W123 AC vs Heat: Monovalve Disconnect - PeachParts Mercedes-Benz Forum

Tanksowner · #1 06-13-2016, 12:45 PM

Hi all,

Haven't posted in a while, hope all of you are well!

I recently had my AC serviced on the '85 300D and it worked fine for a short drive around town yesterday. When I left my parents house for my drive home, whether I used Economy or AC, I only got max heat. I did try cycling the temperature wheel back and forth, as well as powering off the ACC then powering back on to no avail. The compressor is definitely engaging as I can feel the slightest hesitation in the car's power at speed. Thinking it was an electrical connection being faulty (these cars have their share of gremlins), I tried it again this morning and had the same problem. Thankfully my windows still work!

I've done my research and I believe the monovalve is the usual suspect here. However, I have some other questions while I source a replacement kit and the time to install it.

1.) If I were to disconnect the monovalve from the electrical system, would I be able to keep my in-dash ACC on MIN temperature setting and get AC or normal outside air on Economy?

2.) I've heard mixed reports on the auxillary water pump and I'm not convinced this is related, but I've heard disconnecting this might help. Can anyone confirm?

3.) There's only one type of monovalve repair kit available and it's not OEM. I'm positive that I used this brand several years ago to try and fix this issue, and it's looking like that it's resurfaced. Some have done a swap with a W124 monovalve housing since apparently that's a better design. Anyone accomplish this successfully and can attest to it?

Thanks in advance for all the advice and wisdom.

BWhitmore · #2 06-13-2016, 01:23 PM

The 1985 300D is the only w123 model to use a Klima relay to engage the compressor. You might want to visually look at the compressor to make sure it is engaging when the AC is on. The Klima relay is located on the top of the left fenderwell. Quite often the relay will overheat and shut down the compressor. If that happens remove the relay and cool it with a damp cloth and more often than not the compressor will start up. If this is not the problem, more than likely the problem is the monovalve. Clean and check the electrical terminals on the monovalve first, they often get corroded. Unfortunately most of the repair kits for monovalves are not very good and often the electromagnet in the valve goes bad that a kit will not repair. You can remove the top of the monovalve and check to see if the rubber diaphram is intact. If not, get a repair kit and give it a try. The auxillary water pump circulates water through the heater - however, if the pump goes bad it can cause a malfunction in your pushbutton dash controls. The aux pump should have an inline 2 amp fuse installed to prevent a problem with the pushbutton unit.

I would personally try to get a used monovalve from a wrecked car and give it a try.

BWhitmore · #3 06-13-2016, 01:23 PM

The 1985 300D is the only w123 model to use a Klima relay to engage the compressor. You might want to visually look at the compressor to make sure it is engaging when the AC is on. The Klima relay is located on the top of the left fenderwell. Quite often the relay will overheat and shut down the compressor. If that happens remove the relay and cool it with a damp cloth and more often than not the compressor will start up. If this is not the problem, more than likely the problem is the monovalve. Clean and check the electrical terminals on the monovalve first, they often get corroded. Unfortunately most of the repair kits for monovalves are not very good and often the electromagnet in the valve goes bad that a kit will not repair. You can remove the top of the monovalve and check to see if the rubber diaphram is intact. If not, get a repair kit and give it a try. The auxillary water pump circulates water through the heater - however, if the pump goes bad it can cause a malfunction in your pushbutton dash controls. The aux pump should have an inline 2 amp fuse installed to prevent a problem with the pushbutton unit.

I would personally try to get a used monovalve from a wrecked car and give it a try.

Demothen · #4 06-13-2016, 01:46 PM

Your monovalve needs to be connected, energized, and grounded to shut off water flow to the heater core to provide cool air. Disconnecting it is not going to help.

I don't think there are OE quality rebuild kits available anymore unless you can find a NOS one. If your existing monovalve is a Bosch unit, you can swap the diaphragm from the rebuild kit to the existing unit for the best results. If not, just swap the whole solenoid over. You might want to look for a good core in the junkyard and rebuild it.

I actually think that the aux water pump helps keep the monovalve from getting shut closed, but I can't guarantee it. A new one is fairly inexpensive. If the water pump seizes, it will draw excess current which has to be routed through the ACC control panel in the car. This can and will cause fires. I'd suggest buying a new one, and adding a 2 amp fuse at the pump. I made an adapter cable using a fuse holder and some wiring from a junkyard car, or you can just splice it in to the +12v side of the wires to the pump. I think 2amp will work fine, you could also undersize it and see if it fails. Don't oversize it though, then it's useless.

