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  #1  
Old 06-24-2019, 03:34 PM
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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.

Questions:

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?


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Old 06-24-2019, 04:07 PM
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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.
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Old 06-25-2019, 12:31 PM
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Thanks.

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?
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Old 06-30-2019, 12:39 PM
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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.
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Old 07-01-2019, 01:39 PM
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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
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Old 07-05-2019, 06:21 PM
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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.
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