1974 450 SL Jumping Tachometer
 

I have a tach which is now jumping around and above what it seems is the real RPM. It seems to read a few hundred high on average when the oil pressure gauge is reading its usual idle RPM pressure. Up to about 2000, it seems to be reading high from actual. It has been jumping around more recently at all but the highest rpms and it may be a little better after warming up, but it may just be wishful thinking.

I first thought that whatever I touched recently (removing the inst cluster for cleaning) would be the likely candidate for the the cause! I was told it might be the ground strap from the starter (or other main standard electrical path to ground.) But losing a good ground right now would seem be terribly inconvenient and nasty timing, so I doubt it. I've not yet climbed under to retorque the bolts. If I had a bad ground, wouldn't I be seeing other odd intermittent electrical behavior? Would perhaps squirting down the undercarriage with a hose provide a quick "electrical check" to see if the ground can be temporarily improved?

And now I have a new development that probably tells someone something, but not me...

Yesterday, the tach went smooth with no detectable jumping at 1900 RPM or above. This morning, there is slight movement at up to 2100 and then it is perfect above that. Regarding a possible bad ground, since the roadway was wet yesterday, I thought that might have decreased the resistence and made this change in the indication. But wouldn't a bad ground effect all RPMs?? Could this be a gauge or points/condenser problem?

Ideas would be appreciated!

Scott

Jumpin' guages
 

Hello Scott ... fancy meeting you here!
The Capt used to be my boss in HI before he ran off to bigger better things.
My friendly master mech had me clean the three nuts with fingernail polish on the back of the inst cluster ... tis the grounds for the guages. Cured my jumping guages.

Today it is still jumping around and now up into the 2000s, so I bet it has to do with conductivity of a ground somewhere. I'll try to wet down the port then starbord sides and see if it makes any diff. Then I'll dive under. But not til the week after next. We are going to Belize Saturday for a week to find some sun and imported beer. Not much help lately off these boards... Hmmm.
S

We're back from Belize now. Now with the chasis of the car being nice and dry, the tach jumps wildly. I am really going to find time soon to investigate the poor earth ground. At least I think that is the smoking gun. The shop is now almost completed, so I'll have no distractions...

when you say your tach jumps, is it like showing a uniform RPM and then swinging wildly toward redline and sort of 'bouncing' back and forth?

you voltage regulator might be on the way out.

that's what my tach did on my 78 450 SEL

Buy my 1980 450SL. Its tachometer works just fine.

Problem solved!
 

Finally replaced the ignition points and the tach is now solid! No more bouncing around. Obviously, the signal is routed through there.