The automatic climate AC systems in the W123 is complicated and discussed a lot in the diesel forum (I've made several posts on fixing problems with this system). Do not be too specific with searching for info on the diesel forum because the various sub-systems work the same (center vent, floor vent, defrost, side vents, etc.), so a search for say "center vent" may not return info that will help you. You can find a very good M-B AC manual in the Mitchell automotive service manuals - I found copies at my library - and I consider this the best, or eBay for the M-B CD manual or perhaps the OOP print manual for the AC. Tools needed will be the Mighty Vac ($30-$35 basic kit $50-$60 deluxe kit at NAPA, Pep Boys, O'Reilly, etc.) to draw vacuum to check various components (I've found leaks in a 2" length of vacuum line and a plastic Y-fitting that leaked) and a volt/ohm meter. The AC system is a combination of vacuum and electrical components (the swtich over valves are both) so you need both tools to check things out. Finding the problem is a matter of taking the time to trace out and check the components in the sub-system that is not working (here, the center vents). There are not any "silver bullet" fixes on these systems.
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