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Vacuum issues, switchover valve unit?

9K views 9 replies 3 participants last post by  Bob T 
#1 ·
Hi, everyone! Picked up my first Mercedes in early January, a 1986 560SL. I’m finally able to start troubleshooting the multitude of problems on the car and have hit somewhat of a roadblock. Trying to fix the A/C now. Like most posts I’ve read, I’ve got an A/C that just won’t get cold. My suspicion is a number of issues, but probably localized to the monovalve. Separately, I pulled the glove box out and started testing the vacuum system, since most articles I’ve read have mentioned a failure of the vacuum accumulators. I’ve found the following confusing problem - I’m getting good vacuum from the intake, through the check valve (which I replaced with a new one since they’re fairly cheap) and goes into the Switchover Valve Unit (Y34 on the diagram). However, the vacuum does not continue on to all valves. Specifically, the air flap elements (left and right, numbers 42 and 43) never see vacuum and maintain fully in the recirc mode. I’ve put vacuum (via the MightyVac) directly to the elements and they shut just fine, no leaks in any of the four. I’ve found that lines 2 and 3 (for the center vent accumulator) hold no vacuum, so the diaphragm in it is most likely ruptured. Some further tests that I’ve run (car on, drawing vacuum) showed me that in full A/C mode (“b” option), the vacuum that’s present at port 2 or 3 on Y34 immediately drops to 0 upon selection of the option. Vacuum is present in those ports when “c”, “d” or “e” are selected (tested individually, with the other ports blanked off to ensure no other contributing component added to the mix)

Here’s my question - does anyone have a good understanding of how the switchover valve unit works? Does it transfer vacuum continuously to all 4 ports? What are some indications of failure of the unit itself? I’ve tested accumulators 42 and 43 in all ventilation modes - they don’t see any vacuum unless I bypass the Y34 unit.

Follow on question - does anyone have a functioning switchover valve unit for sale? Or can this be a relay problem? I haven’t gone the electrical route yet (my MightyVac came in the mail today, so focus was on vacuum system).

And finally, a big thank you to all the posters on this forum. I’ve already learned a ton that’s helped me fix various parts of the car - odometer included (of course). Thank you!:thumbsup:

- Nick
 
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#3 ·
Robertin,

I've been testing the lines one at a time, plugging the outlet and evacuating. So far, I think i've covered or ruled out all lines - just finished the last this morning. I did, unfortunately, make the dreaded broken heater box mount discovery. Not a priority right now, as I don't foresee needing a defroster option in the foreseeable future (we're at 110 degrees here and climbing up slowly).

The unit is not really getting cold - just pushing air around. Maybe a little cooler than the outside air, but most certainly not the cryo chamber one might expect with "min" setting. I have not tested the mono valve yet, but I think I'm going to swap out the internals since it's a fairly cheap replacement. Any good way to test the mono valve? Did you just use the service manual procedures?

Other discoveries - port "4" for the recirc elements still does not get vacuum in any setting. The pods operate well, since I can apply vacuum to them directly and they close (this is actually my quick solution, bypassing the switchover valve unit entirely). Good news - my defroster pod is actually good - applied vacuum to it directly and it held! I did notice that the lines were backwards - port "3" went to the TEE and port "2" went to the long throw (bottom). I swapped them back - just curious that they were backwards. Other signs include the sound of the compressor kicking in in the appropriate setting - doesn't seem to be an issue with the relays, then.
 
#4 ·
The crack prevents the center vent from opening--this is where your cool air should come from. For now, you can shove a stick in there but be careful so you don't tear the rubber boot.

There is a thread on mono valve replacement/testing. You can rebuild it or replace it, the thread covers both.

In my case it was a combination of the mono valve and the control unit in the dash--the push button control section. I removed it and then proceeded to re-solder all of the soldered connections. I also cleaned and checked the temp wheel for resistance with a multi meter.

