SL/R129: transmission hard shifting
10-24-2020, 06:24 PM
#1
Newbie
Thread Starter
Join Date: May 2007
Location: victoria , british columbia, canada
Posts: 5
Likes: 0
Received 0 Likes
on
0 Posts
1986 ,300sdl, 2002 500ml, 1991 500sl and a 1957 190sl
transmission hard shifting
i have a 1991 500 sl garage queen. highly maintained. but occasionally at about 30 kilometer speed i go to acelerate not hard but motor rev up then transmission ingages very hard really . doesnt do it all the time. can be cold or warm. can be going straight or a city type corner in low speeds .it hits hard when it ingages so im sure not good on rear end. anyone ever have this problem.i will take it in but sure would love to have a better idea what it could be. thank you all fr your help. fluid and filter changed about 1600 kilometers back
10-26-2020, 02:53 AM
#3
Good news: There is no conductor plate inside your transmission 
Do you fill up the right oil last time you changed it? Is the oil level OK now? A broken modulator can cause sucking oil out of the transmission.
Do you fill up the right oil last time you changed it? Is the oil level OK now? A broken modulator can cause sucking oil out of the transmission.
Last edited by AndreasHannover; 10-26-2020 at 02:57 AM.
10-26-2020, 11:44 AM
#4
Senior Member
Your description of the 'hard shifting' mimics that caused by the 'Vacuum Modulator" on the right side near the front of the transmission. A broken/leaking vacuum line will 'tell' the transmission computer to raise the 'line pressure' the shifting force, because the throttle is open wide and the engine is making more power. A broken diaphragm in the vacuum modulator will allow transmission oil to be sucked into the engine intake manifold (and be burned with lots of white smoke. A 'sticking' modulator valve assembly will also cause this hard shifting symptom. If the vacuum modulator does not leak when tested, (holds a full vacuum for 30 sec with no decay) Try rechanging the fluid AFTER using a 'remove gum varnish, slipping transmission solvent. Before doing the 'flush' try holding the brakes, and with the transmission in drive, then opening and closing the throttle rapidly a few times to 'exercise' the modulator piston in its bore. A piece of debris smaller than a human hair can cause this 'sticky valve' symptom. Most people simply do not believe the clearances in the transmission valve body are so fine. The valves are hollow, one closed-end cylinders that slide in honed bores with clearances typically less than 0.001" Lint from a wiping rag or paper wipe, or a spec of dandruff, and tiny spec of dirt are large enough to cause 'sticky valve' behavior.
The 'hard shifting' behavior sounds like a stuck or failed vacuum modulator system. If the vacuum line from the intake manifold is leaking, or the vacuum modulator has failed, this will tell the transmission control system the engine throttle is wide open and to increase the 'shift pressure (actually the 'line pressure.' Sometimes a failed vacuum modulator diaphragm will allow lots of transmission oil to be sucked into the engine intake manifold--with lots of smoke evident.
You can suck on the vacuum line at the manifold and test that it does not leak at full vacuum. If the vacuum line and modulator test leak-tight, try---with the transmission in Drive, hold the brakes firmly and open/close the throttle all the way rapidly to 'exercise the modulator piston in its bore. Tiny specs of dirt, lint, paper fibers or skin cells will cause these precision valves in cylinders to jam or behave sticky.
The 'hard shifting' behavior sounds like a stuck or failed vacuum modulator system. If the vacuum line from the intake manifold is leaking, or the vacuum modulator has failed, this will tell the transmission control system the engine throttle is wide open and to increase the 'shift pressure (actually the 'line pressure.' Sometimes a failed vacuum modulator diaphragm will allow lots of transmission oil to be sucked into the engine intake manifold--with lots of smoke evident.
You can suck on the vacuum line at the manifold and test that it does not leak at full vacuum. If the vacuum line and modulator test leak-tight, try---with the transmission in Drive, hold the brakes firmly and open/close the throttle all the way rapidly to 'exercise the modulator piston in its bore. Tiny specs of dirt, lint, paper fibers or skin cells will cause these precision valves in cylinders to jam or behave sticky.
