ruling out (or in) monovalve - PeachParts Mercedes-Benz Forum

kuene · #1 01-06-2020, 06:04 PM

1982 300D (Turbo).

Trying to do some initial diagnosis. In past few days I've experienced getting heat at all settings and out of my vents - center vents included.

During my ownership of around 4 years my heat has alway worked well and as designed. My AC has been working (completed a partial system rebuild/refresh in summer 18 - including compressor, drier, sensors, O-rings, evap-temp sensor, expansion valve, etc).

Temperatures outside are in 30s. The other day, I was kinda hot so I fiddle with my ACC and tried to turn on the EC mode just to get some average ambient air - and I felt the strong heat coming out of center vents. Turning to regular heat/ac mode and putting temp dial to min(cold) didn't help. I'm thinking monovalve?

The next day, I investigated things a bit while parked (so not at speed). ACC unit seems to work, all the vac pods do what they should be doing, fan speeds correspond. I checked the fuses, they are good. I checked the wire connection at top of monovalve - looked good. During this test, I was able to get ambient air (not heat) to come out of the center vents when I set the ACC to EC. So then I'm thinking maybe not monovalve?

Next day I'm driving around, and I check out my various ACC settings - it's back to full heat from center vents and all others.

I clearly need to do more investigation, but on the surface does this seem like monovalve related problem? Can monovalves fail intermittently or once they go are they gone? Does this sound more like an ACC unit (bad solder) thing?

Thanks for any initial direction pointing....
-kuene

Usaguy · #2 01-06-2020, 06:54 PM

here's how the monovalve works:

when there is no voltage going to the monovalve then its open and it puts out full heat.

when there is voltage then the monovalve is supposed to close.

next time you heater is on full when your selector wheel is on min (cold) remove the plug on the monovalve and check for voltage at the plug - that will tell you whether it's a bad acc or a sticking monovalve

kuene · #3 01-08-2020, 11:12 AM

ok - so in that scenario - if I'm getting full heat at min temp setting (and out of center vents) if I do find that there is voltage going to the monovalve, that would mean the monovalve is not properly closing. If there is no voltage going to the monovalve, it means the ACC unit is not functioning properly - at least in terms of providing power to monovalve.

Diseasel300 · #4 01-08-2020, 12:07 PM

The Monovalve is controlled by the silver box behind the glove box, not the ACC pushbutton unit. The pushbutton unit only tells the silver box what mode it's in and gives a reference resistance from the temperature wheel. There are a few relays on the circuit board that are switched from the silver box. Typically a fault in the silver box shows up as no heat when switched to Min, but full heat in all other positions of the temp wheel regardless of mode button pressed on the ACC pushbutton unit.

The mono valve should be checked for voltage with the plug connected to it, or you can use a test light in the terminals. The switching is done on the ground side, so it is possible to read a phantom voltage with a digital multimeter that doesn't actually exist. The mono valve is energized to close, so if voltage is present, the valve should be closed.

kuene · #5 01-08-2020, 12:26 PM

i'm getting heat even when temp wheel switched to min. (for now). What is the procedure for testing with the pug connected to monovalve? I'm fairly inexperienced with my multimeter -but i'm good at following instructions/learning.

is the silver box behind glove box the "Evap Temp Sensor" thing that has a probe inserted into the evap box? Or something else?

Diseasel300 · #6 01-08-2020, 12:50 PM

Set the multimeter to the 20V DC scale and slightly lift the plug on the mono valve so you can see the silver pins. Slip the probes in and touch them to the pins. You should read voltage if it is switched on. You can also feel/hear it clunk if you plug and unplug it if you have power.

A simpler method is to use a 12V light and plug the leads into the plug. If the light is on, the valve should be engaged.

The silver box is a control box behind the glove box. It's marked "Tempregler" or something similar to that.

kuene · #7 01-08-2020, 02:24 PM

ah, ok - it's just a matter of getting the probes in without fully unplugging - ty!

vwnate1 · #8 01-10-2020, 11:32 AM

To discern 100 % if the Monovalve works or not, unplug it and connect two wires to it directly to the battery and go for a drive .