Also check for loose fuses in the panel. My monovalve failed open and my seatbelt warning system was not working because I knocked one loose in the panel while installing the cover by accident.

I'm curious if you are able to swap the w124 monovalve in. I thought I bought one a while back, but apparently i got the auxilliary water pump by mistake.

Tanksowner · #5 06-13-2016, 02:22 PM

Quote:

Originally Posted by Demothen

Your monovalve needs to be connected, energized, and grounded to shut off water flow to the heater core to provide cool air. Disconnecting it is not going to help.

I don't think there are OE quality rebuild kits available anymore unless you can find a NOS one. If your existing monovalve is a Bosch unit, you can swap the diaphragm from the rebuild kit to the existing unit for the best results. If not, just swap the whole solenoid over. You might want to look for a good core in the junkyard and rebuild it.

I actually think that the aux water pump helps keep the monovalve from getting shut closed, but I can't guarantee it. A new one is fairly inexpensive. If the water pump seizes, it will draw excess current which has to be routed through the ACC control panel in the car. This can and will cause fires. I'd suggest buying a new one, and adding a 2 amp fuse at the pump. I made an adapter cable using a fuse holder and some wiring from a junkyard car, or you can just splice it in to the +12v side of the wires to the pump. I think 2amp will work fine, you could also undersize it and see if it fails. Don't oversize it though, then it's useless.

Also check for loose fuses in the panel. My monovalve failed open and my seatbelt warning system was not working because I knocked one loose in the panel while installing the cover by accident.

I'm curious if you are able to swap the w124 monovalve in. I thought I bought one a while back, but apparently i got the auxilliary water pump by mistake.

Yeah I've done similar research and found there are a lot of crappy aftermarket repair kits out there. I suppose I could get one with the knowledge that I'll have to replace it in a few years, but that's not a fun prospect.

AWP is on my to-do list before the end of the year. Haven't had trouble with it before, and I currently have a 2amp fuse inline with it, got the kit off of a kind user on this forum. Perhaps it was you?

So basically, even if the temp setting is on minimum just to have outside air, the monovalve needs an electrical charge to close off water flow? I think I know the rationale behind this. Engineers wanted heat to be able to flow in case the valve failed so it wouldn't become a safety hazard with frosted windshield glass, right?

Demothen · #6 06-13-2016, 02:29 PM

Yeah. I think the diaphragms in the cheap kit are okay, but my understanding is that the solenoid doesn't do well at high temperatures, where it will expand and bind up. Mine had a tendency to bind closed at 55mph+ this winter, meaning I'd lose all heat. Let me tell you, a 5 hr drive at thanksgiving with no heat was not fun at all.

That wasn't me selling the kit, though I built something almost identical. (if you search for posts by me, I have a picture of what I built with some basic instructions on how to do so)

I think you're right, you'd want the failure state to provide heat to defrost the windshield.

Rogviler · #7 06-13-2016, 03:58 PM

I blasphemed and removed both the aux pump and the mono valve (the horror!), but seriously it gets hot enough to bake cookies now if I want. I just got tired of monkeying with the mono valve and it's going to be harder and harder to find good replacements. The aux pump just sounds like a death trap waiting to happen.

I use a heavy duty ball valve to turn the heat on or off, but I also considered an Audi/VW one, like this:

You'll still have to pop the hood to switch it (the horror!), but it's better than no heat or full heat when you don't want it.

-Rog

Tanksowner · #8 06-13-2016, 04:10 PM

Quote:

Originally Posted by Demothen

Yeah. I think the diaphragms in the cheap kit are okay, but my understanding is that the solenoid doesn't do well at high temperatures, where it will expand and bind up. Mine had a tendency to bind closed at 55mph+ this winter, meaning I'd lose all heat. Let me tell you, a 5 hr drive at thanksgiving with no heat was not fun at all.

That wasn't me selling the kit, though I built something almost identical. (if you search for posts by me, I have a picture of what I built with some basic instructions on how to do so)

I think you're right, you'd want the failure state to provide heat to defrost the windshield.

Yeah that does not sound fun. I love my W123, but I also wanna keep it and I'm not going to put it through a terrible commute to work and waste all the money I've spent to fix it. Plus, I'm only going to be driving it on clean spring & summer days going forward and I'm gonna get a new Civic later this year to assume the beater duties.

