Members,

Patient is my recently-acquired 85 300D, 210,000 miles, retro'd to 134a. At 6pm this evening I was tooling down the road and the AC was blowing cold on high fan speed; at 6:01pm it changed to hot hot hot at high fan speed. Popped the hood when I got home: clutch hub engaged, the tubes that were supposed to be cold, were cold to the touch. I'm no pro on this, but my first guess would be some flap quit in the evap/heater case, ducting the hot heater core air into all four dash vents. Tomorrow I'll check the usual suspects: fuses that might be toast and vac hoses that may have fallen off, cracked, etc.

Does the 85 have a split or independent case? Anyone have a link to a diagram? My few 123 manuals are mum on this.

All comments welcome. I'm just starting to tackle the AC aspect of my love affair with MB old girls.

Thank you.

PS. For all you viewers who are not yet members...what's holding you back? Step on up!

FRANKNBENZ
85 300D 210,000 miles (the patient)
85 300D 270,000 miles
94 E320T 170,000 miles

try checking your monovalve before you take anything else apart.

__________________
Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
---------------------
Section 609 MVAC Certified
---------------------
"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

It is boringly normal to have the monovalve fail like this.

__________________
ASE Master Mechanic
asemastermechanic@juno.com

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D
1984 190D
2003 Volvo V70
2002 Honda Civic

https://www.boldegoist.com/

Thanks for the quick reply Doc and Hunter.

Replaced the monovalve guts in January when then no heat. I pulled the valve just now and checked everything on the bench: diaphram, seals, spring action, C-clip OK; bumped it with 12V DC and got extension. Installed it back on the car and then I checked the monovalve connectors across my cheapie dc volt meter (set to 50V scale): 39 volts with AC off; 35 volts AC on.

For grins (and not really knowing what the heck I'm doing) I tried running the AC with the monovalve connection off the unit. Unfortunately it still blows hot hot hot all four dash vents. Low side tube (crosses back over to the compressor side in front of the valve cover) is cold and sweaty (except I just now stripped the crud insulation and put on a new wrap).

Any new ideas? Thanks.

Still sweating to the oldies...

FrankNBenz

I've not been into the W123 deep enough to be sure...wondering if this can be a symptom of the also common cracked solder joints in the ACC control module?

__________________
Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
---------------------
Section 609 MVAC Certified
---------------------
"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

Default for the monovalve (unplugged) is full heat.

Try blocking off the center hose to the heater core at the firewall (padded jaw vise-grips), and see what happens.

__________________
The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)

To All Members Responding: THANK YOU!

Here's the whole deal:
1. Car blew cold, then hot.
2. Members pointed to monovalve.
3. Monovalve bench tested OK; power in electrical leads tested OK, but...
4. FrankNBenz finally gets his head out of his tailpipe: leads OK and valve OK, HOW ABOUT THE CRUD ON THE CONNECTING POSTS?!?!

Again, just for grins, I took my trusty $3.95 can of Contact and Circuitboard Cleaner (no smoking, vent please!) and gave everything a squirt, waited, probed the plug with a pipe cleaner, and wiped the valve terminals with a rough clean cloth. CONTACT! Blowing cold again!!! Thank you!

So, for the rest of you novices out there -- like me -- the big three first thing to do in my diagnosics book:

a. Check the fuse box, find the slot, and, even if it looks OK, replace whatever is in there with a new fuse.
b. Check for loose and cracked vacuum hoses, connections, etc. (get vacuum service kit and the vacuum diagnostics book from ************** -- I think).
c. When all else fails, clean the contacts.

That last one saved me big bucks several times with the switches on the horizontal plane on the counsol (4 way, windows, radio balance). As each stopped working, I pulled them out, disassembled on a clean paper towel, shot first with air (PC air can) then hit the heavy crud with the electrical contact cleaner, waited till dry, then used a hard eraser to polish and abrade the contact surfaces, dry air again, reassembled. Or buy new ones, of course.

Thanks to all members who set me on the right path. Hope my little $3.95 input helps someone out next time. (The stuff is very flamable; please use with caution in a vented area).

Till my next "emergency"... Thanks again.

FrankNBenz
85 300D 210,000
85 300D 270,000
94 E320 Wagon 170,000

no power to mono valve = full heat.
That was an easy fix.
Very glad you are happy.