Hey guys!

Back once again - just reviewed everything I could find on the 300sd CCU / Air Conditioning threads and I think I've discovered a new (and interesting) problem.

Two summers ago, I had a good mechanic friend go through my 1982 300SD's climate control. He replaced two vacuum spheres, soldered up the CCU, replaced the dryer and pressure switch and wala, I had AC. That lasted until fall, where I assume there was a small leak and once the refridgerant slowly leaked out, the compressor stopped. All buttons still worked, defrost, up/down with A/C, down with A/C, EC and of course, off.

Here's the interesting problem: only half the buttons work, now. I've replaced the CCU with a 'new' unit, one that came out of a car with fully operational climate control. Same part numbers. Defrost, and Up/Down with A/C work; the other buttons (with the exception of OFF) seem to do absolutely nothing.

The reason for the posting is it's getting warmer, but not too warm here, and I'm about to recharge the system - when it's cool at night, I'd love to just use EC and save the gas (err..diesel) when driving home from work. Soo..that would require all the buttons to function.

Anyone else experienced this? I've checked the usual culprits (fuses, etc). Other than the two buttons not working, system runs fine - heat is great, fan steps as it should. The old CCU had the same button issue - so it's clearly not the head unit.

Perplexingly yours,

-James

Please clarify, when you say the buttons "don't work" are you stating that just the compressor doesn't engage or that you get no blower as well?

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Sam

84 300SD 350K+ miles ( Blue Belle )

Sorry for that - with all buttons the compressor is not turning on - I attribute this to the system simply being out of refrigerant. With the first two buttons, I get fan and heat (as I should). With the other two buttons, I get absolutely nothing - no sounds of relays activating, no ducts moving, no heater motor activation.

If I was on Defrost or up/down, the system works normal. Go to floor or EC, and it's like the system was in OFF.

Ok, Here is what I know, and what I would do if my car were doing this. (this is all from memory)

do these tests with the car on, pushbutton on full, that is two white arrows pointing both up and down with the temp dial on full cold, turn until you feel the click.

1st. Compressor test. Jump the two wires going to the receiver dryer(while the car is on) and see if the clutch for the compressor engages. (that is the disc that doesn't spin while the accessories belt is.) If the compressor clicks on, then you are low on refrigerant.

If the clutch doesn't engage then test the evaporator temp sensor. This is the little box up and to the right of the accelerator pedal behind the carpet wall. It will have a sensor wire going into the evaporator. jump those wires and see if the compressor engages. If that is the compressor culprit, then you can do one of two things. Take a screwdriver and turn the tiny screw clockwise until it engages without being jumped, or purchase a new temp sensor without messing with the screw. The replacement temp sensor I bought did not have a screw or it had a tamper resistant cover over the screw. that parts job is to keep the evaporator from freezing which is pretty much death to the cold side of the climate control. the repair will overly total the car.


Now, if those don't activate the compressor clutch then I will defer to the more experienced members of the forum to deal with the compressor.


Blower issues.
If your blower is feeling it's age, it will require more current to spin. the non functioning buttons you are talking about take less current then the working ones. That leads me to believe that you may need to pull your blower motor and replace the brushes.

Test to see if the vacuum actuated flaps are flapping even though the blower is no engaging.

Flap test
With the car on, turn the climate control off. then press the econ button and listen for the vacuum flaps to move. cycle through the buttons specifically listening for the flaps to move. go from defrost to floor, full to EC ect.

If your flaps are moving and the blower is only coming on when set to high current options, I would look at replacing the brushes or the blower.

(I went in to disassemble my blower and found that it was replaced in 2003. I decided it wasn't the blower and moved on.

So, if it is the temp sensor for the compressor and the blower for the push button, then you aren't far from a fully functioning climate control system.

Good luck.

Kent

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77' 300D, "Cartman" SOLD @ 150K (didn't know what I had)
83' 300SD, "The Superdon" 325k+ @ 28mpg
95 E320 wagon, "Millennium Falcon" 231k+ @ 24 Mpg
95 E300D, "Sherley" 308k @ 33.69 Mpg, currently anticipating a head
99 Suzuki Intruder "Trudy" @ 45 mpg

Kent,

Thanks for the help! Finally got some time together today and tried filling the system with refrigerant; lo and behold the compressor did *not* turn on. I'm going to jump the leads on the Evap. temp sensor and see what happens.

Here's hoping!

-James