I bought this 85 SDL six months ago and it's been running well. I've read all of the threads on CCU's, but I can't figure out what's going on with this thing.
Symptoms: Defrost heat is always on. Not bad in winter, sucks in the summer. No other buttons work on the CCU. After 30 minutes of driving everything will turn on and the blower, heat, vents, etc. work fine. Sometimes the fan will have a heartbeat. On, off, on, off. This is after 20 minutes or so, usually.

What I've done: checked blower--it works great hooked up to the battery directly. Replaced porquepine, blower regulator. Removed CCU and resoldered loose soldering (couln't see any problems). Replaced fuses. Chanted strange words.

Does this sound like I need to replace the CCU? It's already been done at least once in the last 320,000 miles.

Brian Carlton, are you out there?

My mechanic friend says it's the blower, even though the thing works when hooked up to direct current. He says it's the commutators that are bad. I don't want to buy a new blower or CCU if I can help it because I don't know when this car will die completely, and I can survive with cold feet.

Thanks.

Keep the friend.........ditch the mechanic.

You have all the symptoms of a CCU that needs to be replaced. Intermittent action of various features and no action of others is certainly indicative of the CCU.

However, are you confident that the vacuum pods are all working correctly?

If you have more than one problem, it's not easy to diagnose over the wires.............

The blower is blowing. It's not the blower.

There are three controllers in your HVAC system. The push button unit (CCU); the Temperature Control Module (TCM), over by the passengers right knee; and the Blower Speed Module.

Your problem could be with the push button unit or the temperature control unit. If either aren't working, you will get heat from the the defrost as the default mode.

The TCM gets input from the CCU temperature wheel and the temperature sensor in the cabin (also from the airbox and the block) and decides when to open the coolant valve. The TCM also is the module that actually turns on several of the vacuum pods. Replacing this module fixed the "hot defrost always on" on one of my SD's.

__________________
Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)

That's certainly a possibility as well. Have you ever found a method to test that module?

How would one go about checking the vacuum pods? Do I access the TCM the same way I got to the blower regulator and blower, or is there another access point from the engine? I suppose I could just try getting a rebuilt or new CCU, but I'd like to eliminate the problems might cost less before breaking out the cash for a CCU. The rest of the car vacuum system seems to work fine, as far as I know.

The best way to check all the pods is to access the set of vacuum switchover valves. These are sitting in a vertical column right next to the CCU and radio. You access them by removing the passenger kick panel, removing the passenger carpeting, and carefully removing the carpeted transmission tunnel piece on the passenger side. You'll see the six electric switchover valves in a vertical row. Remove the top screw to gain some access and you can pull the horizontal hose off each of the valves.

Connect the horizontal hose to a Mityvac and pump it up. Any pod that does not hold vacuum must be rebuilt or replaced. I've got the sequence of which valves to what tasks, so, if you do the checks, just make a note as to which valves have properly working pods and which do not.

It's very common for the dual diaphragm pods to fail. There are three of them.........the defrost pod, and the two main air door pods.