W123 Heater Shutoff valve addition
 

Hey guys. I've been away from the forums for a while, I finished up replacing my windshield this winter, and drove the car for a little bit. I have been spending my time doing woodworking and woodturning lately.

One nagging problem I have had with my 85 300D is the monovalve. My air conditioning system has been rebuilt and is working properly, except sometimes my heater will overpower it. As near as I can tell, the issue is that the monovalve is getting forced open when the engine is at temperature. If i start the car cold, and run the AC for a while it works great. If I then stop, shut off and start the car while the engine is still at temp, then drive off I will frequently get hot air from the vents (not ambient temperature, hot as in the heater is running). This also happens after it heats up and I turn off the AC then turn it back on, or if I use anything other than max cool (which cycles the monovalve). I am 99% sure this is not an electrical issue, but is related to either water pressure or thermal expansion in the cooling system.

So, besides trying another monovalve (currently have a Bosch core - I think - with an MTC solenoid, which i have messed around with repeatedly, my current thought is to install a valve inline with the heater core that I can control manually- close the valve during the summer to ensure I have heat.

My questions are: who else has tried this, how well did it work, what size and type of valve did you use, and where did you get it?

Thanks!

You could install a manual shut-off valve. Many have done so in many U.S. cars, indeed I recall photos and PN's on a Mopar site I frequent (forabodiesonly.com). Your heater hoses are std 5/8"D, at least that is what I used when I removed the "aux water pump". I used silicone hoses which stretches more to fit the unequal sides.
I am dealing w/ a similar "too hot" issue in my 1985 300D. On the "max cool" click setting, I also get AC fine. That tells me that my monovalve works fine (your too presumably) and the problem is in the control loop. I measured the resistance to ground at the temp sensor input wire to the TemperaturRegular box (above glove-box) and found it open. I installed another sensor (behind push-button box, to left of fiber spider) from junkyard, but still "too hot". It read ~12 kohm at room temp and dropped to 8 kohm blowing on it, which is about right. But, the old one checked out too (recall it showed open when I tested it alone prior), so problem might be in my wiring. Anyway, gives you ideas how to proceed. It isn't at the top of my current priorities.

I have a ball valve on mine, and also no aux pump. I would never go back, at least not with what's currently available, which is rubbish. Gets HOT in my car now (and I can shut it off completely in the summer).

I forget the size, but I got the valve at either Home Depot or Ace.

-Rog

I add a ball valve to all my cars that don't have a valve from the factory.

Get a ball valve for 3/8" threaded pipe and a 6" ish long piece of 3/8" black steel pipe. Cut the pipe in half , debur the ends and thread it into the valve. 3/8" threaded pipe is slightly larger than 5/8", so 5/8" hose will stretch over and 3/4 will clamp down.

Install the valve between where the hot water leaves the engine and it enters the heater core. Some OE systems shut off the water as it leaves the heater core so poke around a bit to make sure.

I did learn recently that the 123 has the monovalve on the downstream side. I bet it would work even better (especially for A/C purposes) to put the manual valve on the other side, but mine seems to work fine. I don't have A/C, so you may want to experiment.

-Rog

Why not use a manual heater control valve from say a 240D? If you don't want the manual HVAC panel, you can put a couple throttle control wires someplace inconspicuous like the glove box.