95PGTTech

My brother bought a 1995 C220 which he felt was a good deal because it was "a cheap Benz." We'll table the rest of the issues for now.

There was no A/C when he got the car. I noticed the A/C compressor did not kick on so we bought one of the parts store DIY R134a kits and the low pressure side showed nothing on the gauge. We put 2 cans (about 240z) in it. The compressor clutch kicked on and held it about 35psi while on. The accessory fans (front of engine) did come on as well.

We noticed only cold air out the passenger side a/c vent. Nothing out the centers, hot/warm out the driver side vent. After reading, we found the centers are normal not to blow and a bad duo valve may be causing the left/right issue. We walk outside and hear PSSSSHHHHHHH. After taking some stuff apart, we find a leaking condensor. Neither of us having much experience with Mercedes, we took apart the duovalve and, sure enough, one side literally was exploded.

We ordered the replacement parts and everything seemed to bolt up/go well. We topped off the coolant and bled the system by leaving the cap off of the overflow bottle and the heat blasting. We have since attempted to bleed it by squeezing hoses, etc. a variety of different ways suspecting a air bubble.

After adding refrigerant, we still have the same conditions, and heat is also poor at best (can't remember if it was like that before). I have listed the symptoms here for quick reference:

1. no a/c out driver vent. poor a/c out passenger. pressure is between 55/100 when clutch not engaged, holds 35 when engaged.
2. line coming from compressor to condensor is HOT, line coming from condensor to the sight glass (drier?) is also hot. shouldn't that be cold?
3. diagnostic codes 49, 51. I cannot hear the new duo valve clicking. I have 12v at aux pump with key on, and 12v at duovalve from center to each side with key on. new duovalve does click when 12v applied on bench.

We bench tested the aux water pump and it fails so, for diagnostic purposes, we have bypassed it with a 5/8" elbow. from everything I've read, this simply helps put heat into the cabin when engine is off.

my main question is, why is the line coming OUT of the evaporator hot? shouldn't it be cold? the fans kick on and I can see stuff moving around in the sight glass. I believe the compressor obviously works because low side goes from 70ish (I only have 55 and 100 marks and it's halfway in between) to 35 and holds when the clutch kicks on.

I thought maybe the wiring to the duovalve was bad causing it to be full heat all the time and fighting what is now a properly working a/c system (when you disconnect it you're supposed to have full heat) but I have 12v from side to center post on terminal (both sides). perhaps try and jumper it with 12v while car running? no leaks in a/c system.

thanks for any help you can offer. he did leave me the car so I can do some further diagnostics.

95PGTTech

Important update - I think I'm onto something with the thinking that both systems are now functioning correctly (heat and A/C) and the duovalve is just causing them to fight each other. I just did a full coolant flush including the heater cores. The engine is ice cold. As an experiment, I set it to LO (I would normally do full heat to help burp). Until the engine comes up to temp, I have a/c out both. As engine comes up to temp, it gets hotter and hotter out vents.

I re-checked, with key on engine running I have 12v at both outside pins (1, 3) when using 2 as the ground. As another experiment, I did a jumper alligator wire between tabs 1 and 3 on the valve themselves and ran that to a 12v jump box. Ground the center pin with the box and I can hear/feel both valves click and stay there. Sure enough, a/c in the cabin ice cold.

So what the hell? The connector or pins show no signs of damage.

95PGTTech

Now I am very confused - so close I can almost taste it. for the purposes of discussion, we are talking about the passenger side only.

key on engine off
12.69v between pin 3 and 2
key on engine on
12.59v between pin 3 and pin 2

removed connector, inspected pins. appear good and not spread apart. fits on duovalve tight. added some dielectric grease. reinstall, start car. no a/c.

with passenger window down, hook up external 12v source (jump box) to pins 2 and 3. can feel duovalve side click, hand sticking through window immediately starts to feel it go from hot to cold. if leave on source, full a/c eventually.

...what...the...hell. I know the system works right by jumpering it and if I have 12v and ground from the car at it??? I even tried to take the pins out of the connector and install them directly (very careful not to touch each other or a ground) and it doesn't function correctly, yet measures 12v between outside and center pin