The recirc switch is a function of the other controls. It should light up and close the outer vent flaps. If you have a vacuum leak there (I did, one of the elements on passenger side) then it will not function. It also fails if the control unit is stuck in defrost mode.

I'm a bit fuzzy right now--neighbor started pounding nails at 6AM and I went to bed after 1AM. But there is a link for testing all that I utilized when working on the 560 and I will have to locate it for you later.

A final thing to check is the temp circuit and the hose for the temp sensor. Most people replace with clear tubing, though I got an OEM type from a local dealer. You can access it from the glove box and the stereo bay.
 
#9 ·
I got my yellow jacket manifold in the mail yesterday, allowing the further diagnostic and the confirmation of my first suspicion - insufficient charge. I was getting about 69 psi static on both low and high gages, 15 psi low gage active and 115 psi high gage active - car on, high fan, option "b". Reading many posts on here, I've gathered that even though a pressure gage is not the best way to gage a charge, it can give you a rough estimate. Given our outside temperature of 90-100 F (90 for conservative estimate), I should be seeing at least 190 psi on the high side. I can see that the center vents are open, so I am getting air from those. Next step will be a rebuild of the mono valve, since it is fairly affordable and no reason not to change it out at this step.

On a separate note, I noticed a slight tapping/knocking noise from the passenger side valve cover and investigated the dreaded timing chain. Although I found nothing on the passenger side, I found that the upper timing chain guide on the driver's side was cracked on one stud and allowed plenty of motion - definitely needing a replacement. My priority has now shifted to fixing this first, along with my noisy fuel pump and hot start issue. So, after the timing chain replacement, fuel pump(s) replacement, and new fuel accumulator, I think I'll finally be able to take the car into the local mechanic to possibly do a full system evacuation and charge - still debating if I want to just buy cans online and charge it myself...

Thank you for the thread links - I've been reading every article I can find on this to try and get a better grasp on my way forward.
 
#10 ·
Hi, everyone! Picked up my first Mercedes in early January, a 1986 560SL. I’m finally able to start troubleshooting the multitude of problems on the car and have hit somewhat of a roadblock. Trying to fix the A/C now. Like most posts I’ve read, I’ve got an A/C that just won’t get cold. My suspicion is a number of issues, but probably localized to the monovalve. Separately, I pulled the glove box out and started testing the vacuum system, since most articles I’ve read have mentioned a failure of the vacuum accumulators. I’ve found the following confusing problem - I’m getting good vacuum from the intake, through the check valve (which I replaced with a new one since they’re fairly cheap) and goes into the Switchover Valve Unit (Y34 on the diagram). However, the vacuum does not continue on to all valves. Specifically, the air flap elements (left and right, numbers 42 and 43) never see vacuum and maintain fully in the recirc mode. I’ve put vacuum (via the MightyVac) directly to the elements and they shut just fine, no leaks in any of the four. I’ve found that lines 2 and 3 (for the center vent accumulator) hold no vacuum, so the diaphragm in it is most likely ruptured. Some further tests that I’ve run (car on, drawing vacuum) showed me that in full A/C mode (“b” option), the vacuum that’s present at port 2 or 3 on Y34 immediately drops to 0 upon selection of the option. Vacuum is present in those ports when “c”, “d” or “e” are selected (tested individually, with the other ports blanked off to ensure no other contributing component added to the mix)

Here’s my question - does anyone have a good understanding of how the switchover valve unit works? Does it transfer vacuum continuously to all 4 ports? What are some indications of failure of the unit itself? I’ve tested accumulators 42 and 43 in all ventilation modes - they don’t see any vacuum unless I bypass the Y34 unit.

Follow on question - does anyone have a functioning switchover valve unit for sale? Or can this be a relay problem? I haven’t gone the electrical route yet (my MightyVac came in the mail today, so focus was on vacuum system).

And finally, a big thank you to all the posters on this forum. I’ve already learned a ton that’s helped me fix various parts of the car - odometer included (of course). Thank you!👍

- Nick
Did you ever replace the switchover valve bank? Having same issue with no vacuum.
 
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