No Heat, Not Monovalve
 

My 87 560 SEL has the following problems. Only has heat on defrost mode and only has high blower. No heat in all other modes but blower works normally. I tried a monovalve since I had a similar problem 5 years ago but that had no effect. Any ideas?

Sounds like the heater/air cond. control unit has gone south. The defrost on high only is a safety feature built in so that you won't be stranded with a frosty windshield and no way to get home. I would check the unit first - mine is about ready to go as well. It will sometimes have to be shut off before it will change from heat to air or vice versa. It also does this in the "vent" mode. There are several on ebay right now that will fit your car.

Benzboy87:

Is this the part you're referring to?

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=mercedesshop&baseurl=http://catalog2.mercedesshop.com/&cookieid=1E50U2ODS1E50U2US7&year=1991&make=MB&model=300-SEL-006&category=R&part=Climate+Control+Unit

Thanks.

Thanks for the information. Is it possible that there is a bad solder joint in the control unit like I have had with other electrical parts or should I spring for another unit?

Yes to both of you. You are correct. It usually is a bad solder. There are threads here on how to repair these and there are a couple of places to send them to be repaired if you don't feel comfortable with the job. Salvage yards is another place to find used units - most come with some sort of guarantee. Keep in mind that these units fail with age and use - most used units (unless recently repaired) are on their way to failure at some time. Hope this helps.

Sounds like a good project for this weekend and if I fail I haven't lost much except a little time and a few beers. Thanks again for the info.