Fluctuation in RPMs???

[dldubois]

When coming to a stop in my 280sl I typically get fluctuation in the rpms causing stopping to not be a smooth as it used to be.  

Any ideas on what might be the cause?

Dana DuBois

1969 280sl auto
blue (906)
Corinth, TX

[rwmastel]

With a warmed up engine, place the auto transmission in neutral or park.  Step firmly on the brake.  Do the RPMs rise?  If so, then it's a leaky brake booster, or less likely a leak in the vac line to the booster.

Rodd
Powell, Ohio, USA
1966 230SL, Euro, Auto, Leather, both  tops
1994 E420

[ja17]

Hello Dana,
Tune-up items should be checked first. The linkiage can be out of synch. Makes sure the venturi stop is correct and then check the linkages. After the linkages are correct the idle solenoid or intake constant speed vacuume cell can be set.

Lastly the injection pump "idle control screw" can be adjusted to change the idle (low range) mixture to smooth the idle out.

Does the idle "search" (cycle up and down) or is it just rough?

Joe Alexander
Blacklick, Ohio

[jeffc280sl]

Check the "check valve" and the vacuum hose from the intake manifold to the brake booster.  Sometimes they leak.  I'm trying to connect braking and rpm fluctuation.

Jeff C.
1970 280SL 4-speed

[dldubois]

It's happening when applying the brakes.   I tried applying while in park and get fluctuation in RPMs as a result.  

Also it doesn't seem to be rough.  It just fluctuates when brakes are applied.  The vacuum line is in three pieces and I'm going to check all.

Hopefully, this fixed tonight.



Dana DuBois

1969 280sl auto
blue (906)
Corinth, TX

[jeffc280sl]

My 4 speed does the same thing.  I think it's common to fluctuate some when braking.  A leak would make it worse.

Jeff C.
1970 280SL 4-speed

[Ben]

My 4sp. is the same and I've checked for leaks but found none !

Regards,
Ben in Ireland.
'64 230SL 4sp.

[dldubois]

I haven't found a leak yet but will tell you that years ago when my father owned this car, it did NOT fluctuate when braking.

I'll report results.


Dana DuBois

1969 280sl auto
blue (906)
Corinth, TX

[rwmastel]

quote:Originally posted by dldubois

It's happening when applying the brakes.   I tried applying while in park and get fluctuation in RPMs as a result.  

Also it doesn't seem to be rough.  It just fluctuates when brakes are applied.  The vacuum line is in three pieces and I'm going to check all.

Hopefully, this fixed tonight.

Dana,
Any progress?  To remove the whole brake booster & vacuum line, just pull the vac line from the intake manifold and plug the hole.  Don't drive far if at all, you have no power brakes!  See if this changes your symptoms.

Rodd
Powell, Ohio, USA
1966 230SL, Euro, Auto, Leather, both  tops
1994 E420

[dldubois]

Rodd:

I'll have an answer for you tomorrow evening.  

I checked the check valve and it's ok.  I also couldn't find a leak in any of the hoses.  (I guess I need to pull them and test them individually by capping and submersing).    

Plugged the vacuum line at the manifold and the car purrs.  I'm still just trying to figure out if its the booster or the lines.  


Dana DuBois

1969 280sl auto
blue (906)
Corinth, TX

[rwmastel]

quote:Originally posted by dldubois

I'm still just trying to figure out if its the booster or the lines.

Do you know how old the booster is?  I'm assuming that's the culprit.  Most people replace them, but I think you could find a shop to rebuild it.  I couldn't in 2001 when I replaced mine, but perhaps I didn't look hard enough.  Is yours an ATE brand, T51 model?

Rodd
Powell, Ohio, USA
1966 230SL, Euro, Auto, Leather, both  tops
1994 E420

[dldubois]

Rodd:

I've replaced all the vacuum lines as well as the check valve and still have the same issue.   I also disconnected the vacuum line and plugged (eliminating the power brakes) and still am having the same issue.  

Could it be transmission related?   I'm thinking it can't be the booster as I've removed the vacuum lines and am still getting the same issue.   I go to stop and the car lurches (RPMs fluctuate) as I'm stopping.  

Thanks.

Dana DuBois

1969 280sl auto
blue (906)
Corinth, TX

[Cees Klumper]

Does the same thing occur when you brake using only the emergency brake? Or try disconnecting the brake light switch/circuit for your test run. Reason I suggest this is on the outside superslim (1%?) chance that the electric circuits connected to the braking have anything to do with it (influence on ignition, loose cables, that sort of thing).
Just grasping at straws here.
Cees ("Case") Klumper in Amsterdam
'69 white 280 SL automatic

[dldubois]

Here's an update.   I've ruled out the brakes and vacuum.   I think its related to the transmission.  It seems when coming to a stop the transmission downshifting is causing the lurching.   Could this be a leaking diaphram in the modulator?

Any ideas on how to identify for sure?  Any other ideas?

thanks.

Dana DuBois

1969 280sl auto
blue (906)
Corinth, TX

[rwmastel]

So if you put your car in Neutral, this RPM change does not happen when braking to a stop?

Rodd
Powell, Ohio, USA
1966 230SL, Euro, Auto, Leather, both  tops
1994 E420