No heat in Auto, Lots of heat on defrost - PeachParts Mercedes-Benz Forum

Jadavis · #1 11-25-2006, 12:10 PM

Hello,

I have searched and read a good bit, but I have not seen my exact set of problems.

I have a 1987 300D (Pre 08/1987).

When in defrost I get full fan, full heat no matter what. I have seen conflicting posts on this. One group says this is normal. The other says that they read that this is caused by a bad Aux Water Pump. Anyone else?

When my fan is set on Automatic and the heat is set to something appropriate I do not get any heat. The fan will go into high speed like the ACC knows that the cabin is too cold and that it needs to heat it up, but it is cold. From here I can press the defrost button to heat it up a bit, but clearly this is not right. Any ideas on this?

On an unrelated note, sometimes in the summertime the cold air comes out on the windshield. Eventually I will have to read up on removing the dash and fixing this issue.

So what am I looking at here? A new Aux Water Pump? A new ACC? Take apart and resolder everything that looks loose in the ACC? Does the Mono valve have anything to do with my problems?

Thanks,

Jim

Brian Carlton · #2 11-25-2006, 12:27 PM

Quote:

Originally Posted by Jadavis

Hello,

I have searched and read a good bit, but I have not seen my exact set of problems.

I have a 1987 300D (Pre 08/1987).

When in defrost I get full fan, full heat no matter what. I have seen conflicting posts on this. One group says this is normal. The other says that they read that this is caused by a bad Aux Water Pump. Anyone else?

The group that says it's normal is correct.

Quote:

Originally Posted by Jadavis

When my fan is set on Automatic and the heat is set to something appropriate I do not get any heat. The fan will go into high speed like the ACC knows that the cabin is too cold and that it needs to heat it up, but it is cold. From here I can press the defrost button to heat it up a bit, but clearly this is not right. Any ideas on this?

Since you get good heat from the defrost outlets, when called for, the monovalve is not the issue. Either the CCU is malfunctioning, or the cabin temperature sensor is malfunctioning. But, since the fan is on high speed, it appears that the cabin temp sensor is working properly.

Quote:

Originally Posted by Jadavis

On an unrelated note, sometimes in the summertime the cold air comes out on the windshield. Eventually I will have to read up on removing the dash and fixing this issue.

It's the "sometimes" that's the killer here. If cold air comes out the windshield as a normal occurrence, then the defrost pod or the defrost switchover valve is NG and failing to properly close. But, when the problem is intermittent, you really can't blame the pod. So, the CCU becomes the more likely culprit, again.

Quote:

Originally Posted by Jadavis

So what am I looking at here? A new Aux Water Pump? A new ACC? Take apart and resolder everything that looks loose in the ACC? Does the Mono valve have anything to do with my problems?

Definitely not the auxiliary water pump or the monovalve based upon your stated symptoms. Possibly the CCU.

michael cole · #3 11-25-2006, 12:49 PM

unplug the aux water pump electrical connection.then see if climate control resumes normal operation.the pump could be seized resulting in heavier than normal current draw from ccu circuit

Jadavis · #4 11-25-2006, 02:20 PM

Quote:

Originally Posted by michael cole

unplug the aux water pump electrical connection.then see if climate control resumes normal operation.the pump could be seized resulting in heavier than normal current draw from ccu circuit

Works great with the pump unplugged. (For now...I'll be keeping an eye on everything else. I'm sure eventually the ACC will have to be replaced also.)

Thanks!

-Jim

michael cole · #5 11-25-2006, 03:19 PM

glad that worked.best of luck.after numerous problems with my aux water pump i have just removed it completely form my 87 wagon.we will see how it goes this winter

tenmaz · #6 11-25-2006, 04:13 PM

This afternoon I repaired my CCU on my 87 300 SDL. I was having similar problems. I took the unit apart to check all solder joints, as I've seen discussed on this forum. My CCU is slightly different form the pictures I've seen here, but I did see the temperature control unit was loose where it was soldered to the circuit board. I reheated these three connections and it works fine. Prior symptoms include: off switch usually would not work, sometimes heat would come on as selected and then switch to cold airout any, and or all vents, and then switchback to heat intermittently. sometimes cold out one vent and hot out another. It's working fine now

I do have another problem now

I had left the radio connected for a few days with CCU out. It had been working fine. Today before working on the CCU I noticed dotted line across radio screen and it will not turn on. Is there a safety switch or something I may have tripped?

