Failing mono valve?

[Dan O.]

Nothing more frustrating than when you ACC system decides you're gonna get both hot and cold air. Things usually start off fine, after initial start up I'll get hot air out of the side vents for a short time then all cold air. If I make a stop and restart a short time later I'll get blast furnace heat out the side vents and what seems to be hot and cold out of the center (weird, right?); sometimes things will go back to normal but most times lately not.

I did test the air sampler and that's working, I also swapped in a spare push-button console because I had one - no change. I'm guessing most of the problem here is a failing mono valve, I have yet to verify whether or not it's seeing voltage though. I have the late production valve so the replacement is going to be pricey, before I bite the bullet I want to be sure. Is there anything else I should be looking at? Has anybody ever had one of the late production valves apart?

 

[gerryvz]

All US-spec 036 models should have the early style 124 mono valve design with separate mono valve and aux coolant pump assemblies. They are located in front of and slightly behind the CAN box.

Are you sure you have the late-style "integrated" valve/pump assembly?

If so, then I did a HOW-TO on how to replace the late style integrated assembly. This could also largely be applied to the early style, though part of it is a PITA to get to around the CAN box.

Can you confirm / post pics of your setup? My 1994 E500 still has the separate (early style) components, whereby my 1995 E320 wagon had the integrated assembly, hence the HOW-TO.


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[gerryvz]

All that said, yes most often when the mono valve fails, you will either get all heat all the time (most common) or no heat any of the time.


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[Dan O.]

My bad, talking '94 E420, posted here thinking the vin-split situation was the same as the E500. I have the 4 pipe valve:

http://c1552172.r72.cf0.rackcdn.com/376019_x600.jpg

 

[gerryvz]

OK yes, like the .036 that is the early style.

 

[samiam44]

I have experience in this- my a/c got colder when the mono valve was replaced. I would say it was leaking and not "on". Vent temps were ~ 50-55 and dropped to 40-44 F with a new mono valve. From my experience the EPC is incorrect on the VIN break. You need to see if your mono valve has 3 or 4 ports. Later are 4 ports and are EXPENSIVE new.

I use a Hazet flexible hose clamp tool to remove the clamps- it's not hard. May have used a long screw driver for one.



Michael

 

[Jim]

I have a 3 port duo-valve on my face-lifted car.

This video is helpful...

https://www.youtube.com/watch?v=8jhPEfLI45s

There are 3 electrical connector pins on the valve. Centre pin to outer pin read about 12.5 Ohm. Across the two outer pins I saw 25 Ohms.

I applied 12 V from centre to each outer pin and the valve moved. Conclusion defective control system.

Putting a meter in the plug I saw 12 V to one valve and not the other. That told me that the drivers side (temperature wheel?) was the problem.

I think 0V gives valve open which gives you heat. I bridged the two outer pins so that the passenger temperature wheel worked both valves as a temporary repair until I have to go in to the dash for something else. So cold wheel setting gives 12v and the valve(s) close.

It would be good if someone can confirm my commentary above...just in case I got it wrong.

 

[clarkz71]

This is a "duty cycle" type of valve, it uses pulse voltage to control coolant
during automatic settings off minimum to just before max heat.

Max heat there is no ground, min temp there is full ground

The very early style valves (pre V8) may have used the on/off valve like the W126

These are considered a wearing part. Either try a known good valve, or just replace it

There's also a possibility the pushbutton CCU is defective,
that sends the ground signal to the mono valve.

The other pin is + 12v all the time

So put the A/C on full minimum temp and check for 12 volts between the 2 pins of the plug

 

[clarkz71]

Here's the info from WIS

So you should be testing for ground from the pushbutton at the mono valve plug

Putting 12v to the wrong pin can fry the CCU pushbutton, just as bridging wires is
not a good idea.

The ground comes from the pushbutton left connector, pin 9 yellow/green wire


.

 

[clarkz71]

Just thought of something else, have you checked the ACC codes?

Pin 16 at the 38 pin diagnostic connector

Code 31 is a mono valve code

 

[gerryvz]

I know when my monovalve went bad in the E320 wagon, there were no codes thrown. It's kind of like coil pack codes ... sometimes it will throw one, other times not.

 

[clarkz71]

I know when my monovalve went bad in the E320 wagon, there were no codes thrown. It's kind of like coil pack codes ... sometimes it will throw one, other times not.

True, that's why the power & ground test is first, but the sensor chain can cause issues as well
if your not on full minimum max A/C

 

[Jim]

Thanks Clark...

I have no push buttons...

OK so the later push button Tempmatic System used Pulse Width Modulation to control the duo-valve to partially open positions and hence control temperature with greater accuracy?

I assumed the earlier 2 x temp wheel approach was simply cycling the mono valves on or off depending on a thermostat (temp wheel) setting.

Jim

 

[Klink]

Thanks Clark...

I have no push buttons...

OK so the later push button Tempmatic System used Pulse Width Modulation to control the duo-valve to partially open positions and hence control temperature with greater accuracy?

I assumed the earlier 2 x temp wheel approach was simply cycling the mono valves on or off depending on a thermostat (temp wheel) setting.

Jim

There is no "partial" position in this valve control system, or any 124 heater valve system that I know of. When correctly functioning the valve is fully open or closed. The circuit is closed (as is the valve) by switching the negative side of the valve solenoid circuit. When the system is in a temperature regulation mode, the valve will be alternately be pulsed open and closed at clearly audible and tactile intervals varying over several seconds time. The most common fault is the valve mechanically sticking open or closed. This electric circuit and valve solenoid is almost always without fault. Only when this circuit is faulty, as in open or shorted, will the pushbutton unit produce a fault code.

 

[Dan O.]

I apologize for disappearing but I did successfully rebuild the mono valve while I was away. Although the "full heat" with a/c problem hasn't returned I'm not entirely convinced it was being caused by the mono valve, however I did find some interesting things and confirmed my suspicions that the later 4 port valve can be rebuilt with soft parts from the earlier 3 port valve. Most importantly, it wasn't necessary to shell out upwards of $300 to get things back to normal; I have a ton of pics that I'll get around to uploading as soon as I find some time. In the interim here's a link to a really exciting video I made during the rebuild process:

https://www.youtube.com/watch?v=Ra9bkFqWHCQ

 

[QuintBenz]

Heater/Mono Valve leaked & no longer functions (probably shorted).
-Heat from dashboard vents whenever car is driven.-
CCU disconnected at present.

Question:
What is the harm to the: M119 engine, wiring harness, other parts
driving the car this way ?