Parts Catalog Accessories Catalog How To Articles Tech Forums
Call Pelican Parts at 888-280-7799
Shopping Cart Cart | Project List | Order Status | Help




Go Back   PeachParts Mercedes-Benz Forum > Mercedes-Benz Tech Information and Support > Diesel Discussion

Reply
 
LinkBack Thread Tools Display Modes
  #1  
Old 07-13-2019, 12:25 PM
Registered User
 
Join Date: May 2011
Posts: 43
Transmission Vacuum Nightmare

Another issue with my cursed low-mileage, rust-free 300d.

Driving yesterday, all is fine. Shut off. Get back in 15 min later, canít get out of second gear. Shut off, unplug vacuum from transmission, drive home totally fine, all gears good.

Get in this morning, canít get out of second. Driving 20 mph. After 15 min, I have 3rd and fourth again.


This car is a mess where the vacuum in concerned. I currently have to shut off the engine by pressing the two ignition vacuum lines together (which are otherwise loose and disconnected under the dash.) the valve in the ignition assembly is bad so this is the temporary fix. see attached image

I need to make a 2 hour drive tonight. Am I asking for trouble? I really really need to make this drive.
Attached Thumbnails
Transmission Vacuum Nightmare-f83c0af3-1666-4ca1-b342-7a8fc7c53406.jpeg  
__________________
1984 300D Surfblau "Blueberry" 250K R.I.P
1984 300CD Manila Beige "The Banana" 238K R.I.P
1984 300TD Cypress Green "Olive" 390K
1984 300D Oriental Red "Steak" 195K
Reply With Quote
  #2  
Old 07-13-2019, 05:21 PM
JHZR2's Avatar
Registered User
 
Join Date: Jan 2002
Location: New Jersey
Posts: 4,029
I wouldn’t drive with an open vacuum line. May affect vac pump duty cycle and wear.

Was under the impression that vac at the transmission affected shift hardness not the shift itself. You sure the Bowden cable is OK?
__________________
Current Diesels:
1981 240D (73K)
1982 300CD (162k)
1991 350SD (113k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)
Reply With Quote
  #3  
Old 07-13-2019, 06:25 PM
Registered User
 
Join Date: May 2011
Posts: 43
Bowden is fine, I’ll reconnect and see what I get when I’m out of work. I haven’t lost a gear before on this car, never a slip or an excessively hard shift even. I know it’s vacuum but so much to troubleshoot when I really need to get on the road once I get out of work.

The vacuum SHOULD just control the quality/timing, but it was an immediate fix yesterday. I know I’m going to spend awhile chasing this down. Going to disconnect the kickdown for good measure.

Any other suggestions for a quick fix to get me through the evening?
__________________
1984 300D Surfblau "Blueberry" 250K R.I.P
1984 300CD Manila Beige "The Banana" 238K R.I.P
1984 300TD Cypress Green "Olive" 390K
1984 300D Oriental Red "Steak" 195K
Reply With Quote
  #4  
Old 07-13-2019, 06:28 PM
Registered User
 
Join Date: May 2011
Posts: 43
Has it been unwise to drive it with my....improvised shutoff method (ignition lines open) I teed them off a couple times but I swear it made for a rougher idle.
__________________
1984 300D Surfblau "Blueberry" 250K R.I.P
1984 300CD Manila Beige "The Banana" 238K R.I.P
1984 300TD Cypress Green "Olive" 390K
1984 300D Oriental Red "Steak" 195K
Reply With Quote
  #5  
Old 07-13-2019, 07:38 PM
Diseasel300's Avatar
Registered User
 
Join Date: May 2016
Location: Texas Hill Country
Posts: 4,904
Vacuum has nothing to do with gear shifting or timing other than softening shifts when at light throttle. Unhooking the vacuum line was most likely a coincidence.

Vacuum has absolutely NOTHING to do with idle or fuelling of the engine. This is not a gas engine, vacuum leaks do nothing to affect fuel mixture since the suction is created by an external pump, not the intake manifold.

You haven't mentioned where the fluid level is on the dipstick. I'd be checking that before anything else. Sticking in gears or losing gears is typically due to low fluid pressure in the valve body, valve body wear, or internal problems with the transmission (broken spring assemblies in the valve body, worn or broken bands, etc).

Making a 2 hour drive is your decision. You may be walking home or calling an Uber. Poor shifting is one thing, losing gears or hanging up in the shift sequence is a different animal.
__________________
'11 Honda Accord EX - "The Daily" 69K
'83 500SL Euro - "The Money Pit" 116K
'91 350SD - "Diseasel Jr." 173K
'94 BMW 530i Touring - "BMWtf LoLwagen" 233k
Reply With Quote
  #6  
Old 07-15-2019, 02:45 PM
Registered User
 
Join Date: Feb 2016
Location: North New Jersey
Posts: 414
chiming in to second other's opinions - the only issues I've ever noticed when totally disconnecting the vac line from my 82 300d trans modulator have been very harsh/clunky downshifts at 2-1. I don't think it would impact upshifts, or generally shifting through the gears at all.
__________________
Andrew
Montclair, NJ
--------------
1982 300D Turbo
120k
Petrol Blue Green
Reply With Quote
  #7  
Old 07-15-2019, 04:33 PM
JHZR2's Avatar
Registered User
 
Join Date: Jan 2002
Location: New Jersey
Posts: 4,029
Quote:
Originally Posted by lbj.lbj View Post
Has it been unwise to drive it with my....improvised shutoff method (ignition lines open) I teed them off a couple times but I swear it made for a rougher idle.
Maybe. There have been threads with theories regarding vacuum pumps and “parking” pistons when a certain level is reached. In this circumstance the leak will cause maximum flow and thus maximum work. Pulling a hard vacuum is energy intensive, but these pumps don’t do that. But moving max air mass/flow is guaranteed to push you to the right:



Not exactly your pump but used to illustrate.

IF our pumps operate on such a principle, and more flow = more work and energy, then it could cause accelerated wear. More load = shorter life.

So unless you’ve fully capped your vac system so the pump sees a “leak free” situation, it may be seeing higher mass flow and thus more work. That may result in shorter life.
__________________
Current Diesels:
1981 240D (73K)
1982 300CD (162k)
1991 350SD (113k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)

Last edited by JHZR2; 07-15-2019 at 04:43 PM.
Reply With Quote
  #8  
Old 07-15-2019, 07:03 PM
tyl604's Avatar
Registered User
 
Join Date: Feb 2008
Location: Atlanta, GA
Posts: 3,235
The long and short of it is that your vac is not causing the tranny to miss shifts; you probably have something inside the tranny going bad.

Are you sure the ignition switch is bad? It could just be the rubber connectors on the plastic lines. From what I hear on the forum those switches just almost never go bad. I pulled my lines about a year ago and the rubber connectors were still perfect after 38 years (failure to cut off was a bad shutoff valve instead). You can get new rubber connectors from Pelican.
Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Trackbacks are On
Pingbacks are On
Refbacks are On




All times are GMT -4. The time now is 08:00 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2018 Pelican Parts, LLC - Posts may be archived for display on the Peach Parts or Pelican Parts Website -    DMCA Registered Agent Contact Page