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

LinkBack Thread Tools Display Modes
Old 06-24-2019, 03:34 PM
Registered User
Join Date: Aug 2009
Location: Seattle
Posts: 387
Weak vacuum signal to W124 300TD climate control

I tested the climate control vacuum pods for my 1987 300TD this last week. (PO claimed he had replaced them, but the climate control seemed not to function as I expected). The pods actually checked out ok, however I noticed that vacuum coming to the switch-over valve unit seemed low as the vacuum gauge fluttered up and down around the 10 inch mark.

I did a quick test coming off the vacuum pump and the gauge held steady at the ~22 inches.

I then blocked off the two lines running to the EGR system (red X's in picture) and re-tested vacuum at the switch-over valve under the dash. The gauge went up a bit but still fluctuated a lot.

I then blocked off the line running to the vacuum control unit (purple x in picture). Now the vacuum was ~20 inches at the switch over valve.

Since I plan to delete the EGR, it seems that weak vacuum would be caused by the vacuum control unit or something else connected to the VCU.


1. Is the expectation that vacuum measured at the Switch-over valve would be close to 20 inches? Am I interpreting correctly that 10 inches or less is a weak vacuum signal?

2. Is there common culprit around the VCU I should be looking at?

Reply With Quote
Old 06-24-2019, 04:07 PM
Diseasel300's Avatar
Registered User
Join Date: May 2016
Location: Texas Hill Country
Posts: 4,653
Make sure the restrictors are present in the VCV system and that the VCV is plumbed correctly. Orientation on the cap is important and the restrictors are critical. If the restrictors are missing, you'll be bleeding off the vacuum to all vacuum accessories.
'11 Honda Accord EX - "The Daily" 64K
'83 500SL Euro - "The Money Pit" 116K
'91 350SD - "The Diseasel Jr." 168K
'86 300SDL - "The Diseasel" 189K (Totaled 1/31/19)
The Diseasel Thread - Everything You Didn't Know You Wanted To Know
Reply With Quote
Old 06-25-2019, 12:31 PM
Registered User
Join Date: Aug 2009
Location: Seattle
Posts: 387

Am I correct in assuming that vacuum measured at the climate control switch-over valve should be close to the ~20 inches measured at the vacuum pump? Or is there a reason that the vacuum would be significantly less by design?
Reply With Quote
Old 06-30-2019, 12:39 PM
Registered User
Join Date: Sep 2004
Location: Las Vegas, NV
Posts: 623
You might try disconnecting various vacuum lines and blowing them out to clear debris. I've noticed a bit of dust when doing this over the years. Those little restrictors can get clogged. Anyway, I've also always replaced the rubber lines as they age more than the hard lines.
87 300D latest project!
11 GLK 350 So far, so good
08 E350 4matic, Love it.
99 E320 too rusted, sold
87 260E Donated to Newgate School - check it out.
12 Ford Escape, sold, forgotten
87 300D, sold, what a mistake
06 Passat 2.0T, PITA, sold

Las Vegas NV
Reply With Quote
Old 07-01-2019, 01:39 PM
Registered User
Join Date: Aug 2009
Location: Seattle
Posts: 387
I've been slowly working through the system trying to figure out at what point does the vacuum start dropping. I found that if I block the line coming off the VCU and leading to the vacuum amplifier section (purple X in picture) then the vacuum measured at the climate control holds at ~20 inches. When I reconnect that line to the vacuum amplifier the vacuum drops to ~7 inches.

So I think I am bleeding vacuum somewhere within the vacuum amplifier area.

What I still don't know is whether there is a baseline vacuum that should be measured at the climate control? Should it be close to the same as measured at the vacuum pump?

Last edited by 240dddd; 07-01-2019 at 01:42 PM. Reason: added pic
Reply With Quote
Old 07-05-2019, 06:21 PM
Registered User
Join Date: Aug 2009
Location: Seattle
Posts: 387
After some more testing I'm pretty sure the problem, if there is one, is somewhere within the BFS (blue flying saucer).

Is there a testing procedure to for the bfs? I.e. would removing all the lines and plugging the connection points and testing to see if it holds vacuum tell me anything?

Also, I know people have bypassed the BFS before, but I haven't been able to dig up any specific diagrams or procedures on how to best do this.

Thanks for any further suggestions.
Reply With Quote


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 07:51 PM.

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