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 04-21-2004, 10:34 PM
JHZR2's Avatar
Registered User
 
Join Date: Jan 2002
Location: New Jersey
Posts: 5,265
hairtrigger downshift?

Hi,

I am curious if these old non-electric transmissions dont kick down automatically a gear when going from high speeds to moderate speeds.

For example, Ill be driving 60 mph. Ill then coast/lightly brake to 25. At this speed, I ought to be in 2nd or 3rd gear (most likely). However, the trans remains in 4th. If the terrain is flat and I am very light, the car will slowly accelerate up from 25 in 4th gear. If I am heavier on the throttle, it will be like a hairtrigger and shift hard into 3rd or 2nd. If Im going up a hill and am light on the throttle, it will be extremely slow (cant get boost up because RPM is too low), and if Im too heavy on it to get it moving, it wont kick down unless I lean on it to press the button.

Overall, often the trans will stay in 4th and not go back to a lower gear, although roadspeed requires it. Typically then, the throttle is a hairtrigger, and the slightest touch will set off a downshift.

Is this normal?

Also, what happens if the vac. modulator is set for too firm of shifts? My whole vac system is great, but because I was having some funny shift problems, I firmed it up two clicks.Now, sometimes, like when hot, its just right, but when going slow or when cold, the shifts are really hard, like more or less a clunk into gear. Is this hard on the trans?
I think Ill soften it up by one click and all wil be well; the real hard clunk into gears is a bit too much early in the morning when Im trying to drive the car lightly. Worse is when the car downshifts, as it is quite hard of a shift and it almost feels like the car is jarred around.

Is there any damage from having too hard a shift?

Thanks for the help,

JMH

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)
Reply With Quote
  #2  
Old 04-21-2004, 11:33 PM
wolf_walker's Avatar
Zen And The Art Of Diesel
 
Join Date: Dec 2003
Location: Oklahoma City
Posts: 2,050
Hard shifts are better for the clutches and bands, worse for the rest of the driveline. Have you checked the vacuum going to your modulater recently?

Mine is either too soft or 3-4 is too hard. There is no inbetween. I'm really sick of it.

The downshift thing seems to be normal, and exagurated with a weak engine or one with the ALDA turned down so that there is no extra fueling at those lower rpm's. I tightened up the bowden cable and got the 4-3 part throttle downshift to come on a little more readily, and it's not so harsh when it does because your not so far into the throttle. It's a balancing act, and is probibly great when everything is just *perfect*(just like the cruise control). But there are not many quarter million mile drivetrains that are that perfect. Now it's just anoying. I either just put my foot in it, manualy shift to 3rd, or let it slug along till it's in a happy RPM area to accelerate in 4th.

I'd like to know how other non-computer controled auto trans'd diesel vehicles controled shifts, anyone?
__________________
One more Radar Lover gone...
1982 VW Caddy diesel 406K 1.9L AAZ
1994 E320 195K
Reply With Quote
  #3  
Old 04-22-2004, 12:21 AM
BoostnBenz's Avatar
Benötigt Mehr Druck!
 
Join Date: Aug 2002
Posts: 1,855
While my IP vacuum valve doesn't seem to work perfectly (flares 2-3 cold, clunks into and out of 1st warm) my car as a whole shifts pretty good. I can make it downshift on a fly if I want but if I'm light on the throttle around 25mph it'll shift into or stay in 4th. If I give it a little more juice it'll drop to 3rd and I can hold it there until redline if I so desire. If I floored it at this point it'd probably drop into second, but normally won't go 4-2 unless I'm going slower than 25mph.

Clunking into and out of first can be brought on by having less than 13"hg going to the tranny at idle.
__________________
Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k
Reply With Quote
  #4  
Old 04-22-2004, 05:43 AM
JHZR2's Avatar
Registered User
 
Join Date: Jan 2002
Location: New Jersey
Posts: 5,265
OK, so overall I think that my setup is OK, I just need to soften up the modulator a bit, and have a little less slack on the bowden, and Ill be fine... Or so I think.