10-27-2020, 12:57 AM
#5
Newbie
Thread Starter
Join Date: May 2007
Location: victoria , british columbia, canada
Posts: 5
Likes: 0
Received 0 Likes
on
0 Posts
1986 ,300sdl, 2002 500ml, 1991 500sl and a 1957 190sl
i like to thank everyone who has posted things to test and try. so gratfull for all your help. as soon as it warms up a bit ill heat shop and lift car and try your suggestions. thankyou again ill let you know cheers mike
11-01-2020, 01:00 PM
#6
Member
Join Date: Feb 2017
Location: L.A., Moscow, NYC, San Diego, Baja
Posts: 245
Received 36 Likes
on
32 Posts
1998 E320 Wagon, 2003 W211 E500, 2003 W211 E500, 1999 W210 E55 AMG
Your description of the 'hard shifting' mimics that caused by the 'Vacuum Modulator" on the right side near the front of the transmission. A broken/leaking vacuum line will 'tell' the transmission computer to raise the 'line pressure' the shifting force, because the throttle is open wide and the engine is making more power. .A 'sticking' modulator valve assembly will also cause this hard shifting symptom. If the vacuum modulator does not leak when tested, (holds a full vacuum for 30 sec with no decay) Try rechanging the fluid AFTER using a 'remove gum varnish, slipping transmission solvent. Before doing the 'flush' try holding the brakes, and with the transmission in drive, then opening and closing the throttle rapidly a few times to 'exercise' the modulator piston in its bore. A piece of debris smaller than a human hair can cause this 'sticky valve' symptom. Most people simply do not believe the clearances in the transmission valve body are so fine.
The 'hard shifting' behavior sounds like a stuck or failed vacuum modulator system. If the vacuum line from the intake manifold is leaking, or the vacuum modulator has failed, this will tell the transmission control system the engine throttle is wide open and to increase the 'shift pressure (actually the 'line pressure.' Sometimes a failed vacuum modulator diaphragm will allow lots of transmission oil to be sucked into the engine intake manifold--with lots of smoke evident.
You can suck on the vacuum line at the manifold and test that it does not leak at full vacuum. If the vacuum line and modulator test leak-tight, try---with the transmission in Drive, hold the brakes firmly and open/close the throttle all the way rapidly to 'exercise the modulator piston in its bore. Tiny specs of dirt, lint, paper fibers or skin cells will cause these precision valves in cylinders to jam or behave sticky.
The 'hard shifting' behavior sounds like a stuck or failed vacuum modulator system. If the vacuum line from the intake manifold is leaking, or the vacuum modulator has failed, this will tell the transmission control system the engine throttle is wide open and to increase the 'shift pressure (actually the 'line pressure.' Sometimes a failed vacuum modulator diaphragm will allow lots of transmission oil to be sucked into the engine intake manifold--with lots of smoke evident.
You can suck on the vacuum line at the manifold and test that it does not leak at full vacuum. If the vacuum line and modulator test leak-tight, try---with the transmission in Drive, hold the brakes firmly and open/close the throttle all the way rapidly to 'exercise the modulator piston in its bore. Tiny specs of dirt, lint, paper fibers or skin cells will cause these precision valves in cylinders to jam or behave sticky.
It appears, since I've already replaced a number of compromised vacuum lines on this car, it would be best to simply replace the line from the manifold to the trans, yes? Is the line apparent, easily accessible, or a bear? Any trick to it?
Could there be indication of an issue if when replacing/testing the vacuum line there is fluid in it, smoke notwithstanding?
When using a 'remove gum varnish, slipping transmission solvent' which I presume is an additive to the fluid, should a complete flush/extraction be performed, or will simply replacing the fluid when dropping the pan suffice? (Likely a stupid question)
As an aside, can someone tell me approximately how much fluid is released when the pan is dropped? (Asking for a friend)
Many thanks
