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1993 500sl
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Discussion Starter #1
Just had work done on my 1993 500SL to address unstable idle speed. My technician replaced leaking vacuum hoses at throttle body and small diameter vacuum lines as well as ignition wire set, and alternator. But when it came out of the shop, it had a new problem that baffled my technician, who has always been top-notch. Now, transmission shifts to each gear only after letting off the gas at appropriate speed and tac revolutions. It's similar to driving a straight stick, but without a manual shifter. When we put the pedal down, we want to go, not let off the gas each time for the next gear. Never had this problem before. Is it the transmission, or something else? Any suggestions on what the problem and solution might be?
 

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R129 500 SL 1991
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Problem is surely vacuum related. Check that you have vacuum in small dia line to transmission. Did your technician also replace the rubber elbow to vacuum modulator at transmission?

Some small dia lines are taken off the throttle body, others from the inlet manifold. Check that all lines have been put back in correct positions and also that some have not simply been capped off.
 

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1993 500sl
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Discussion Starter #3
Problem is surely vacuum related. Check that you have vacuum in small dia line to transmission. Did your technician also replace the rubber elbow to vacuum modulator at transmission?

Some small dia lines are taken off the throttle body, others from the inlet manifold. Check that all lines have been put back in correct positions and also that some have not simply been capped off.
Good, logical suggestions. I hope that only a vacuum leak is the culprit. Many thanks for your thoughts!
 

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2008 E320 OM642 Bluetec1992 300D 2.5 Turbo (Sold @ 248k running strong), 1985 240D (sold)
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What robm.UK said. The little elbow at the vacuum module is often dry rotted or oil soaked, depending on the the conditions of the individual car. Basically, old rubber fails and leaks.
I'd start there and check/replace all the rubber and plastic vacuum lines going from the trans back to the source of vacuum on the engine. Usually a fitting on the manifold near the carb or the carb itself. Diesels have a mechanical switch in there too since the vacuum isn't dependent on load - I don't think gassers have one/are more direct.
 

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Doesn't that 500sl have the 4-speed hydraulic tranny still instead of electronic 722.6? I bet the vacuum that goes to the transmission modulator also. Shifting behavior can be adjusted/fine tuned with that small valve on the modulator under its rubber boot.
 

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2008 E320 OM642 Bluetec1992 300D 2.5 Turbo (Sold @ 248k running strong), 1985 240D (sold)
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I forgot about adjusting the modulator. It can make it harsher or less so. But I'd look for a vacuum leak and renew any deteriorated lines before I started adjusting things.
 

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Discussion Starter #8
dkmenefee,
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jim jones
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1993 500sl
Joined Jul 30, 2006
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Discussion Starter • #1 15 days ago
Just had work done on my 1993 500SL to address unstable idle speed. My technician replaced leaking vacuum hoses at throttle body and small diameter vacuum lines as well as ignition wire set, and alternator. But when it came out of the shop, it had a new problem that baffled my technician, who has always been top-notch. Now, transmission shifts to each gear only after letting off the gas at appropriate speed and tac revolutions. It's similar to driving a straight stick, but without a manual shifter. When we put the pedal down, we want to go, not let off the gas each time for the next gear. Never had this problem before. Is it the transmission, or something else? Any suggestions on what the problem and solution might be?


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robm.UK
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R129 500 SL 1991
Joined Jun 1, 2009
946 Posts
#2 15 days ago
Problem is surely vacuum related. Check that you have vacuum in small dia line to transmission. Did your technician also replace the rubber elbow to vacuum modulator at transmission?

Some small dia lines are taken off the throttle body, others from the inlet manifold. Check that all lines have been put back in correct positions and also that some have not simply been capped off.


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jim jones
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1993 500sl
Joined Jul 30, 2006
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Discussion Starter • #3 14 days ago
robm.UK said:
Problem is surely vacuum related. Check that you have vacuum in small dia line to transmission. Did your technician also replace the rubber elbow to vacuum modulator at transmission?

Some small dia lines are taken off the throttle body, others from the inlet manifold. Check that all lines have been put back in correct positions and also that some have not simply been capped off.
Good, logical suggestions. I hope that only a vacuum leak is the culprit. Many thanks for your thoughts!


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dkmenefee
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2008 E320 OM642 Bluetec1992 300D 2.5 Turbo (Sold @ 248k running strong), 1985 240D (sold)
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#4 14 days ago (Edited)
What robm.UK said. The little elbow at the vacuum module is often dry rotted or oil soaked, depending on the the conditions of the individual car. Basically, old rubber fails and leaks.
I'd start there and check/replace all the rubber and plastic vacuum lines going from the trans back to the source of vacuum on the engine. Usually a fitting on the manifold near the carb or the carb itself. Diesels have a mechanical switch in there too since the vacuum isn't dependent on load - I don't think gassers have one/are more direct.


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1985 240D (sold, Comfy but couldn't get out of it's own way)

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jim jones
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1993 500sl
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Discussion Starter • #5 14 days ago
Thank you, @dkmenefee!


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karma4jake

robm.UK,
I forgot about adjusting the modulator. It can make it harsher or less so. But I'd look for a vacuum leak and renew any deteriorated lines before I started adjusting things.

There are some very good long-distance diagnosticians on this blog. Thank you to all who suggested fixes.

The solution to my transmission failing to shift probably won't surprise you. My technician replaced the shift modulator valve and adjusted the kick-down cable. Runs beautifully again. There are headaches, and there are challenges, all looking for solutions.
 
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