Another heat issue

[psaboic]

Now that it is getting colder, I figured it is probably time to look into my HVAC issue on the 83 300D. Here is the issue, the heat (temperature wise) is fine, as is the cooling air. The blower fan works fine on all fan settings too. My problem is I can only get heat/cool air flow at all with the defrost button, or the button immediately to the right of it pressed.

If I press the "EC" button next to the "off" setting, or the one immediately to the left of the "EC" button, all air flow stops no matter the temperature wheel setting or fan speed setting. Once in a great while (1 try out of 100) these buttons will work.

So, in a nutshell, all climate control functions work ok (fan speed, auto setting, temperature wheel, defrost, etc). I just get nothing at all from the system (acts like I shut it off) if I press the "EC" button or the one to the left of it on the climate control panel. Defrost button and the one to the right of it work fine.

Probably been discussed here before, but I could not find anything using the search. Any help greatly appreciated.

Thanks,

Glenn

[Brian Carlton]

The CCU needs to be replaced or resoldered.

[rocky raccoon]

on my 1988 560SL. Exactly as you described. I found that if I turned the system on first with the buttons you describe, I could then press the EC button and it would work as desired. I decided to live with it.

[tangofox007]

The engine low temp cut-out switch (on the thermostat housing) disables the blower in the EC and Norm modes until the engine warms up. I would suggest that you jump that wire to ground and see if that solves your problem.

The other likely culprit is one of the fan relays in the CCU; the EC and Norm buttons use a different relay than Bi-level and Defrost. A common situation is that the fan will not run unless started on one of the two left-most buttons, but will continue to operate if the EC or Norm mode is selected while the fan is already running.

[Brian Carlton]

Originally Posted by tangofox007

The engine low temp cut-out switch (on the thermostat housing) disables the blower in the EC and Norm modes until the engine warms up. I would suggest that you jump that wire to ground and see if that solves your problem.

Point.

Check this first prior to replacing CCU.

It could be that simple.

[kerry]

Originally Posted by rocky raccoon

on my 1988 560SL. Exactly as you described. I found that if I turned the system on first with the buttons you describe, I could then press the EC button and it would work as desired. I decided to live with it.

X2. And it's unrelated to the coolant temperature switch.

[ascalise]

my money is on the CCU

[psaboic]

Can someone post a picture of the low temp switch, just as an aid?

Thanks for the tips

Glenn

[tangofox007]

The first picture in this post shows the switch. Yours might be in a different port, however.

http://www.peachparts.com/shopforum/diesel-discussion/309634-thermo-vacuum-valve-electrical-sensor-its-place.html

If your blower is running in Defrost, will it stop if you push the EC button? (Do not select OFF before selecting EC.)

[Brian Carlton]

Originally Posted by tangofox007

The first picture in this post shows the switch. Yours might be in a different port, however.

This is the photo you're looking for (in the other thread):

[psaboic]

Originally Posted by tangofox007

The first picture in this post shows the switch. Yours might be in a different port, however.

If your blower is running in Defrost, will it stop if you push the EC button? (Do not select OFF before selecting EC.)

Yes, if the blower is running full blast in defrost, 97% of the time if I push the EC button or the one to the left of the EC button, the fan shuts off.

[psaboic]

Thanks for the pic!

[tangofox007]

Originally Posted by psaboic

Yes, if the blower is running full blast in defrost, 97% of the time if I push the EC button or the one to the left of the EC button, the fan shuts off.

The 3% success rate suggests a problem with a blower relay in the push button unit. There are several relays which will be in plain view when the PBU is opened. Cleaning the contacts would be a good plan. (Buttons 1-2 use a different relay than 3-4.)

http://www.peachparts.com/shopforum/attachments/diesel-discussion/48579d1192079194-diy-page-introduction-resoldering-climate-control-unit-a12_small_0012_sf.jpg

[psaboic]

Well, I tried disconnecting the low temperature switch and grounding the lead....no change, fan still dies when switching from Defrost to "EC"

[tangofox007]

Then it's time to clean some relays.