Need help diagnosing Mono Valve & A/C (84 300TD) - PeachParts Mercedes-Benz Forum

byronnash · #1 06-10-2011, 09:23 AM

My A/C is blowing only hot air, as it does if I were to turn the dial to heat.

Here is a list of what I know or have checked:

New compressor and dryer bottle this time last year
compressor does come on when I turn on the air
mono valve membrane seems intact, but there was some slight surface corrosion on the metal washers
aux fan is not working, I will take it apart this weekend to attempt a repair
not sure about the freon level (R134a)

I will try to get a photo of the mono valve up soon. What other items should I check out?

Dee8go · #2 06-10-2011, 09:49 AM

Yeah, I think the monovalve is one of the first things you need to check. I have the same problem and am assuming that is the most likely culprit.

byronnash · #3 06-10-2011, 10:10 AM

What do I need to look for other than torn membrane?

Jeremy5848 · #4 06-10-2011, 11:10 AM

Try pinching the hose near the monovalve (either side) to mechanically interrupt the flow of hot coolant. Use a clamp and a couple of pieces of soft wood or pad the jaws of a Vice-Grip® with tubing and use them as a clamp. If the problem goes away, replace the monovalve (I believe the rebuilt kit is NLA).

Jeremy

toomany MBZ · #5 06-10-2011, 04:12 PM

You can simply remove it and inspect. It's very high up in the system, no need to worry about losing coolant.

vstech · #6 06-10-2011, 04:20 PM

I'll be on south side this weekend, I'm going to convert a car to 12...
gimme a call.

Yak · #7 06-10-2011, 04:23 PM

Put a voltmeter on it and see if it's getting +12. It'll stay open if it doesn't. Then doublecheck that the fittings are snug onto the pins so that the solenoid is actually receiving power. If the rubber's intact, and there's +12 going to it, then I'd recommend the pinched hose test along with a temp probe at your vents. If the temp goes down with the coolant blocked, then something is preventing the solenoid from closing and still allowing coolant into the heater core. If the temp stays is close to ambient at the vents with/without the clamp on the line then it's probably working okay.

It may just be damn hot out and your A/C isn't working so you're getting uncooled hot ambient air.

byronnash · #8 06-10-2011, 04:38 PM

I took out the mono valve last night and it looked OK to me. I'll check the voltage and do the pinch test over the weekend. The air is definitely heated and not just recirculated air. I had to use it in the rain the other day and was about to have a stroke from the heat. The outside temp was probably in the 80's that day.

vstech, I'll call you but I won't be around most of Saturday. I did find out that the oil I have is compatible with R134a or Freeze12 but not regular R12. That's a bit of a bummer.

byronnash · #9 06-13-2011, 09:20 AM

I was able to do a little troubleshooting this weekend. Here's what I found.

Aux fan is definitely shot. The bearings are shrapnel and I can't get it to turn when connected to the battery directly
Pinching the hose near the monovalve didn't seem to change anything. Maybe I didn't get it pinched all the way?
Was not able to determine if the mono valve is getting any voltage. I'm not sure exactly where to test so I tried some different things and got nothing
No matter what button I hit on the CCU, it seems to just want to blow from the defrost and the vent next to the drivers window
Fuses seem to be OK

vstech · #10 06-13-2011, 09:56 AM

missed your call, sorry.
the CCU when bad often defaults to Full heat, Defrost settings.
pull yours out and see if it's got classic or excessive overheated/burned board on it.
I've got several good ones if yours is toast, and I can show you how to solder it if it's just weak.

leathermang · #11 06-13-2011, 10:14 AM

Quote:

Originally Posted by vstech

I'll be on south side this weekend, I'm going to convert a car to 12...l.

UH... where is the video posted ?

byronnash · #12 06-13-2011, 10:15 AM

NO problem. I just looked up how to apply voltage to the monovalve to test. I'll try that tonight just to make sure that's not the culprit. After some more reading, I'm also wondering if the vacuum issues aren't an issue as well.

piccolovic · #13 06-13-2011, 12:00 PM

I have the same problem. In fact, I have had to vacuum stuff behind the dash replaced when I got my car back in '96, as none of the vents worked. I have had the monovalve checked several times . . . but here is what happens to mine:

I had it recharged with Freeze-12 (with a new compressor and Dryer) and it was working . . . so-so. If I put on the A/C when the car was cold, it worked great, but if I shut down the car, and got back into it within an hour, it would out hot/heated/humid air. So, I think we may in the same situation. I will be watching this thread closely, as I do not know if it's vacuum issues (door-locks are weak with apparent leak) or if it's the mono-valve (although I was told my model didn't have a mono-vavle . . . and am ready to bi-pass the valve altogether for yr. round except for January (as I live in Florida).

I may just have to bite the bullet and take it across the state to Pierre to get the issues resolved, along with the PM and minor other work.

Torie