Hello all,

New poster here, but lurker for months. Have Google Fu'd until my Googler was sore, including on these forums, for an answer to this question, which I decided to post here.

So, I've inherited my father's old '81 300D, that he "hybridized" into an '84 with a turbo engine and trans swap. Don't believe that's relevant to my question, but, my father, now at 90, says he should have left well enough alone, as the original '81 really didn't have any problems. Oh well, left for me to sort out, I guess.

Anyway, what I want to know is this. Even though this car has the ACCII, or second version of the wunderbar Automatic (read "$HITE") Climate Control, can the monovalve be simply bypassed??? Yes, I've read seemingly a thousand threads on the fact that, yes, it can be bypassed, IF you have the older ACC system with a servo, but I've never seen a thread or post anywhere that describes how you could do so on the newer models with ACCII. And yeah...I know..."swap the monovalve!"..."will cure all your ills on the newer system!"---I call Horse_hit to that, because I just swapped out the monovalve with a new MTC one. Here's what I got for that repair---still an "oozing" of radiated heat coming from both the side and center ducts, even with the "Off" button depressed. And...for added bonus...heat that would rival the Gates of Hell, no matter which "On" button is depressed, and this is with the Temp control in the "Min" position! I tried going all the way to the "Max" position, then reversing to "Min" to see if it would somehow reset...umm, no. So, yeah...that's where I'm at.

What I'd like to do is this; bypass the $%^$%g monovalve, and either put a Pex ball valve in to stop the hot water flow into the heater core...OR (better yet), simply put a manual heater control with Bowden cable in the cabin. What I don't know is how this will affect the A/C, which is why this question has become of more importance for me. I have a good local mechanic willing to replace the crapped-out R4 compressor, etc. to get the A/C to work, but...if the damned monovalve is calling for "SUPER HEAT FROM LUCIFER'S DUNGEON", he may have a hard time getting it to work.

Basically, I'd like to shut off the heat to the heater core and hope that the A/C will work with the Temp dial in the "Min" position. Oh, and for the record, I've replaced that goofy-assed foam pipe thingamajig above the glovebox with a piece of recommended PEX, so I don't think that's causing this silly issue with the temperature.

My father used to cuss this Climate Control up one side and down the other when I was a kid; now I know why.

I wouldn't use a PEX valve, a brass one would be much better. A 3/8 pipe will hold a 5/8 hose a bit tight and 3/4 hose will squeeze down with a clamp. I'm not sure what metric size you have but it is bound to be close.

I do this on every other car I have that doesn't have a heater water valve. I've even done it in car that have a heater valve. The default position for an electric or vacuum controlled automotive HVAC is hot defrost. When the car is off the valve opens letting hot water in to the heater core, when you get in before everything cools off, you are now greeted with hot air until things cool down again.

Look through a good automotive HVAC catalog ( 4seasons is one ) . You can even get an inline cable controlled unit, hook it to a choke cable and have in car manual heat control. This type of valve was found on 70's Fords , AMC. Some Japanese cars used them in to the 80's like a 86 ish Mazda RX-7.

You can shut down the line at the monovalve with a valve. With the auxiliary water pump disconnected and a valve to shut the circulation of the coolant , your AC will have no issues of working. Except you will no longer be able to control the temperature. I agree a brass valve would work best and if you can control the coolant flow in the cabin , it would be helpful.

Btw- ACC-II is the servo style, ACC-III is what you have.

A 3/4" pex brass ball valve will be perfect Shop Apollo 3/4-in dia Brass PEX Ball Valve Crimp Fitting at Lowes.com

It is short in profile and the diameter will be perfect to splice into the W123 heater hose.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Thanks, kind folks! I think that pretty much settles what I will do later this week. Today, my nephew will help me with...installing a new steering damper, replacing some slightly leaking fuel hoses, and replacing a leaking brake booster O-ring, the rubber buttons on the Master Cylinder and the seals on the bottom of it. Plus, bleed and fill the brakes.

Say, is there anyone that can show me a pic of where exactly to put the brass valve (or perhaps the inline cable controlled unit)? I will take a pic of what my ride's "plumbing" looks like, and perhaps one of you good people can tell me where its location would be best served.

