max heat all the time, but ccu switches vents fine. - PeachParts Mercedes-Benz Forum

tony597fitter · #1 02-20-2011, 10:33 PM

Yes I have searched.

Im just unsure where to go from here.

I replaced the CCU unit with a rebuilt a couple years ago. Everything was working fine UNTIL:

The vacuum pump shaft snapped and a piece of the pump hung up in the timing chain and bust the cam and the towers.
This was several years ago so the car sat until I recently put it back together.

Now I have MAX heat all the time, but the CCU will switch the venting, but ZERO control over temp.

The monovalve looked OK, and I put 12v power to it and I would say the plunger mover 3/16"-1/4". Should it move more than that?

I used my tester on the plug and I get 12.5v on the defrost setting and 12.5v on the a/c setting, with nothing in between.

Sorry so long, but Im starting to cook!!

rhodes2010 · #2 02-20-2011, 10:57 PM

This is a really good puzzle.
Would you mind updating your location/signature so we know where you are.
Maybe someone close to you can help a little.

Other than that . I have a 1983 300SD, and that is the only MB in your sig.

You looked at some of the things I would think of first.
That mono-valve for instance.
I have to go look and see if we have an auxillary pump on these.

Worst case, you could cut the hot water supply to the cabin.
And maybe control it with a valve until the problem is solved.

My experience with the later climate control is that it works great.
Normally no trouble.

Your mechanical problems are something I have never experienced.
It is assumed by me that your vacuum is now fuly operational.
Those vents working properly would sort of signify that.

vstech · #3 02-20-2011, 11:52 PM

if you've got voltage in defrost, and a/c settings, the ccu is not working right. it should not have power to the monovalve in defrost.
the monovalve is open when power is removed, and closed with voltage applied.

tony597fitter · #4 02-21-2011, 06:14 AM

Quote:

Originally Posted by vstech

if you've got voltage in defrost, and a/c settings, the ccu is not working right. it should not have power to the monovalve in defrost.
the monovalve is open when power is removed, and closed with voltage applied.

Right, after researching this, thats what I thought, but the monovalve is ALWAYS open.

shadetreemechan · #5 02-21-2011, 07:35 AM

Start by wiggling your fuses. Poor connections at the fuse panel and unlimited heat often go together.

tangofox007 · #6 02-21-2011, 10:25 AM

Quote:

Originally Posted by vstech

if you've got voltage in defrost, and a/c settings, the ccu is not working right. it should not have power to the monovalve in defrost.

That is absolutely incorrect. The monovalve is controlled via a switched ground.

You need to understand how things work before you attempt to diagnose the problem.

polarisrmk · #7 02-21-2011, 11:57 AM

Quote:

Originally Posted by tangofox007

That is absolutely incorrect. The monovalve is controlled via a switched ground.

You need to understand how things work before you attempt to diagnose the problem.

This is correct. And you cannot use a volt meter to determine if it is working. The ground is switched at different frequency depending on the temperature. U need an ocilliscope to determine if the ccu is sending the proper signals.

tangofox007 · #8 02-21-2011, 04:02 PM

Quote:

Originally Posted by polarisrmk

This is correct. And you cannot use a volt meter to determine if it is working. The ground is switched at different frequency depending on the temperature. U need an ocilliscope to determine if the ccu is sending the proper signals.

Also incorrect. The monovalve ground is switched by the electronic unit for temperature control, not the CCU. No ocilloscope is required; the cyclic rate is quite slow. The only "signal" sent by the CCU is system voltage, any time the key is on.

You need to understand how things work before you attempt to diagnose the problem.

tony597fitter · #9 02-21-2011, 08:36 PM

So how do I go about diagnosing?

polarisrmk · #10 02-21-2011, 09:39 PM

Quote:

Originally Posted by tangofox007

Also incorrect. The monovalve ground is switched by the electronic unit for temperature control, not the CCU. No ocilloscope is required; the cyclic rate is quite slow. The only "signal" sent by the CCU is system voltage, any time the key is on.

You need to understand how things work before you attempt to diagnose the problem.

we use the scope at work ... yes a mercedes-benz dealer

ccu (climate control unit) can refer to the pbu (push button unit) or the temp control module just an FYI.

Paulc66t · #11 02-22-2011, 08:44 AM

Check your fuses. I have had the same issue on my 83sd. I believe it was #12 fuse that was blown. Had something to do with the kick down switch circuit etc. I don't recall the details.