Thanks,

JMH
__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)
Reply With Quote
  #5  
Old 04-22-2004, 07:44 AM
Registered User
 
Join Date: Aug 2001
Location: Milford, DE
Posts: 1,558
Steve Brotherton has written an article about these issues, as with everything he writes this is worth a close look.

http://www.continentalimports.com/ser_ic20242.html

Last edited by whunter; 06-16-2010 at 01:17 PM. Reason: replaced bad link
Reply With Quote
  #6  
Old 04-22-2004, 10:30 PM
Registered User
 
Join Date: Nov 2002
Location: Wakefield, RI
Posts: 2,145
My car is exactly as you guys describe. These trannys seem to take forever to warm up. Once warm mine shifts firm but nice in all gears. Cold, an occasionall slight 2-3 flare and hard shifting unless I am light on the pedal. I don't like how it stays in 4th unless floored at low speed so I give it some pedal and manually downshift. Its quite impressive how well these cars accelerate if you do this and keep the engine in its powerband. RT
__________________
When all else fails, vote from the rooftops!
84' Mercedes Benz 300D Anthracite/black, 171K
03' Volkswagen Jetta TDI blue/black, 93K
93' Chevrolet C2500HD ExCab 6.5TD, Two-tone blue, 252K
Reply With Quote
  #7  
Old 04-22-2004, 10:53 PM
BoostnBenz's Avatar
Benötigt Mehr Druck!
 
Join Date: Aug 2002
Posts: 1,855
Perhaps but if you let the rpms stay lower then you get better mileage. My 84 was adjusted so that under normal acceleration it'd stay in 3rd until around 48mph, my 83 on the other hand is set such that it shifts about 40mph. It is a little low by time it makes it into fourth but if I want to keep it in gear I just push the pedal down a little harder.

I blame my cold 2-3 flare on the vacuum valve, as well as the harder than optimal 1-2.
__________________
Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k
Reply With Quote
  #8  
Old 04-23-2004, 03:10 AM
Registered User
 
Join Date: Apr 2004
Location: Wisconsin
Posts: 1,009
This is a very interesting thread to me -- I've got an '82 turbo coupe that can drop very suddenly into a lower gear. I've got the Bowden cable pretty tight because I like it there! I want a quick kickdown, and the car accelerates smartly as a result. Last summer I bought a 300td that's a dog compared to the coupe, and I think that part of the issue is the transmission's willingness to downshift. I plan to deal with that later -- it also had a load of carbon in it, which I'm working on!

The main issue I wanted to address is the vacuum control valve. I've tried to adjust the VCV on the coupe by the numbers, complete with the adjustment disk. But it doesn't hold to the vacuum specs over time, and I get those classic symptoms: a little 2-3 flare if I'm not careful and a clunk when the trans downshifts to first while I decelerate to a stop from highway speed.

I suspect that the VCV is toast on this car. A friend whose opinion I respect highly on Mercedes mechanics tells me that many, many W123 turbo diesels on the road today have bad vacuum control valves.

Of course, the part is expensive, so I'm cautious about laying out the dinero. But I think my car would benefit from a new VCV. Thoughts?

Russ Maki
Ixonia, Wis.
Reply With Quote
  #9  
Old 04-23-2004, 03:38 AM
wolf_walker's Avatar
Zen And The Art Of Diesel
 
Join Date: Dec 2003
Location: Oklahoma City
Posts: 2,050
I'd like to know what kind of curve the VCV is suposed to have. There is the spec at closed throttle, 0 vac at WOT, and the spec with the gauge in it at it's position, but that is not very precise it seems. When set to gauge spec mine is at 5 inches of vac during most acceleration, which is too little to prevent hard upshifts. Downshifting off throttle is seamless and vacuum at cruising speed is reasonable. If it were electrical i'd use a graphing meter as you do with a TPS to see if there any spikes and what the overall curve is.

