W116 Climate Control Help - PeachParts Mercedes-Benz Forum

victoriacoffeeroasters · #1 04-06-2020, 10:06 PM

Trying to take advantage of the unwanted extra time on my hands by working on my daughter's 1980 300SD a/c system. It was working flawlessly until my compressor when out. Had a new system installed and it was working great until just before Winter hit. Now, only the defrost button works. Here's what I've done so far:
1. traced each vacuum line coming out of the rubber multi-port and resolved all major leaks (see orange lines);
2. repaired vacuum line feeding the climate control system;
3. tested each blower and compressor master switch, they work.
4. tried to test vacuum at push button switch but could not register any. In picture #2, there are 2 empty ports. I used a plug on one and my finger on the other.

The problem is when everything is connected, I get no vacuum at the test port (pic #3 & 4). But, using my handheld pump connected to test port, I can cycle through each push button - auto low/high/bi-level and each work fine. Blower and compressor kicks on and temperature changes as I move the temp wheel. As soon as I stop manually applying vacuum, the system turns off.

Although I can toggle on/off the compressor using the a/c switch, the compressor did not cycle when adjusting the temp wheel.

I'm at a loss as to what next needs checking. I'm thinking either the rubber multi-port is not sealing, causing a loss of vacuum or the push buttons are leaking. But if that were the case, why was I able to cycle the system using the manual pump?

I'm considering pulling each of the vacuum lines from the multi-port and connecting them directly to the push button circuit.

Based upon what I've described, did I miss something? Am I on the right track?

Don't knock the temporary translucent orange lines. All I had at the time.

Thanks in advance - John

vwnate1 · #2 04-07-2020, 06:55 PM

Oh, boy ~

I went through this in my first two NA W123 coupes twenty years ago, I don't envy you .

There's lots of really good how to DIY info on this site, you'll have to read out and print it to use as cheat sheets .

I suggest testing the Evil servo first as that's where most problems occur .

Once it's done you can begin chasing the myriad vacuum leaks and so on .

I wish I remembered what all I did, it was a steep learning curve .

victoriacoffeeroasters · #3 08-03-2020, 10:18 PM

Just a quick update on my 300sd.

With vac tester in hand, I went through each circuit, repairing/replacing rubber connectors, cracked lines and faulty check valves. But every time I went to run the system, nothing worked. There was good vacuum coming into the control panel but nothing after. So, I ordered a new push button panel, installed it, and boom! system worked. But no cold air. Compressor was bad too.

Now I have a functioning A/C system but have one final questions. Is the compressor supposed to cycle? I've observed it running but have never seen the clutch release. The cabin air modulates just fine.

ROLLGUY · #4 08-04-2020, 03:38 PM

Quote:

Originally Posted by victoriacoffeeroasters

Just a quick update on my 300sd.

.........

Now I have a functioning A/C system but have one final questions. Is the compressor supposed to cycle? I've observed it running but have never seen the clutch release. The cabin air modulates just fine.

The compressor should only cycle off when the evaporator temp regulator senses it starting to ice up. This is a very rare occurrence with these systems. Maybe when they were new, but I have yet to see an R4 and tube and fin condenser get cold enough to ice, and thereby cycle the compressor. This will NEVER happen with a 134a conversion. With a perfect compressor (Sanden), R12, clean as new evaporator, and a parallel flow condenser, I could see the scenario where the compressor would cycle......Rich