Hope all had a good Christmas.

I am almost positive that this subject had been posted before but I am sorry I can’t find the same exact problem I can only find similar issues.

The Patient is a 1993 400 SEL W140

Environment is Chicago, very cold this time of year.

Symptoms.

Heat only works when out side temperature is above freezing 35 or so. If it is 35 or so out side no problem.
Heat will work at colder temperature after driving car for about 2 hours. The blower will just come on, nothing to move, nothing to shake, no bumps in the road.
If temperature is above freezing heat will work as soon as you start the car.
You will feel hot air from the vents when blower not working.
Rear vents always blowing cold air when heat is working.

Fuses all fine, I turned the ignition switch in every possible way to check that. I know the blower works.

Any help will be greatly appreciated.

The first thing to check is engine temp, must be at least 180F (80C). If that is good then probably the heater valve is stuck closed. "DUO" style behind the left headlamp under plastic cover. ONLY replacement on this car is a new valve assembly. Part number is 001-830-14-84 ($436.)

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

The temperature almost never goes to 80C in this cold weather. Even after driving for over an hour. At least that’s what the temp. gage says. The gage is always a bit below the 80 mark often half way between 40 and 80.

Start with a new thermostat! Engine needs to be at 80C for heater to preform properly.

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

Thanks

Even though I doubt this is the case, but I see the relation.

For example today I driven the car back from work, traffic was a nightmare and the temp shot past 80 and the heater still did not come on. And last night I started the car (also driving back from work) the heat started fine all the way home, I parked the car, went back out after 20 minutes to go grab some stuff from the near by store and guess what it did not work.

It's hunted I tell you.

I am going to replace the thermostat since it is an easy fix and worst-case scenario it will not hurt.


Thanks a million, I will be sure to post the solution once this one is for the books

If the engine is at 80C & you still don't get heat take a look at the FIRST post that I made!! as we replace heater (duo) valves on these cars all of the time!
NO reason to replace t-stat if the engine gets that hot.

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

Mr. M.B. Doc, thanks for all your help.

I finally got some time to work on this.

That valve you mentioned,

What does it look like?
Hoses, wires coming out of it?
How Can I tell if it is open or stuck closed?
Can that valve assembly be diagnosed?
Sorry "DUO" ? I am lost

The part you mentioned 001-830-14-84 ($436.) I can’t find on FastLane, I can find

R3022-88891 ACC Mono Valve $436.00

For that year, and that model, I think it just may be the difference in wording.
Sorry to be such a pain but, I am sure you understand, that a part that costs that much money and probably not returnable, I don’t want to replace and find out later that it is not the one. I just want to be sure.


Thanks

Mono heater valves are for single heater core control. "DUO" heater valves are for cars with 2 controls, like your car. Also the valve IS under the hood beneath the plastic cover that is just behind the left headlight. IF the heater valve is stuck in a partially closed position testing WON'T reveal any problems.

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

To help clear things up, call the 1-888 number in the partsShop. We have Phill on the phone to help.

If you are standing in front of the car with the hood up, the duo-valve is located to the right side of the engine under the plastic cover...above and behind the light assy. It has rubber hoses hooked to it and two wires. Also it is made of plastic and has two metal round things on top.

We are not saying 100% that it is the cause, but it is where to start.

__________________
Benzmac:
Donnie Drummonds
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS

Don't know what it is, I have just changed the valve and no heat still.

Any other ideas?

Thanks for all who helped.

__________________
1993 400SEL (W140) 281K
1999 S500 (W140) 163K (Sold)
2002 S500 (W220) 72K
1999 S500 (W140) 88K
You cannot simultaneously prevent and prepare for war.
Albert Einstein (1879 - 1955)

__________________
1993 400SEL (W140) 281K
1999 S500 (W140) 163K (Sold)
2002 S500 (W220) 72K
1999 S500 (W140) 88K
You cannot simultaneously prevent and prepare for war.
Albert Einstein (1879 - 1955)

Solved, it was the ignition switch.

Let here be heat

__________________
1993 400SEL (W140) 281K
1999 S500 (W140) 163K (Sold)
2002 S500 (W220) 72K
1999 S500 (W140) 88K
You cannot simultaneously prevent and prepare for war.
Albert Einstein (1879 - 1955)