Parts Catalog Accessories Catalog How To Articles Tech Forums
Call Pelican Parts at 888-280-7799
Shopping Cart Cart | Project List | Order Status | Help

Go Back   PeachParts Mercedes-Benz Forum > Mercedes-Benz Tech Information and Support > Mercedes-Benz SL Discussion Forum

LinkBack Thread Tools Display Modes
Old 05-10-2005, 11:39 AM
Captain Scott's Avatar
Registered User
Join Date: Apr 2005
Location: Coos Bay Oregon
Posts: 90
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!

Reply With Quote
Old 05-10-2005, 11:57 AM
'81 380SL
Join Date: Mar 2005
Location: foot hills east of Redding CA
Posts: 20
Thumbs up 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.
Reply With Quote
Old 05-12-2005, 11:29 AM
Captain Scott's Avatar
Registered User
Join Date: Apr 2005
Location: Coos Bay Oregon
Posts: 90
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.
Reply With Quote
Old 05-23-2005, 04:17 PM
Captain Scott's Avatar
Registered User
Join Date: Apr 2005
Location: Coos Bay Oregon
Posts: 90
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...
Reply With Quote
Old 05-23-2005, 07:43 PM
Registered User
Join Date: Aug 2004
Posts: 232
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
Reply With Quote
Old 05-24-2005, 02:21 PM
Registered User
Join Date: Jan 2005
Location: Kingsport, TN
Posts: 140
Buy my 1980 450SL. Its tachometer works just fine.
Lawrence Coppari

2002 SLK32 AMG
2005 Acura TL
1987 328GTS
1986 944T
Reply With Quote
Old 09-04-2005, 12:09 PM
Captain Scott's Avatar
Registered User
Join Date: Apr 2005
Location: Coos Bay Oregon
Posts: 90
Problem solved!

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


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Trackbacks are On
Pingbacks are On
Refbacks are On

Similar Threads
Thread Thread Starter Forum Replies Last Post
1970 300 SEL 6.3 & 1974 450 SE For Sale M.A Razzaq Mercedes-Benz Cars For Sale 2 07-24-2006 09:08 PM
1974 mercedes 450 sl pjm Tech Help 1 04-28-2005 11:09 PM
1974 450 SL--newbie questions phango13 Vintage Mercedes Forum 12 09-14-2004 08:13 PM
1974 450 sl driver window ted Tech Help 4 11-28-2003 01:56 PM
Which oil in a 1974 450 SL??? ytown Vintage Mercedes Forum 1 06-22-2002 06:22 PM

All times are GMT -4. The time now is 09:57 PM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2018 Pelican Parts, LLC - Posts may be archived for display on the Peach Parts or Pelican Parts Website -    DMCA Registered Agent Contact Page