AC works then off and HOT
 

It's HOT in Georgia - somebody please help.
Car: 1982 300D Turbodiesel 156000 miles
Problem: AC works pretty good half the time. The other half it blows from the "Gates of Hell".

I suspect the Monovalve is not closing. It was replaced with a new one last year and I didn't have this problem. I checked the fuse and the Monovalve connection this evening. The fuse is OK. The Monovalve connection seems tight and is not corroded. The connection was cleaned up when I replaced the Monovalve.

Example: I went to lunch at work today - about a 10 mile I-75 drive at 70 MPH. AC worked fine. I left the restaurant and all I get is brutally hot air. I turn everything off for a minute or two then back on - still very hot. Close the vents and open the windows for the rest of the ride. Four hours later it's 90+ degrees outside. I leave for home and it blows nothing but "dragon breath" for the 25 mile trip. I could cook things at the vent. I was pretty cooked myself.

The car had extensive AC work before I bought it in 1/2008. I saw the receipts ($1600) but don't have them to know the specifics. Based on the performance, I think it was converted to r134a and the components I can see look new. Last year it would blow air at the vents about 25-30 degrees cooler than the outside air. This was adequate. It does the same this year when it works. I''m glad they put windows in these cars.

I gotta take two teenage boys on a road trip in about 9 days and I'm gonna need earplugs and a baseball bat if the AC doesn't work.

If anybody has any suggestions, I would appreciate a post.

Mark

Check the fuse first. I cannot recall which fuse controls the monovalve. It may have a hair line crack. It happened to me once. It is a white fuse for sure.

It is likely the monovalve. You need to get 12V to it when AC is called for. I once hardwired a 12v to it until I found out the hairline crack in the fuse.

If it does not solve it then it is something else.

Good luck.

I had similar symptoms and it seemed like the AC had a mind of it's own. Resoldering the CCU fixed the problem. I assume it was not sending the 12V to the monovalve part of the time when it should have.

Did you read the post above yours from yesterday? same problem it appears. Turns out to be a corroded/ cracked fuse The post title is "300D heat always on"

okay you checked the fuse, but did you check the MONOVALVE? Simply take the four screws out of the top and see if it is flooded above the diaphragm. If it is, get a kit and replace the diaphragm.

What scenario involving hot air would not involve the monovalve?

There should be system voltage at the monovalve regardless of whether a/c is "called for" or not, provided that the key switch is in the "run" or "start" position.

More AC work
 

I worked on the AC problem last night and believe I have the problem narrowed down.

I replaced the fuse. The monovalve was replaced last fall. I checked the dash temperatuire sensor and foam hose. Everything looked good there but I didn't use a meter on the temp sensor. I had everything back together before I thought about it (doh!).

I started the car and observed the following behavior for about 45 minutes.

With the CCU temp wheel in the full cold (past the click) position, I get twelve volts at the monovalve and cold air at the vents. When I adjust to full cold but automatic temp control (other side of the click), I get warm air at the vents that eventually becomes very hot. As expected, I do not have 12 volts to the monovalve when this is occurring. Moving the temp wheel back to full cold past the click gets cold air after the heat has dissipated and I have 12 volts at the monovalve. I repeated the adjustments above at 10 minute intervals and got consistent behavior.

It would seem that some component in the automatic temperature control feedback loop is not working and this is allowing the monovalve to remain open providing generous amounts of scorching heat.

What should I do next? What components are involved in the feedback loop that controls 12 volts to the monovalve? What are common failures?

Thanks,
Mark

Which fuse, exactly?

Voltage to the monovalve is not controlled by the CCU. You should have system voltage at the monovalve any time the key switch is in the "run" or "start' position. The monovalve is modulated by switching the ground side of the circuit as controlled by the solid state temperature control module (except in "off," "max heat" and "defrost," where the ground is controlled by the pushbutton control.)

Plugging or clamping a heater hose should provide immediate relief.

A bad solder joint in the CCU would deserve a top spot on the list of probable suspects.

14
 

I replaced Fuse 14 - the lower one on the right.

As to the monovalve. If I understand correctly, ground is switched in response to the CCU.

I believe I will follow the DIY instructions for CCU re-solder this weekend and see if that helps.

Meanwhile, the AC seems to work OK at full cold past the click.

Is there a simple way provide continuous 12 v to the monovalve?

Thanks,
Mark

There are other scenarios. Burnt fuse is one. CCU not putting out 12V is another. The monovalve controls the water flow but the problem leading to the monovalve not in the right position can be caused by other factors. I suppose one can start from the monovalve and work backwards.

The monovalve voltage is 12V for A/C, 0V and anything in between is for heat when the key switch is in the "run" or "start" position. I do not know what 'System Voltage' :confused: you meant. The 300SDL service manual does not have that term.

Hardwire a 12V to the monovalve. NOT from the battery but from a fuse which is ON when car is running, like the window fuse or others. You only want it to energize when car is running. I went to a junk yard and got the same connector plug, measured the polarity of the plug and wire it to a window fuse. Remove it during winter. I did this for a short while until I realize I had a hairline crack with the fuse.

It seems your A/C is working now as the voltage at the monovalve is what is should be at cold/hot thumb wheel position. The only issue is that does not work in auto position ( fan switch ). It is glass half full and is good enough for me personally. I do not have access to the A/C manual, it is a separate manual, and may be other members can help.

I suspect it may be the temperature sensor inside the car is giving false reading to the CCU or the CCU itself is faulty. Try re-solder all the dry joints and see how it goes.

No. You might benefit from a little study on the concept of a "switched ground." If you look at an electrical schematic for the subject vehicle, you will see (or should see) that the monovalve is hardwired to fuse 14, which is powered in "run" and "start," regardless of which climate control mode is selected.

The electrical system does not operate at 12 volts.

That happens to be precisely how the vehicle was originally wired.

I am only trying to help a member with what I know about MBZ A/C and my experience. I am not an electronic or A/C whizkid and never claim to be.

This is what I know and I have successfully fixed all my A/C problems in my 300SDLs and 300D, and a soon-to-be 190D ( 5 sp ).

1) When it blows hot air, a lot of time is the fuse, monovalve, CCU, in this order.
2) MBZ cars operate on 12V battery, if it does not then I must be missing something. It might have electronics to modulate it to different voltage rails but it is 12V source nonetheless.
3) I measure the voltage across terminals, the differential voltage. How it arrives there by 'switched ground', voltage modulation, etc is not that important to me. It would be nice to know but for DIY like myself, it is good enough.

Given that voltage is applied via fuse 14 when in run or start, would providing an unswitched ground wire to the body at the monovalve connection accomplish closing the monovalve when the car is running?

It seems too simple...

Thanks, Mark