I've experimented with retrofitting the blue vacuum amplifier from a later model with some success, but the problem is still there really. Especialy that hard 3-4 upshift in my case.
__________________
One more Radar Lover gone...
1982 VW Caddy diesel 406K 1.9L AAZ
1994 E320 195K
Reply With Quote
  #10  
Old 04-23-2004, 10:59 AM
BoostnBenz's Avatar
Benötigt Mehr Druck!
 
Join Date: Aug 2002
Posts: 1,855
Yes the VCVs go bad in time, their effective range becomes smaller and smaller. A good VCV would have 15-17"hg of vacuum at idle, 5-6.5"hg 10mm away from the stop (where mercedes considers normal acceleration), and preferably as close to 0"hg as possible at WOT. I'm also quite hessitant to spend $140 on that cheap looking/build part.

Here is a test for you, if you remove the throttle linkage from it and get out the MityVac if it holds that 17"hg (or close to) odds are it is still savable. The slots inside oval out which is fixable with a little plastic epoxy, and from there it should be able to be adjusted out. But the truth of the matter is most won't hold over 13"hg like this even though the bleeder should be completely closed!
__________________
Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k
Reply With Quote
  #11  
Old 04-23-2004, 11:37 AM
Registered User
 
Join Date: Apr 2004
Location: Wisconsin
Posts: 1,009
Excellent tip on the epoxy repair. I just happen to have some of the 5-minute stuff left over from the valve cover baffle job! I think I'll have a go at it. Updates to come.

Russ
Reply With Quote
  #12  
Old 04-24-2004, 01:40 AM
BoostnBenz's Avatar
Benötigt Mehr Druck!
 
Join Date: Aug 2002
Posts: 1,855
I used the little straw from a spray bottle as the hole filler then put epoxy around it and left it there until dry. It seems like that was the right size, better check first though. At least now there is no twisting inside before the valve moves.
__________________
Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k
Reply With Quote
  #13  
Old 04-24-2004, 08:21 AM
Registered User
 
Join Date: Apr 2004
Location: Ojai CA
Posts: 181
Are you guys talking about the white vacuum bleed valve on the I.P.? I hope you are because I need some advice. On my beater SD the valve starts showing normal vacuum for the first few seconds, then starts buzzing and drops to 10 to 12 on the Mityvac and doesn't drop to zero at WOT. Replaced it with one from a doner car and it does the same thing. What does the vac line that goes into the side of it do?
__________________
'84 300SD veg-oil bliss
'83 300SD veg-oil beater
Reply With Quote
  #14  
Old 04-25-2004, 10:14 AM
BoostnBenz's Avatar
Benötigt Mehr Druck!
 
Join Date: Aug 2002
Posts: 1,855
Yes. The buzzing is caused by the valve leaking, it is trying to seal but it can't. It is a hard plastic ball in the center of a metal housing, either it started off soft rubber and has hardened or the metal is pitting. I've taken several apart. This implies that I've tried several donor vehicles to find them just as bad. The only good one I found I ended up breaking on seeing how it worked. It was good because the vehicle it came off of had a vacuum pump failure and covered the valve in oil so no corrosion could happen.

The black line in the side of it is just a vent, rather than letting your vacuum pump suck normal dirty air in it has to suck it through this vent line which I presume has a filter on the end of it somewhere.
__________________
Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k
Reply With Quote
  #15  
Old 06-16-2010, 01:25 PM
whunter's Avatar
Moderator
 
Join Date: Dec 2003
Location: Metro Detroit, Michigan
Posts: 17,390
Answer

PROPER VACUUM SYSTEM "TUNING" ACHIEVES OPTIMAL SHIFT CHARACTERISTICS AND AN ACCURATE REPAIR
http://www.continentalimports.com/ser_ic20242.html

Bowden cable, is this the adjustment
Bowden cable, is this the adjustment

Bowden Cable
Bowden cable...

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 02:58 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2024, 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