A/C Question
 

I have been through the A/C section of this forum a few times which has led me to test my A/C the way I have and led meto now understand a little about what it going on with my 1983 300D (w123 617 engine). But I am kind of stumped and if someone could point me in the right direction for the final solution, that would be great.

So I had the system charged with 2 pounds of R134 (I think i will switch to Freeze 12 but for now....) and it holds a vacuum. Pushing buttons on the climate control unit, I couldn't get any cold air to blow in the car. So I cut the wires to the compressor and grounded the brown wire, and jumped the Blue/green wire to battery and viola, the compressor engaged, the pipe in front of the valve cover (usually covered insulation but mine is lacking it currently) got cold and my mechanic said the high pressure was good but the suction was a little high (low side?) and said probably because the front fan was not on. However even after pushing every button on the air temp console, I still could not cold air into the cabin of the car. The buttons seem to appropriately direct air to the defrost, floor and front vents - although it appears that my two center vents do not operate. One other point, I did re-splice the compressor wires which I had cut and I am pretty sure I could here the compressor go on, but I do not think my front fan every went back on AND that A/C pipe that runs in front of the valve cover seemed to get hotter and hotter and of course there still was no cool air in the car.

Thanks
Doug

So, you cut wires, did this, did that, but you still want to change to Freeze 12? I don't get it.

BTW, it sounds like your problem is in the air handling or the monovalve. Your climate control system for whatever reason is not managing the air properly or the monovalve is having problems allowing hot air to offset the cold air available at the evaporator.

Freeze 12 is basically R134A... so it's a waste of time. Just do it properly the first time out.

Ok, I have some more data, so I hope someone could send me in the right direction so I do not have to pull the dash apart only to find out this is not the problem.

1. So, the heater controls seem to work. If I adjust the temperature dial, I can get the heat to increase or decrease. Pressing the various air blower directional buttons 'seems' to work except that the center vents do not blow.

2. The AC clutch engages and with nothing pressed on the climate controls, the the upper AC pipe gets cold, but no cold air in the cabin.

That is about all that I know right now. Does this give anybody a heads up to what to replace/repair next?

Thanks
Doug

Did you put the compressor wiring connections back the way the were? When the system is in the off position (last button on the CCU), the fans and compressor should be off.

Did you check the mono valve? It takes 12 volts to close the valve. This block coolant flow from the engine through the heater core.

You need to take pressure readings, post them along with the ambient temp at the time, and we will have something to diagnose from.

Ok, got some gauges hooked up, cheap ones, but... At about 80 degree ambient.... I was getting 90lbs pressure on both the high and the low side. Any ideas?

Doug

Roll down the windows,
Turn the AC on HIGH blower,
Set the temperature on MAX COLD.
have a fan directed into the radiator / condenser front....
have the engine at high idle...

It sounds like your compressor has not engaged yet...when it does there will be some kind of difference on the two sides...low and high sides of the system...

so momentarily jump it and watch the inside front of the compressor clutch...to see if the actual compressor is rotating... although you should also feel it engage if you are standing there..

With all ac work.. wear goggles...

Let it stabilize about 10 minutes and then tell us the temp shown on the center vent pencil thermometer you have installed there...later you will want to check that temp at low blower speed just for reference...

Let us know what is happening... pressure wise on the gauges...

Hot Air In AC
 

Since you said the high pressure pipe is cold but there's hot air coming out the vents , the most likely culprit is the monovalve , this controls the flow of hot water through the heater matrix and fails often ~ it's easy to repair , four flat headed screws and it slides out in your hand , no need to even drain the coolant . look for a tear in the rubber diaphram or test it electrically or sometimes there's accumulated crud & corrosion inside the well once you've removed the coil & plunger assy .

Lastly , turn the AC on and go lok under the hood , if the front of the AC compressor is a blur it is working fine , post that so you won't get so many random , incorrect answers .

Your failure here is Uber cmmon and 95 % of the time , it's that damned monovalve .

The compressor is not engaged. Find and fix that problem, then you can move to the next step.

Ok, here is the A/C update.

1. Replaced the monovalve.

2. After re-soldering all the connections in the climate control unit, if I click on the AC, I get 12volts at the clutch (well the wires (brown and blue/green that I cut and put connectors) and the clutch seems to engage - starts spinning. However, that pipe coming out of the compressor now, never seems to get cold any longer and since I have 90psi at the high and low side, I am taking that to mean I might have a bad compressor now. Anything else I could check before jumping in and taking on a ~$350 repair... $275 compressor, ~ $25 drier, ~$25 expansion valve, ~ $15 high low switch .

Ok.. just be sure you are looking at the interior section of the front of the compressor to see that it is spinning.... if it is and you still have no difference between the high and low something IS suspect...
However , to have absolutely no difference between the sides with the compressor actually engaged is very unusual.... not Enough difference on a worn out compressor is one thing...but absolutely no difference makes me suspect some other variable is in play and if you do not do more research may cost you a lot of money ... even perhaps without fixing your problem... hate to see you waste money and effort without just cause...
The type two and three AC systems by MB are really too complicated..
Notice that I own a TYPE ONE..... It seems to me that RLeo may have switched one of his cars back to the simple valve on the firewall my car has... but even that still leaves some messy controls on the panel which I refuse to troubleshoot... I have the MB AC manual on those... something like 80 pages of diagnosing procedure JUST for the dash push buttons on the vertical wheel system....

Ok, anybody have any guess to what 'may be suspect?' I might have a few psi difference between the high and low like 87 and 90 psi - the gauges I have are not accurate 'top of the line'.

It is a closed system right? I mean basically, you power it on and that front pipe should get cold even if the interior is overwhelmed by a malfunctioning mono-valve and the heater core. There is not function I am missing that closes the system - except when there is a fault right? So with the front metal pipe not getting cold, I either have a block in the system or the compressor is not compressing. (I think I do have R134 in the system.)

So if anybody has a next course of action let me know. I am leaning toward replacing the drier and both its switches (high/low and temp) and the expansion valve. In the process, blow out with an air compressor all hoses and the condenser and evaporator. Put it all back to together and see what happens. If it still doesn't operate, put on a new Compressor.

How many cans of R134 does a system like this take?

Doug

Doug

If your compressor is indeed engaging and turning, then either the compressor is completely shot or the expansion valve is WIDE open. Neither of these scenarios are likely, but things happen.

Or both service valves are open on the guages, causing hi-lo pressure to mix.

Happy Motoring, Mark