Sometimes No Heat - PeachParts Mercedes-Benz Forum

shertex · #1 11-18-2011, 09:14 AM

Minor quirk...thought I'd see if anyone has any ideas. 92 300D. What I've noticed recently, on occasion, is that only cold air is blowing when heat is on. I'll shut the car off, turn it back on, and heat comes right back.

I know auxiliary water pump needs to be replaced, but that's a separate issue.

Monovalve replaced 22,000 miles ago, CCU is original, 231,000 miles on the car.

Any ideas?

dablk1 · #2 11-18-2011, 10:25 AM

If it's intermittent I would suspect the monovalve. At least give it a look. Once you replace the auxilliary water pump that will also make a huge difference while idling.

shertex · #3 11-18-2011, 10:34 AM

Quote:

Originally Posted by dablk1

If it's intermittent I would suspect the monovalve. At least give it a look. Once you replace the auxilliary water pump that will also make a huge difference while idling.

Is it common for a monovalve to fail in that short a period of time?

tangofox007 · #4 11-18-2011, 10:35 AM

Quote:

Originally Posted by shertex

Is it common for a monovalve to fail in that short a period of time?

What was the period of time?

shertex · #5 11-18-2011, 10:43 AM

Quote:

Originally Posted by tangofox007

What was the period of time?

Replaced monovalve 22,000 miles ago, in Jan. 2009.

dablk1 · #6 11-18-2011, 10:47 AM

3 years can be more than enough time. You never really know when and if it's going to fail. Besides it the easiest thing to check. Unplug it and see if you get max heat, it should actually default to defrost mode.

shertex · #7 11-18-2011, 10:49 AM

So, when a CCU starts to go bad, what symptoms do people typically notice?

tangofox007 · #8 11-18-2011, 10:53 AM

Quote:

Originally Posted by dablk1

Unplug it and see if you get max heat, it should actually default to defrost mode.

If "it" means the monovalve, unplugging it will have no affect on the ventilation configuration.

dablk1 · #9 11-18-2011, 11:05 AM

I stand corrected Tango. Unplugging the monovalve should give you max heat. As far as CCU issues sometimes it just becomes unresponsive. I forget which model maybe all of them....it was stated in the forum that a seized auxiliary water pump can damage the CCU.

shertex · #10 11-18-2011, 11:15 AM

Quote:

Originally Posted by dablk1

I forget which model maybe all of them....it was stated in the forum that a seized auxiliary water pump can damage the CCU.

Not a problem on 90-93 300D.

dablk1 · #11 11-18-2011, 11:19 AM

That's good to know.

shertex · #12 12-02-2011, 04:11 PM

Looks like the monovalve is the culprit. Swapped out the CCU since I had another....same symptoms. Then unplugged the monovalve and still no heat.

And, to add to my fun, I discovered that the sampler blower isn't working either.

So I'm replacing monovalve, aux water pump, and sampler blower....and I just did the CCU. Hopefully that'll do it for a while!