A/C in W123 blows cold ONLY when temp set to full cold

[tgantos]

Hi all you benzers!
I have read all the A/C related threads before posting just in case this was a FAQ, cause I hate when folks are lazy. Anyway....

W123 1985 300D turbo sedan (Uhh, it's white, Ray)
A/C converted to 134a correctly with flush, replaced parts, new aux coolant pump, new aux fan, brand new compressor, etc. Worked ok last summer, but now the system acts really weird after recharging. Only blows cold when the temp dial is set to full cold click-stop. I have a second climate control pushbutton unit to test with, same problem. When I pull it off the clickstop it blows hot air. Replaced monovalve plunger, brand new temp control unit, nada.

The linkage on the center pod did fall off so I have a freely moving flapping center vent flap. Have not tested the vacuum system completely, but am going to pull the dash to replace all the diaphragms, center flap linkage, and clean out the evaporator core for it's 20 year anniversary. I will test the vacuum lines then, but it's probably not a vacuum problem is it? Could it have something to do with the temp sensor in the center of the dash going ka-fluey? I was also going to test that with a my multimeter, the manual says its a thermistor, different temps are supposed to give different resistance values. I appreciate any ideas you might have on this. I do plan to take complete photos of the dash removal to illustrate the procedure for all those who will follow in my footsteps, and will have BC post that for all somewhere, as I could not find a good illustrated dash removal procedure.

I am also considering replacing the (original) evaporator valve when I pull the dash, as I have heard it can be problematic in the 134 switchover, plus gets crud in it.

***SHAMELESS PLUG***
My A/C mechanic is a genius by the way, I was really lucky to have found him, a 3rd generation auto A/C specialist, his grandfather started the shop he owns/runs in Flint, Michigan, Colberg Radiator. Consistently undercharges me, even after scolding me for putting in HC 134 one time, only nicked me $30 to evacuate and refill the system!
***END OF SHAMELESS PLUG***

Thanks to everyone for all the excellent info sharing that goes on at this board! Often dealers are clueless on the older vehicles. Great resource for old Benz owners. Too bad MBZ.ORG is offline, but it might be back someday Rusty tells me... :-(

Regards,

Theo

klakka klakka klakka klakka...Arf arf arf...
The dog always knows when Theo's home!

[engatwork]

I am having the exact same issue with a W123 that I am replacing the evap in. Initially, the fan would stop blowing when I turned if off of min but I changed a blower regulator out and resolved that. I'm wondering if it could be an issue with the monovalve.

[tgantos]

If it was the monovalve, then wouldn't it keep blowing hot? I thought when the monovalve dies failure mode results in full on heat? Oh yeah, come to think of it, doesn't the schematic show when A/C is in full cold detent then the heater core airflow is cut off? That might make sense then. Isn't the monovalve just a solenoid with a plunger on the bottom of it? Should be easy to trouble shoot and repair.
Yeah I never swap out a $140 part if I can fix a component. I was really proud of myself the other day, I fixed a stereo remote control with a bad switch in only an hour, probably could have bought a new one for $5. Oh well... Too much time on my hands this summer I guess.

Regards,

Theo

klakka klakka klakka klakka...Arf arf arf...
The dog always knows when Theo's home!

[Brian Carlton]

I've been chasing some issues on the SDL lately and came across two sensors that feed the PBU:

The evap temp sensor

The cabin temp sensor

Now, what is interesting is that I always assumed that the evap temp sensor was the device that regulates the compressor. If the evap temp gets too cold, the PBU dumps the compressor. Makes sense.

However, the evap temp sensor was found to have a resistance of 2.2K at ambient temperature without the system running. This is far below the specs of 5.5K for this temperature. So, clearly, the evap temp sensor is not regulating the compressor, otherwise the a/c would never work correctly with this much of a deviation in the resistance in the thermistor.

So, this leaves the cabin temp sensor. The SDL regulates the cabin temp with near perfection as compared to the temperature wheel. I'll bet it's within five degrees. So, I conclude that this sensor is working properly.

This leaves the conclusion that it is far more likely that the cabin temperature sensor is regulating the compressor, and, if faulty, it will dump the compressor if it believes the cabin is too cold.

While I have no data to make a conclusion with regard to this, maybe this anecdotal evidence will help you fellows determine the problem. I'd be real curious to see if the cabin temperature sensor cures this issue.

Of course, I am assuming that you have "ambient" air blowing when the temp wheel is off the stop. If it is truly "hot" air, meaning air above ambient, then all of the above won't help at all and you have a monovalve issue of some type.

Have you verified the monovalve is actually closing when the AC should be on? If you run the AC and unplug the connector on the top of the monovalve it will open. If you then reconnect the plug you should hear/feel it click shut. Make sure the electical connections on the valve are clean.

Also, is the AC compressor clutch still engaged when the AC should be on, and is the top AC hose cold? That may help tell you if the AC is working and just being overpowered by the heat.

[Brian Carlton]

Originally Posted by Craig

Also, is the AC compressor clutch still engaged when the AC should be on, and is the top AC hose cold? That may help tell you if the AC is working and just being overpowered by the heat.

That's an excellent idea. Just roll the wheel off the stop and take a look at the compressor. If it is still spinning, then the problem is not the a/c system. However, if the compressor stops, then seek out and test the cabin temperature sensor.

[tgantos]

OK, I'll test the following things as some of you suggested and add to my list of things I was already going to try, then report back to you in the thread.

