01-11-2005, 12:38 PM

I had that type of problem on my 107. It was cured when I replaced my ignition control module. I had a lot of idle control and drivability problems but cured them all over time. Any other symptoms?

Darrell Woods · #4 01-11-2005, 09:35 PM

Mine's a '75 450SL. No other major problems, just wondering about the rpms. Thanks for the replies.

Steve Gutman · #5 01-12-2005, 10:10 AM

I have been told "they all do that". THe 450 & 500 series engines with the electronic idle control are the ones he was referring to. At idle, mine will run perfect and silent at certain tempratures and at others it will slightly cycle up and down but so little I can't see it on the tach. I figure if your tach is moving then the RPM must be going up and down. I think you car has a manual idle control. If the tach chcles but the motor rpm doesn't change it is most likely the tach. Best luck...