I’ve taken my 1985 300D Turbo to three different shops over this. My heater, when heat is demanded, throws out a great blast of hot air for 15-20 seconds, then abruptly switches to cold. So far, I have taken it to three shops. They replaced the mono valve (repair kit), climate control module, heater valve, and a relay...no change. I am taking it back in to the latest shop next week.
I’m not a mechanic by any means, but I did study a bit of A/C many moons ago. Armed with that, I did a bit of thinking about the problem. Here is what my puny little brain came up with.
Since heated air does come out, if only briefly, the blower is ok.
Since the temperature goes from hot to cold so abruptly, there must be some sort of damper in the ducting. Since there is automatic climate control, there must be some sort of thermostat to control the delivery of air to the cabin. That is likely the problem.
My question (assuming my little brain has it right) is: where is the cabin thermostat and can it be replaced?
Thank you in advance.

There is no damper in the ductwork. Heating is controlled by the monovalve. It opens and closes to allow coolant into the heater core. The heater core is ALWAYS exposed to the air stream, it's temperature is controlled by coolant flowing through it.

The definitive way to test the monovalve is to simply unplug it. It energizes to CLOSE, not to open. If you unplug the monovalve, you should have full heat 100% of the time.

If you don't, the monovalve cartridge is bad (you didn't say if they used genuine MB or aftermarket), or the heater core is clogged. It's really that simple. If they used a crappy aftermarket cartridge in the monovalve, shell out the $35 or so it takes to get an OEM one from the dealership or from this forum's sponsor and change it out. It's 4 screws and very easily DIY.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

+1 on the cheap monovalve. lotsa scuttlebutt amongst the pros about the same problem. rumor has it the oe valve is again available from the dealer. good luck, chuck.

As he said, unplug monovalve and run it to see what happens. Also, as stated, I've yet to see a non MB monovalve work for any length of time.

I got a shop set up to repair these cars. Unfortunately it is about 2 hours to Soperton from Tybee but let me know if I can help.

__________________
Jim

Unplugged the mono valve...still blasts out cold air. Will order new mono valve (good one) and change it out myself. Still, if the fail safe position is full on heat, there must be something else wrong.
HELP!

The aftermarket valves are reputed to have a spring that's weak enough that the engine coolant pump can suck them closed. Considering the relatively weak flow through the heater core, that's a pretty wimpy spring.

Another possibility is a failed or clogged auxiliary water pump if your vehicle has one, though you should still get heat even if it is seized.

Start with the Monovalve cartridge. It is the most common cause of no-heat and one of the cheapest parts you can replace. DO NOT waste money on the aftermarket brands. Go with OEM or don't bother.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Also, put your hand on the tubing after the mono valve and before the heater core to see if it's cold. If that section is hot then there could be a clog at the heater core.

__________________

1990 190E 3.0L

Moral of this story is to use MB parts. It was easy to do myself and works great. Thanks for advice.