Appreciate the awesome replies; feel like I've won the lottery joining this forum! Lots of info here, for sure!

Pic 1---Monovalve from passenger side
Pic 2---Monovalve and other Heater plumbing from front of car
Pic 3---Closeup of Monovalve
Pic 4---Heater Plumbing from driver side
Pic 5---What exactly is this...the auxiliary water pump (if not, where is it)? This is on the passenger side, under the air cleaner, near the fender

So, there ya go. Here's what I'll do for everyone; once complete, I'll take pictures and post what I did. Likely do the quick-and-dirty Lowe's Pex brass ball valve fix first (thanks funola!), then do the controllable fix later as per 97 SL320's suggestion.

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So...bought the Pex valve at Lowe's, and I think, after looking around on the Interwebs, I've figured out where to put the Pex valve for maximum effectiveness.

If you see in picture 1 (found via Google search), the hose noted as #4 would be the same as the location where the person in picture 2 (also from a Google search) placed their Pex valve. That location would also be the same as what I've noted in Picture 2 from my previous post, which has a hose snaking next to the top of the oil filter housing. Basically, I can either cut my existing hose and jam in the Pex, or (better idea) get a same size hose cut to size, then cut it in the middle and insert Mr. Pex Valve. Probably much better for clearance and potential vibration on top of the oil filter housing otherwise.

Finally, from this post's picture 1, it appears the reason the Aux Water Pump in the upper left hand corner would be best disconnected is that no water could be circulated from that side of the engine (to the monovalve or otherwise) ergo totally eliminating hot water from entering the heater core. In picture 1, there are 3 entry points (left, middle and right) to the heater core---the Pex valve takes care of one of the items, the Aux Water Pump disconnect the other two, if I'm understanding the heat flow correctly.

This is how I intend on tackling the issue. Before I do, I'll let others "check my work" on here before I document this. Thanks, everyone!

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The stock system is a bit strange, as I learned recently that the mono valve is on the downstream side of the heater core. I put my ball valve where the auxiliary pump used to be (and deleted the mono valve of course), thinking that this was on the upstream side. As a result, there does seem to still be some heat getting to the heater core even when the valve is off. Seems like you would want it on the upstream side, unless there's a compelling reason not to.

I'm also curious as to why the stock system didn't seem to have any heat soaking into the core when the flow was off. Doesn't really seem fundamentally different.

-Rog

I would put it in place of the monovalve if the hoses will mate up without kinking. This way you do not cut any hose.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Of course, the problem is there, if you look closely at the monovalve pics, the hoses are really coming at the monovalve in an L shape, if you will. Think it would be quite hard to match the Pex valve up with those hoses without seriously bending the existing hoses out of whack. Ya'd almost have to use some type of L adapter to make the straight connection for the Pex valve.

And, interesting what Rogerviler said as well; I think his issue is because of what I mentioned in my last post. He covered two angles of "entry" for hot water into the core, but not all 3. Makes me also think that the "ultimate" hot water shutoff setup would be 1)simply unplug the Aux Water Pump, 2)Place Pex valve at existing monovalve location, and 3)Place 2nd Pex valve at hose 4 that I mentioned in my earlier post. My view is that all of that could easily be reversed come winter (ball valves opened, aux valve plugged back in).

I may not get to this tomorrow, but for sure in a couple of days I'm going to try my method, and simply remove the #4 hose for a replacement piece I can cut up from any local auto parts store. Will most certainly post my results. What I don't know about is what size the hose will need to be; will play that card as it comes off the deck.

My 97 SL320 has the mono valve at the exit of the heater core. I ended up putting a ball valve on the inlet side to prevent hot water from making it's way into the heater while the engine is off. ( Thermo syphon )

I would not block both ends with a ball valve as I would not want the heater core to be a closed system and either over pressure in the heat or collapse when things cooled off.

Remove the monovalve and both twisty hoses off the pipes, buy a short length of straight 5/8 i.d. heater hose to install the ball valve. Simple! When you find another monovalve, you'll still have the hoses to install it.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

So, yeah...the first try at this...wasted an hour of my time, lost lots of coolant and...reattached the original hose (explanation below).

1. In picture 1 below, you see the original hose that I was trying to "adapt" to, it's the one snaking over the oil filter housing.