1. Test resistance of Evap Temp switch at ambient temp (with system off) and when running cold.

2. Test resistance of Cabin temp sensor at ambient temp. Try to see if it varies with temperature. If not, try swapping in different resistor values to see if I can get the temp mix to change. Book says just a few hundred ohms, I have several resistors in that range handy to test with.

3. Check to see if the compressor still runs when the temp wheel is backed off the full-on detent position.

4a. Test monovalve plunger function by removing plug while system is on. Hey that won't overload the monovalve control circuit in the pushbutton control will it? That monovalve pulls a lot of juice. I heard someone say just fooling with the aux pump power connection can blow circuit board traces in the control unit.

4b. Test power output to monovalve when in cooling and heating modes and see if it's within specs. Test disconnected monovalve for shorts/opens. Scream loudly so the all neighbors can hear if I have to buy a new one right after springing $40 for a new plunger.

Additional question: extra credit to help out a poor pre-med student
Anyone know of any way to rebuild a monovalve coil? Or do I have to stand up and pay for a new one just to get the darned coil? My #$%^&! plunger's brand new. I'd be great working for Mr. Scott on the Enterprise, no warp drive parts stores in the neutral zone. "I'll rig something up but I dunno if it'll hold for long, Captain". The mythical legendary engineer, that guy could fix anything with nothing, anywhere, in no time. It always seemed like it was in the nick of time. Oh well, that's the difference between fiction and fact.

Regards,

Theo

klakka klakka klakka klakka...Arf arf arf...
The darned dog always knows when Theo's home!

Originally Posted by tgantos

4a. Test monovalve plunger function by removing plug while system is on. Hey that won't overload the monovalve control circuit in the pushbutton control will it? That monovalve pulls a lot of juice. I heard someone say just fooling with the aux pump power connection can blow circuit board traces in the control unit.

I can't make any promises that unplugging and plugging in the monovalve can't damage the control unit, those things seem to self-destruct without much help. All I can tell you is that I've cycled mine by unplugging and plugging it without any apparent damage. I've also cycled my monovalve by putting a 12V jumper on it from the battery without any problems. Your mileage may vary.

If you don't want to mess with the connector with the system on, you could have someone adjust the temp controller from heat to AC while you listen to and/or feel the monovalve. It should open for heat and close for AC. If the valve does not seem to be changing position (clicking) correctly you can check the voltage between the two contacts in the connector. You should see voltage to the valve for AC (valve closed) and no voltage for heat (valve open). The point of this exercise is to verify that the valve is actually closed when the AC is operating. If the valve is staying open the heater can overpower the AC and blow hot air. There is a post here someplace where someone replaced the monovalve then found out it was just a dirty connector causing it to stay open.

Good luck

[engatwork]

valve is actually closed when the AC is operating

If we are getting cold are with the temp wheel set to min then I would surmise that the monovalve is closing off.

I'm going to look into the temp sensor good and see if I can tell what is going on with it.

[Brian Carlton]

Originally Posted by tgantos

Additional question: extra credit to help out a poor pre-med student
Anyone know of any way to rebuild a monovalve coil? Or do I have to stand up and pay for a new one just to get the darned coil?

........well, you wouldn't be standing up and paying for a new one. It would more likely be bending over for a new one.

I'm not sure if anyone has rebuilt the coil. If the wiring has broken, it would be a difficult task. However, these can readily be located in salvage yards. Someone on the forum might even have one. Put a post over in parts. Just state what you need.

[Johnhef]

Unless I missed something, since this is an 85, has anyone suggested swapping out the klima relay? I have a spare, but I forgot what was wrong with the car it came out of.

On the other hand, I had a similar problem in winter with my 82, only blowing heat on full clicked position. My problem was the climate unit itself. I still have no auto speeds between high and low but I believe that should be the regulator box.

Anyone know which box is which? Black or Silver? I may swap them out this weekend.

[Brian Carlton]

Originally Posted by Johnhef

Unless I missed something, since this is an 85, has anyone suggested swapping out the klima relay? I have a spare, but I forgot what was wrong with the car it came out of.

On the other hand, I had a similar problem in winter with my 82, only blowing heat on full clicked position. My problem was the climate unit itself. I still have no auto speeds between high and low but I believe that should be the regulator box.

Anyone know which box is which? Black or Silver? I may swap them out this weekend.

If the Klima is bad, you won't get the compressor to engage, independent of what you do with the PBU. This is also true if any of the signals received by the Klima are not to spec. A perfectly good Klima will fail to start the compressor if all input signals are not proper.

It's definitely a PBU situation. Might be the PBU itself. Might be one of the thermistors that feed the PBU. I am presuming the monovalve is OK.

[engatwork]

In reviewing the schematics it looks like I will start out by checking the resistance values in the in car temp sensor and heater air temp sensor. I suspect one of these circuits is causing it. I'll let you know what I find.

[engatwork]

Looks like the temp senser that is located behind the center dash vents is bad. I checked it this afternoon and it is very, very flaky. The only way to get a reading on it is to push on the terminals hard (and then it read 8 Megohms). Otherwise it is an open circuit, which the way I understand the schematic, should make it open up the monovalve when the temp thumbwheel is anywhere but at minimum setting. It will be getting replaced.

[tgantos]

I got a little sidetracked this weekend helping the wife with gardening projects, so I didn't get a chance to check mine, but I will check it tomorrow and report back.