I've been reading up on past posters' problems and want to pass this by you guys to be sure.

Suddenly I'm not getting any heat from the floor or upper vents but the defroster works. I swapped out my CCU with a known-working one and still have the same problem. I looked at the vacuum lines running behind the radio and they look good. I looked at that manifold on the passenger side wall behind the radio and all the vacuum lines seem ok and attached. Should this be a monovalve issue or could it be a TCM issue and how to determine? Thanks guys.

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Tony
(Actually, a member since 2000)

I just replaced my monovalve in my 560SEL; if you're getting cold air out of the vents, its most likely the monovalve.

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1987 560SEL (210,000 miles)

Some findings from current troubleshooting.....
OK, I pulled the connector off the monovalve and I get 13v when the CCU is in the off position. When any of the other buttons are activated and the temp wheel is full-hot I get 0v.

When I activate any of the CCU buttons (not fan speed) it takes about 20 seconds for anything to happen. If I switch the fan speed buttons from min-max or max-to-min it takes a while for the fan to change speeds. When the speed finally changes I can hear something click on the back wall behind where the radio resides. I think it's that black box but unsure what that is.

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Tony
(Actually, a member since 2000)

You'll need to define "heat".

When you state "heat" do you actually mean "heat" or do you simply mean "airflow".

If you have airflow without warmth, that's a completely different scenario than if you do not have any airflow.

Kindly clarify the post as nobody can give a proper response without it.

Your vacuum lines look good, but have you checked for vacuum at the source with a mighty vac? Mine does the same thing on any temp setting other than min temp. Fan speed varies as commanded and I get hot air, just not in the right commanded registers - defrost only. I have a vac leak somewhere at the changeover valves on any heat setting and my trans shifts hard while on anything other than min temp. So, like me, you may need to start hunting leaks.

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Thanks,
Mark in NC

"Spark plugs?...We don't need no stinking spark plugs!"
1985 300SD "Der Silberne Schlitten" 420,000 mi


Wish these were diesel:
2003 Ford Club Wagon 130,000 mi

Thanks Brian. No airflow coming from the lower vents, nor the upper middle vents. The outer-upper vents have heat coming out but very little, if any airflow.

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Tony
(Actually, a member since 2000)

If you fail to have airflow from any of the vents other than defrost (default is open), you'll have to get out the Mityvac and start testing the vacuum pods. There are six pods on the vehicle, one for the footwells, one for the center vents, one for the defrost, one for the 'scoop', and two for the main air door.

You can access all of them via the switchover valves. The group of six is located just to the right of the CCU. You access them by removing the right kick panel, the right floor mat, and the carpeting on the side of the transmission tunnel. The six valves will then be quite visible. Remove the two screws to move the group out away from the CCU so you can do some testing. The horizontal legs of each switchover valve goes to a pod. Start testing to see if they hold vacuum. If they don't, you're in for some work to get them out and replaced or repaired.