If you get heat the valve is kaput .

If you get no heat the vale is fine, look else where .

Someone posted a good photo of the silver aluminum box behind the glove box, take it and have the cruise control guy replace the caps in it (forget "programma", they're the bunk) and your Klima II should begin working as it should again .

BillGrissom · #9 01-10-2020, 06:20 PM

The aluminum box is termed "TempRegular" = "Temperature Regulator". It is above the glove-box. Screwed to the same plate is a black plastic box which has ~6 relays to control blower speed. They switch in different resistors which are in the engine bay (passenger side). Another component is the cabin air temperature sensor which is under the tiny black vent in the top of the dash pad. A large rubber hose sucks air thru that vent past it, into the suction side of the blower wheel, so follow that hose w/ your hand to feel the sensor which can be unplugged. I recall it is a thermistor (solid-state component, resistance varies w/ temperature).

ykobayashi · #10 01-11-2020, 09:25 AM

I had both a leaky monovalve and a messed up silver box on my 300d. The leaky monovalve gave me heat when I didn’t want it. I removed the valve and found it was torn. Later the heater would sometimes go full heat when I wanted cool for no apparent reason. I measured my voltage on the valve and it was zero meaning the valve was being called to open even though I had not commanded it to.

I touched up the solder in the silver box and replaced some transistors and caps. I also retouched the solder in the pushbutton unit. It was better but now it had a different problem. The fan could not turn on when I pressed EC.

I got frustrated and bought programa silver box and a programa pushbutton unit. Ouch. It was like $400. Fan still wouldn’t come on.

Turned out the wire on the thermostat mounted switch had broken.
Blower not turning on with EC button CCU/Programa works with DEF SOLVED

Hope this helps. Though I’m kind of mad to have fixed it in the most expensive way possible, I’m really happy with a fully functioning system. I’m happy with programma for now but I’ve been warned of their durability.

kuene · #11 01-13-2020, 12:51 PM

thanks ykoayashi - I will def investigate that sensor now!

well, I've yet to pull my monovalve for inspection or tested the voltage to it - but I was able to do more road testing this weekend with the unseasonably warm temps for NJ. While I'm still getting seemingly random (from short trip to trip observations) heat from center vents when on EC, one symptom/behavior is clear and consistent. I'm not getting AC at all. My compressor will get turned on and sounds fine - but the line in the engine bay isn't even getting slightly cool or sweaty. There are also some bubbles in the sight glass but I can't honestly say they haven't been there when it is working (I'm currently running r134a). So I don't know if this means that in addition to my probs with erratic heat, that my systems is undercharged, but now I have that to sort out. I thought that when I first set it all up that my compressor wouldn't even turn on if the system was too low on refrigerant given that there is the pressure switch on the receiver drier (both new).

going to check monovalve tonight....

Diseasel300 · #12 01-13-2020, 02:59 PM

Conditioned air runs through the heater core and A/C evaporator at the same time, one before the other. If the heat is stuck on, it will overpower anything the A/C is trying to do - the result is hot air. Fix the mono valve issue and the A/C will probably work fine.

vwnate1 · #13 01-13-2020, 06:04 PM

Bubbles always means low charge but I'd suggest doing the positive test provided to eliminate it or not then move forward .

Re : the Aluminum "TempRegular" box above & behind the W123 glove box, I looked at it in two junkyard cars, didn't see how to remove it.......

I don't want to damage it, I want to buy a spare and have it re capped then install it and use my old box for re capping for my other Klima II equipped W123.....

TIA,

kuene · #14 01-13-2020, 06:08 PM

Yes, I do understand that the mono valve malfunction would cause the heat to overwhelmed the AC. Important to note that the heat is not stuck on ALL the time - and therefor I'm wondering if the monovalve problems can be random, working/not working from trip to trip? Or do they just stay broken when they break? I'm currently getting no AC even when I'm not having the heat-always-on problem.