HVAC / Monovalve diagnosis - 1987 300D - PeachParts Mercedes-Benz Forum

renaissanceman · #1 01-20-2015, 03:51 PM

Hello again, folks!

I finally got around to doing a little diagnosis on my HVAC woes. I have an "always hot" condition. I turn the wheel to heat, I get heat. On a cold day with a hot engine, I put the fan control on auto, and turn the wheel to say 22C. The heat comes on full blast, and as it approaches 22C, the blower slows almost to a stop. Then it passes 22C, and the CCU tries to cool, but HOT air continues, causing runaway with full blower speed. The temperature sensor grill on the sunroof switch area pulls air.

That led me to believe that the CCU is working fine, but the monovalve is stuck open. So I did a little DMM diagnositcs:

With the CCU on full heat, the monovalve has no voltage across it.

With the CCU on full cool, the monovalve has 11.71V across it. The air still blows HOT.

(but the center vents blow air, so my vacuum motors seem intact! Woohoo!)

How should I proceed from here? Jump battery voltage to the solenoid and see if it clicks (or have a helper turn the control wheel from hot to cold?)

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Also, I glanced at the A/C compressor, and the clutch was not engaged (as I suspected). I pulled the connectors from (what I assume is) the pressure switch, and it was open.

Seems that the system is not pressurized or the switch has failed causing no clutch engagement.

It's an R12 system, but I have seen several R12 replacement refrigerants that are affordable (hydrocarbon based -- Isobutane/propane, I think).
ie:
http://www.amazon.com/Cans-R12a-Refrigerant-Freon-Replacement/dp/B00DJDYMEA/ref=pd_sim_auto_5?ie=UTF8&refRID=0BFWFQP1GZFX4Y2BHNYQ

or

Enviro-Safe Refrigerants

I'd like to keep it R12 to keep the efficiency up for the hotter than hell CA summers I'm probably moving back to at some point. Should I test other things (common leak points, etc?), or just gas it up with a drop in replacement refrigerant and some leak dye?

Thanks!

JamesDean · #2 01-20-2015, 04:08 PM

A few comments:

1) The CCU switches the ground on the monovalve. When the monovalve diaphragm tears you will get full heat always. I wonder if this has happened to you?

2) I use ES-12A stuff in my 124 and 201. It works well at speed but at idle, in standstill traffic not so well. Its cheap too. Before doing anything though I would grab a set of gauges and see what the system pressure is it.

renaissanceman · #3 01-20-2015, 11:10 PM

1.) Thanks, That's helpful. So I should be able to read +12v from either pin when the CCU is requesting hot water, and then read ground on one pin and +12v when the CCU is requesting the valve to close.

Do you happen to know what the resistance across the coil should be? assuming it's not open or shorted, should I then disconnect the wire that grounds through the CCU and apply my own ground to check solenoid function?

2.) I have a R134a manifold set and vacuum pump from Harbor Freight (I know, POS Chinese junk...), I may just get the fittings that are used in the conversion of R12 to R134a to simplify the matter. I'm the only one that will be servicing this AC. Unless there is a better option for the DIY guy.

renaissanceman · #4 01-23-2015, 02:27 AM

Went back out to the garage, and found my hood liner laying on the engine.

The monovalve solenoid had 5 ohms of resistance.

funola · #5 01-23-2015, 08:46 AM

"With the CCU on full cool, the monovalve has 11.71V across it. The air still blows HOT."

Take the monovalve apart to get your answer.

JamesDean · #6 01-23-2015, 08:48 AM

I was doing some reading the other day and evidently the 124 does not have a monovalve in the same sense that the W126 does. The monovalve in the 124 is called the heater valve and does not have a replaceable insert kit like the W126's do.

w124 heater - monovalve - how it works

Quote:

The controller in many Mercedes cars of this vintage uses Pulse-width Control Modulation (PCM) to control how much hot coolant is allowed to enter the heater core. That is, the electronic circuit pulses the coil in the monovalve so that the valve is open for a small amount of time and then closed. The ratio between the "open" time and the "closed" time determines how much hot coolant gets into the heater core and thus how warm the air is that blows out of the climate control vents.

One of these days, I need to put an oscilloscope on the monovalve and measure the frequency of the pulsations. Can't be more than a few Hertz since the monovalve has considerable inertia.

Jeremy

W124 Mono Valve replacment

W124 1991 300D Mono Valve - Repairable ?

funola · #7 01-23-2015, 09:37 AM

Quote:

Originally Posted by JamesDean

I was doing some reading the other day and evidently the 124 does not have a monovalve in the same sense that the W126 does. The monovalve in the 124 is called the heater valve and does not have a replaceable insert kit like the W126's do.

w124 heater - monovalve - how it works

W124 Mono Valve replacment

W124 1991 300D Mono Valve - Repairable ?

Never touched a W124 monovalve myself and maybe for the W124 Mercedes went to PWM (pulse width modulation) control, but the myth that the W123 monovalve is PWM has been debunked.

JamesDean · #8 01-23-2015, 09:58 AM

Quote:

Originally Posted by funola

Never touched a W124 monovalve myself and maybe for the W124 Mercedes went to PWM (pulse width modulation) control, but the myth that the W123 monovalve is PWM has been debunked.

I've never analyzed either so I really can't comment. My understanding--from what I've read on here--is that the temperature regulator unit in the W123/W126/R107 system switches ground at different rates based on the heat demand etc.

I was not aware that there was a myth/controversy over the older style monovalve system?

renaissanceman · #9 01-23-2015, 12:39 PM

Yeah, eventually it'll get removed and such. For now I was trying to isolate the problem to the controls side or the valve side. Controls seem fine, now I want to isolate electrical vs mechanical on he valve. All signs point to the electrical being fine so far.

If it's PWM, might it just hold the valve partially open against the spring rather than cycling it?

It's low on my list, since it's winter and I have other more pressing repairs (vac pump, belt drive rebuild, glow plugs, etc, etc). I guess I'm a multitasker. And now I get to do a hood liner to save my beautiful original bronze paint.

funola · #10 01-24-2015, 02:06 PM

Quote:

Originally Posted by JamesDean

I've never analyzed either so I really can't comment. My understanding--from what I've read on here--is that the temperature regulator unit in the W123/W126/R107 system switches ground at different rates based on the heat demand etc.

I was not aware that there was a myth/controversy over the older style monovalve system?

Light bulb is connected in parallel with the monovalve terminals. Does that look like PWM?

CLICK TO PLAY VIDEO
Monovalve Duty Cycle Light Video by funola | Photobucket