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Old 12-30-2020, 03:03 PM
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Changeover valve

1985 W123 300TD
How do you test the changeover valve for proper operation? I am talking about the valve with three vacuum ports that is located behind the brake booster next to the fuse box; not the one near the oil filter. I am trying to trace a vacuum leak. Should it hold vacuum on any of the 3 ports with a mighty vac? It has an electrical connection so is it tested with the key on or off? Vacuum leaks are frustrating to trace for me!

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Old 12-30-2020, 09:17 PM
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A vacuum switchover valve works exactly like an electrical switchover relay. The power connections control the electromagnet winding, on or off. There are three vacuum connections. One connects to the vacuum source. The valve will connect the vacuum source to one or the other of the outlet ports (never both), depending on whether the winding is powered.

All you need to test is a power source, and a vacuum source. A small 12V battery and a mityvac are fine. First connect the mityvac to the source nipple. Then pump it and make sure that it transmits vacuum to only one of the outlet nipples. Finally, attach your battery, probably a good idea to mind polarity. Then check that the mightyac now connects only to the other nipple, in other words, vacuum has "switched over". Simple.

Last edited by Mxfrank; 12-31-2020 at 08:50 AM.
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Old 12-31-2020, 09:06 AM
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Quote:
Originally Posted by Mxfrank View Post
A vacuum switchover valve works exactly like an electrical switchover relay. The power connections control the electromagnet winding, on or off. There are three vacuum connections. One connects to the vacuum source. The valve will connect the vacuum source to one or the other of the outlet ports (never both), depending on whether the winding is powered.

All you need to test is a power source, and a vacuum source. A small 12V battery and a mityvac are fine. First connect the mityvac to the source nipple. Then pump it and make sure that it transmits vacuum to only one of the outlet nipples. Finally, attach your battery, probably a good idea to mind polarity. Then check that the mightyac now connects only to the other nipple, in other words, vacuum has "switched over". Simple.
Thank You!
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Old 01-02-2021, 12:57 PM
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Yes, "switch-over valve" is one of the many quirky German translations. Even stranger, the similar 5 electric vacuum-valves in the climate system are termed "codes". Effectively, it is like a Single Pole Double Throw (SPDT) electrical switch. Describing its function can be clumsy since when a port is "open" that is equivalent to an electrical contact being "closed", so maybe term that state "on".

As I recall, that switch-over valve is only in later OM617 cars. I know it is in my 1985 300D and recall it isn't in my 1984 300D. I recall that it blocks the intake manifold pressure signal (Pman) from going to the vacuum piston on top of the injection pump under certain conditions. There are other switch-over valves in the EGR system.

1985 cars also have the "blue moon" vacuum-amplifier for the VCV vacuum signal to the transmission (modulator on side). I added one to my 1984 since seems like a good idea. 1985 cars also have the Klima Relay which disables the AC when the pedal is floored (and other functions?). A reasonable question is why throw those new things into the last year of the W123 300D. I recall both continued in later models, so 1985's were apparently guinea pigs.
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Old 01-10-2021, 04:50 PM
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My 1983 300SD had one, notice I said “had” one. Took that thing off within the first week of owning the car. Always trouble for leaks and with an EGR delete or not I recommend you do the same.

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