Jadavis · #7 11-26-2006, 10:06 PM

OK, I was wrong. It may have improved things in the parking lot, but on my 2-1/2 hour trip tonight it simply did not work. Symptoms are exactly as they were originally described. Looks like I'll be playing with the ACC. I'm also going to pull the Aux Cooling Pump and test/replace it.

-Jim

tobybul · #8 11-27-2006, 10:41 AM

Quote:

Originally Posted by Brian Carlton

........... But, when the problem is intermittent, you really can't blame the pod. So, the CCU becomes the more likely culprit, again.......
Definitely not the auxiliary water pump or the monovalve based upon your stated symptoms. Possibly the CCU.

Brian's right about the AWP and the MV...

Brian, when referring to the CCU, can you please point out which specific part this is? Is it the temperature control knob? or is it something else? Thanks

Brian Carlton · #9 11-27-2006, 10:56 AM

Quote:

Originally Posted by tobybul

Brian's right about the AWP and the MV...

Brian, when referring to the CCU, can you please point out which specific part this is? Is it the temperature control knob? or is it something else? Thanks

I just refer to the "CCU" as the climate control unit. Attempting to go into the unit and fix the issue is usually unsuccessful without a lot of additional information and diagnostic equipment. Once the CCU is determined to be the culprit, I just change it. I've used at least five of them in the last four years..........they're not a very durable item once they've reached this age.

Jeremy5848 · #10 11-27-2006, 12:17 PM

The CCU is the entire assembly of temperature dial, five mode buttons, and three fan speed buttons, plus the circuit boards inside. There are numerous soldered connections that sometimes come loose over the years. If you know how to solder, you can fix it yourself. Otherwise, buy a rebuilt unit.

tobybul · #11 11-27-2006, 01:06 PM

Quote:

Originally Posted by Jeremy5848

The CCU is the entire assembly of temperature dial, five mode buttons, and three fan speed buttons, plus the circuit boards inside. There are numerous soldered connections that sometimes come loose over the years. If you know how to solder, you can fix it yourself. Otherwise, buy a rebuilt unit.

Thanks, Jeremy!! You answered my question.... I assume you get to the "CCU" by taking off the wood grain cover and whatever's behind it? Now I am curious about this... soldering should not be that difficult but I gotta see what I'm dealing with. I am well equipped with soldering tools.

It would be a good weekend project if I can remove the Unit.

RROSIN1 · #12 03-03-2007, 05:30 PM

does the faceplate just pop off on the ccu the way it does on the 300d. how do i get the ccu out?

babymog · #13 03-03-2007, 05:42 PM

Post year and model, replies will be more useful.

If it's a 124 / '87 300Dt as in the original thread, there are two screws at the bottom of the Zebrano panel, then the panel swings out and down to remove.

The CCU is held in by screws you'll see with the panel removed.

RROSIN1 · #14 03-04-2007, 02:28 AM

i have an 86 300sdl, i bought a new ccu. just cant figure how to get the faceplate off??

Brian Carlton · #15 03-04-2007, 09:25 AM

Quote:

Originally Posted by RROSIN1

i have an 86 300sdl, i bought a new ccu. just cant figure how to get the faceplate off??

Pull the ashtray out. Remove the tray. Two screws secure the ashtray housing to the vehicle. Then pull the entire ashtray housing out.

Remove the radio.

On the underside of the faceplate are two phillips drive screws. Remove them.

Slide the faceplate downward........carefully........about 3/16" so that the top will clear. Once it clears, the faceplate and CCU will pull forward to allow you to begin the massive disconnect job on the back.