@Rogviler, that's definitely an interesting idea. I don't wanna monkey around with the system too hard since I'm a novice mechanic at best. I feel like a monovalve replacement job isn't too difficult, plus one would hope that someone steps forward to manufacture a higher quality replacement...

sleepstar · #9 06-13-2016, 07:42 PM

to me this sounds more like a problem with the temperature regulator behind the glovebox. i had a problem with full heat all the time and switching that unit out returned the system to normal operation. part # 0008221003

ROLLGUY · #10 06-13-2016, 09:43 PM

I suggest you rule out a bad monovalve by pinching the hose with a pair of long nosed vice grips with rubber hose on both jaws. If the heat is competing with A/C, the heat will win. Cut of the coolant flow to the heater core, and you should get cold air if your A/C is working correctly.

Demothen · #11 06-13-2016, 10:18 PM

If you have HOT air from the vents, the monovalve is open/leaking. If you pinch that line, regardless of what condition your valve is in, your heater will stop working. This doesn't really prove anything.

Before you even think about testing the AC, you have to fix/replace the monovalve. You can either make it work correctly, replace it with a mechanical valve, or pinch the lines. I wouldn't recommend pinching the lines long term.

If you want to make it work correctly, here are some tests I would run:

(You take all responsibility for this, use your own judgement, if you are not comfortable working witb electronics, contact a qualified technician, use proper safety gear, etc)

First, make sure all of your fuses are in good shape.

Next, hook up your multimeter (you DO have a multimeter right?) and check the voltage across the two pins of the monovalve with the AC turned ON and the temp set to max cool. If memory serves, the monovalve is provided with 12v at all times, and the ground is switched, but I am not sure about that without my diagrams handy. If you have 12V across the pins (not from one pin to ground) the monovalve is getting energized correctly (assuming you dont have a bad connection somewhere). If you do not have 12v across the pins with the AC system on and set to max cool, there is either a grounding or electrical problem somewhere else. (A test light would also work for this)

If your monovalve is being supplied 12v and ground, but not sealing either the diaphragm is bad or the solenoid is bad. You can check that the solenoid is working by energizing it. An easy way to do this is to make sure the car is shut off, then find a 12v source. I prefer to use a UPS backup battery, but your car battery will work as long as you are careful. Note which wire is brown on the connector, then unplug it. Connect ground (-) to that pin, and 12v (+) to the other, making SURE your wires dont contact each other or the other pins. I have several small jumper wire sets made with alligator clips and/or various other connectors and thin wires (speaker wire, scraps, etc). When you connect power and ground, you should hear a thump as the solenoid engages and will probably see sparks. Only connect the wires for a moment. If nothing happens, your solenoid is bad.

Finally, you can open up the monovalve to see if the diaphram is in good condition.

Now, once you have your monovalve working/replaced/blocked, you can test your AC system. There are many people here more qualified than me to help you with that.

If you determine that your monovalve is functional but not receiving power, let me know and I can try to help you troubleshoot that further.

BWhitmore · #12 06-13-2016, 10:51 PM

I have also had the symptoms described by the OP (mixed heat and cool air) when I had vacuum leaks in the system (1985 300D). Fixed the vacuum leak (under the hood vacuum leak) and AC was returned to normal cold air.

Tanksowner · #13 06-14-2016, 09:38 AM

Quick update,

So I ran Economy air this morning after getting under the hood last night. I disconnected/reconnected the monovalve, and wiggled some fuses/cleaned connectors. Perfect operation this morning with no heat on MIN setting. Now, of course, I'm remaining cautiously optimistic, since I'm pretty sure my car is home to a family of gremlins, and I fully expect the problem to resurface again later.

@sleepstar, I replaced the temperature sensor several years back after the previous one crumbled in my hand (messing with center vent vacuum pod), but it's possible this could be playing a role. The quality on available parts for these cars varies so widely that it's not unreasonable to think it needs to be replaced again.

My thought process would be to test the monovalve for failure points again. Might need to buy a new repair kit, but I'll take any small win I can get.

Demothen · #14 06-14-2016, 09:43 AM

Did you replace the tube, or the sensor itself? The sensor is rebuildable, just FYI - I can help you with that if you need to (look inside it, if there's a broken circuit board, it's broken)

Tanksowner · #15 06-14-2016, 09:55 AM

Quote:

Originally Posted by Demothen

Did you replace the tube, or the sensor itself? The sensor is rebuildable, just FYI - I can help you with that if you need to (look inside it, if there's a broken circuit board, it's broken)

Circuit board crumbled in my hand. It was very brittle from over 2 decades of heat, so I had to replace the whole thing. That's interesting to know that the sensor itself is fixable, I'll keep that in mind