Rough (low) idle in gear

[bpossel]

Hi All,

Christmas is close!  Hope all your shopping is done....  I havent even started yet.....  only purchased a few gifts for my 113 so far...   kidding...  

Problem:
I am still sorting out an issue that I have with a rough idle when I put the car in gear.  Car runs good overall.  When in Neutral, the car idles around 700-800.  When I drop it into gear, the rpms drop to almost 0, car doesn’t die out...  and then rpms go back up to ~400.  This low idle is rough...

I read that the idle switch (on the venturi) and constant speed solenoid work together.  If the idle switch is not adjusted correctly, or is faulty, then the constant speed solenoid will not work correctly.

How do I adjust the idle switch?  How do I test if it’s working correctly?  Note: the constant speed solenoid does move forward when I kick on the air-conditioning.

Thanks all!
Bob


bpossel
Memphis, TN.
1971 280SL
1997 E320

[Cees Klumper]

Sounds like your switch is operating fine (no adjustment that I am aware of), but the constant speed solenoid is either worn, not adjusted properly or meeting too much resistance in the linkages.

First try adjusting the solenoid (there is a small counterlocking nut on the shaft that allows you to increase/decrease the idle RPM's). Also, make sure your linkages are all clean, lubricated etc. so that there is minimal resistance there. Check whether the solenoid meets with the linkage 'properly'.

Cees Klumper in Amsterdam
'69 white 280 SL automatic

[bpossel]

Hi Cees,

Can you expand on the adjustment on the solenoid.
I have the solenoid adjusted so that it touches the linkage when in idle.  Please explain adjusting the counterlocking nut to effect the rpms?

Thanks!
Bob


bpossel
Memphis, TN.
1971 280SL
1997 E320

[Cees Klumper]

Hello Bob - loosening the counterlocking nut allows you to move the end of the shaft outwards sufficiently to increase the RPM's to the desired level. If not (if the adjustment is already at its limit), then you can try bending the post that the solenoid sits on, towards the linkage, thus giving you the required range of adjustment. This should (hopefully) all be pretty much self-explanatory once you check out how it all works together.

Cees Klumper in Amsterdam
'69 white 280 SL automatic

[George Davis]

Bob,

(leaping in where I know nothing...) I thought the CSS was activated when the transmission was placed in gear, and that the switch was somehow connected to the transmission (either via a cable, or a pressure switch on the trans).

(crawling back in my corner now)

George Davis
'69 280 SL Euro manual

[tuultyme]

With the automatic transmission in gear the CSS is grounded through the pressure switch on the side of the transmission.  So when the transmission is in gear the circuit is commpleted and the CSS pushes the linkage to increase the engine speed a little.  My CSS was activated all the time the engine was on.  I found that the wire that goes over the top of the transmission was somehow grounded to the transmission itself; so for now I just added a second wire and disconnected the original one.  The idle switch also has some adjustment that Cees went though in one of his earlier posts on that subject.

Bruce; 268Blaugrun(green) 1970 280SL; IL

[jlennon3]

Another possibility is your idle mixture is a bit too lean. Try making the idle mixture a bit richer by "clicking" the idle adjustment a few "notches" in the clockwise direction (be sure to write down exactly what you have done so you can get back to the original setting). If your idle improves, you know the mixture is too lean and may need be adjusted with help of a CO meter to proper settings. In my opinion these cars seem to run better idling a bit richer than factory spec (say around 6% or so). Note if you're only experiencing this problem during warm up, you need to adjust warm up device as per service manual. I hope this helps...

John
113 chassis euro
108 chassis US

[bpossel]

Update!

I have finally gotten my idle in gear to be ~700 rpms.  Neutral runs ~800 or so....  This is sooo cooool!

Thanks to Joe A.!!!  It appears that my car, later '71 model (automatic trans), retains the proper idle in gear with the vacuum throttle control, not the constant speed solenoid (CSS).  The CSS on my car only kicks in when I turn on the air-conditioning.  The vacuum throttle control is mounted on the venturi.  See pics below...


Download Attachment: vacuum-gov1.jpg
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Download Attachment: vacuum-gov2.jpg
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What I finally ended up doing was to, start over again, adjusting all of my linkages per the info from this site.  Then I removed and cleaned up the vacuum throttle control unit, reinstalled, and then adjusted the "shaft distance" and "spring distance" per the factory diagram (pics attached) using a ruler.  In testing the car in gear, it was nice to see the "throttle valve opening" right at about 1.5mm.

The car now runs great when I put it in gear and no longer drops to ~0 rpms...  It does drop a little, but the vacuum throttle picks it right back up and holds it.

Another note: I also notice now that when the car it turned off, the "throttle valve opening" gap is quite large (havent measured exact mm).  As I start the car, the vacuum throttle control unit sucks the spring in and the throttle opening closes completely, ensuring that it is resting against the stop screw, throttle plate fully closed...  The reason I mention this fact is that if I were to start adjusting the idle, not knowing that, when the engine is running, it compresses the spring on the vacuum throttle unit, thus closing the throttle plate...  I would adjust the large spring on the vacuum unit to the point it would be too far compressed and not be able to push up on the idle when in gear....  I think this was my problem all along...

If anyone has any questions or wants to add to this, please do.

When did our cars start coming with the vacuum cells?  Using these cells for idle vs the CSS?

Regards,
Bob

bpossel
Memphis, TN.
1971 280SL
1997 E320

[n/a]

I have a very similar problem and would like to know if others agreed that I should take a similar approach. My car, 70 280sl automatic, has a problem of stalling when brake is applied. The car would maintain an idle of between 800-1000 after warm up. The rpm would drop to zero and the car stalled when the transmission is in "Drive" and brake applied. I have been shifting it to neutral before applying brake and the rpm would hold and the car will not stall. The car has just had a major tune-up by an experienced 113 mechanic and was running fine for awhile. What are the steps that I should take to correct this problem?

Thanks,
Louis

[zanone]

Louis,

I've been having the same problem of the car stalling when coming to a stop. I put the car in neutral before stopping and car doesn't stall. Bob Possel feels the linkage adjustment may help, will keep you updated.

Dennis Zanone

[rwmastel]

quote:Originally posted by bpossel

....adjusting all of my linkages per the info from this site.  

Bob,
Do you have a nice collection of links to the specific discussion threads that have this info?

Thanks,

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

[n/a]

Dennis,

Would very much interest in your progress in correcting the problem. Thank you for sharing.

Louis

[n/a]

My car won't start today. I have to admit that my battery is at it's last leg and I am going to replace it tomorrow. After hooking it up to the family car, it started after long period of cranking.  It would hold idle at around 1100 rpm but from time to time the idle would drop-off although the car never die. The car would stall as soon as I move it out of Park (does not matter if it is neutral or in gear). Would like some comments on what cause the idle drop-off. Should I focus on the constant speed solenoid on the stall after shifting into gear?

Thanks,
Louis

[hands_aus]

Louis and Dennis,

The start problems you both are having were discussed only very recently on here.

If you do a search for "cold start" or "cold idle" you are sure to find the posts.

We all eventually have these problems.

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto