Mercedes-Benz Forum banner

1 - 8 of 8 Posts

·
Registered
1974 450 SL
Joined
·
55 Posts
Discussion Starter #1
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. It's jumping around more recently (and might be worsening) 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 have to guess that whatever I touched recently (removing the inst cluster) 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.) 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. Can I use a volt meter to check the tach output to verify the input to the gauge? If I had a bad ground, wouldn't I be seeing other odd intermittent behavior? Or shouldn't I be able to bring a "new" ground path to the cluster ground to test fixing the tach? This is not yet proven. Since I work like a dog all week, my workweek is good for investigating things and the weekend is good for diving under the girl and trying to make progress!

Here is the background... This 1974 450 SL with 210k is a "family heirloom" that I took custody of in February. It has seen good mechincal maintanance but no TLC and certain things have been let slide. The undercarriage is filthy but Gunk should fix that. The hardtop has leaked so the inside was pretty stinky! I pulled the console, seats and carpet and cluster to clean and freshen it up and maybe get the frozen clock running. Then after reinstalling everything the car wouldn't start due to an interlock issue that I have yet to solve. Nopw, the clock is frozen dead, too. The usual checks of the 4 plug wires under the seats revealed nothing out of order. I'll try to jump them out of the circuit. Or maybe I messed up an interlock connection in the ignition switch wiring lume? I put in an ignition switch bypass so I can at least start and drive it while I trouble shoot! Someone sent me the PDFs for the wiring system so I have some good references with which to do the troubleshooting. The connections at the tranny interlock terminal bar looked clean so I ruled it out just because I didn't touch it (yet...) and it made no diff where the shift lever was (D or N or inbetween.)

So here is my laundry list of things to fix:
1) tach jumping around
2) starter interlock
3) leak at front rt hardtop seal
4) infrequent rough idle (ongoing)

Thanks for listening and maybe commenting,
Scott
 

·
Registered
1974 450 SL
Joined
·
55 Posts
Discussion Starter #2
OK, 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 points/condenser problem?

Scott
 

·
Registered
1975 450SLC
Joined
·
7 Posts
Hi Scott

I wish I could help but I have not been able to figure out a similar problem with my 1975 450SLC. I have cleaned the contacts on the Instrument Panel but have not had any improvements.

I will post something if I figure it out.

Mark
 

·
Registered
'80 450SLC Afro RHD Ikonengold
Joined
·
2,884 Posts
No matter how strange it may be, check negative terminal that connects engine to the chassis. As a matter of fact, if no trouble, run a wire from heads to the chassis and see if there is any change.
 

·
Registered
1974 450 SL
Joined
·
55 Posts
Discussion Starter #5
Tach Problem SOLVED

Well, it was a simple as changing the ignition points. They were a little pitted and dirty. This also cleaned up some of my idle issues. Two injectors are now not firing, so I have more work to do, but I'm certainly happy about looking at the tach and not seeing it bounce around!

Would the can of Sea Foam i put in the tank cause the injectors to NOT fire??
S
 

·
Registered
2017 Police Interceptor Utility, 2017 Police Interceptor Sedan
Joined
·
7,742 Posts
RE: Tach Problem SOLVED

Captain Scott - 9/2/2005 9:38 AM

Would the can of Sea Foam i put in the tank cause the injectors to NOT fire??
S
Highly unlikely.
 

·
'89 300E Turbo project, ‘85 Audi Coupe Quattro, ‘71 BMW turbo 2002, '73 BMW 2002tii, ‘67 Kaiser M725
Joined
·
17,725 Posts
I would say that the circuit is defective.If it is only two injectors not working,then that rules out bad
trigger points.If the circuit checks out,then it is possible that the injectors are faulty.

In regards to Sea Foam:

If it contains any methane products,it will KILL your
fuel pump,plain and simple and highly recommend that
you never buy the transmission tune that they sell.
I have had two transmissions fail shortly after adding
it to them.[:(!]
 

·
'89 300E Turbo project, ‘85 Audi Coupe Quattro, ‘71 BMW turbo 2002, '73 BMW 2002tii, ‘67 Kaiser M725
Joined
·
17,725 Posts
Nutz 4 Benz - 9/3/2005 6:01 AM

I would say that the circuit is defective.If it is only two injectors not working,then that rules out bad
trigger points.If the circuit checks out,then it is possible that the injectors are faulty.

In regards to Sea Foam:

If it contains any methane products,it will KILL your
fuel pump,plain and simple and highly recommend that
you never buy the transmission tune that they sell.
I have had two transmissions fail shortly after adding
it to them.[:(!]

Harness I meant. To late for an edit.[:)]
 
1 - 8 of 8 Posts
Top