2. So I first take off that hose at the firewall. Success! Very little coolant loss. Good, right? Umm...

3. Getting at the hose clamp near the engine...well...let's just say there's a lot of things in the way there. But, I was finally able to loosen it. That said, it was a bugger to pull off, but when I did...the floodgates opened. I just let the thing ooze its coolant life away on my garage floor (and thought of the song "Mack the Knife" while it was happening).

4. Well, I decided, can't quit now, let's get crackin' with the new hose. I bought both 5/8" and 3/4" (2 feet worth) at a local auto parts place, and the Pex valve per suggested by funola. And then found out that...

5. Neither size really worked---the 5/8" I couldn't finagle on the engine and firewall barbs; the 3/4" was loose. I surmised that IF I tightened down the 3/4" with the hose clamps, it MIGHT work, so...off to the next part.

6. Well, rats. The 3/4" ball valve snaked nice and tight...down the 5/8" hose, but was rather loose with the 3/4" hose. So what...I'll tighten 'er down with the hose clamps! and...

7. Found the issue perplexing me in Picture 2 below. There was no real way to bend the hose without kinking it; I'm also not real comfortable with loose-fitting hoses that I've found in steps 5 and 6, so...I decide it's 4th and 12 from my own 2---time to punt from the back of the end zone and give up by...

8. reinstalling the original hose and refilling the radiator plastic tank with 50/50 lifeblood. Good news is, no leaks upon returning the car to it's original design.


So...not sure how to tackle this, or if I'm even going to again. Looks to me like you need some kind of "funky type" hose; it needs to bend, but can't be too big a hose/ball valve combo, else it will get tangled with the throttle linkage, or contact the hood from the underside. Further, I have no idea exactly what size that original hose is; must be metric, something between 5/8" and 3/4". After I've done this, I read funola's post about doing this work on the monovalve side of things and what to use---perhaps I'll try this. Or just wait until my mechanic tries to fix the A/C only to find the "super heat" issue I'm dealing with.

What I thought was going to be easy has really frustrated me; after all, the guy in the "WTF?" picture did it, I just don't know how.

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Discussed the situation today with my mechanic; he's basically going to use a "pinch off" method to figure out where the majority of the heat is coming from, and where best to put some kind of controllable valve with Bowden cable.

What I have noticed is this; when switching on the ACC, when the car is not up to operating temp, the air coming out of various ductwork is ambient (normal or cool, if you will). Once the car reaches operating temperature, THEN you get the Gates of Hell effect coming from the ductwork. Perhaps there's a clue hidden in there. My repair is Friday; will report back with photos when complete.

I did this on my w123 (fixed the monovalve later) and my w116 while I still had it.

I put the ball valve in place of the monovalve on the grey w123 in an emergency (I was in 40 degree weather and the heat stopped), and in place of the evil servo on the w116 when it decided to pour all my coolant out...

worked great heat was either on or off. tempted to do this on my troublesome monovalve on the blue car.

I'd use these:
https://smile.amazon.com/SharkBite-UC134LFA-Threaded-Adapter-4-Inch/dp/B007AHF9VQ/ref=sr_1_1?ie=UTF8&qid=1470277714&sr=8-1&keywords=3%2F4+pex+fitting
https://smile.amazon.com/Everflow-PXML3434-NL-4-Inch-Brass-90-Degree/dp/B00N8YZHQ2/ref=sr_1_9?ie=UTF8&qid=1470277762&sr=8-9&keywords=3%2F4+pex+fitting+90
https://smile.amazon.com/Mueller-107-824NL-Threaded-Forged-Packing/dp/B002WEG3IU/ref=sr_1_1?ie=UTF8&qid=1470277780&sr=8-1&keywords=3%2F4+ball+valve

If I could find 1/2 NPT stuff instead I'd use that, but I didn't see any.

__________________
1982 300D (w123, "Grey Car")
1982 300D (w123, "Blue Car")
2001 Ford F150 "Clifford" (The Big Red Truck)
1997 Dodge Ram 2500 12V Cummins
1996 Dodge Ram 2500 12V Cummins
Previous Vehicles:
1995 E300D, 1980 300SD, 1992 Buick Century, 2005 Saturn Ion