Mercedes-Benz Forum banner
1 - 7 of 7 Posts

· Registered
1998 S320 W140 120K+
43 Posts
Discussion Starter · #1 ·
Hi Everyone!
My W140 had a bad week.
1. Climate control started only blowing out of the defroster and a little out of the side vents. This happened before and it was the Evap Temp Sensor. I was lazy and it is winter so I was going to do the Evap Sensor at Thanksgiving.
2. Climate Control will only blow COLD air!!! (see note on winter above!).
1. I pulled ACC Ebl codes and got the expected 230 for the Evap Sensor but I also pulled a 235 Emmissions or Smog sensor??? Could this have anything to do with the cold air? I thought the smog sensor was only an issue in smog on recirculate?
2. I checked the DuoValve and it's hot, but the bottom 2 pipes (heater cores) are not hot.
3. When I did the climate control test, I did not hear all the doors opening and closing.
4. Door assist vacuum stopped working at the same time.
Front Vacuum Pump?
Valves behind the glove box on the cores?
Clean DuoPump?
I am trying to avoid as much trial and error as possible. THANKS IN ADVANCE!

· Moderator
93 SL500, 95 SL320, 96 S320, 98 S500, 2002 E320 4Matic Wagon & A little 91 5.0 FORD Mustang
9,056 Posts

Post the climate read-outs. that will be helpful.. See the process below..

Reset the pump, I want say it is fuse 11 in the back.

The DUO valve you can be tested with a 12v power source, but the best I've found is to remove it, and test it outside the car.

What I did in pinch last year once I figured it was the duo valve. I removed the two valves(so it is always blowing hot air in the car If I got to hot I just opened the window, and adjusted the temperature) inside the duo valve, and I went looking for a working one at the junk yard.


· Registered
1998 S320 W140 120K+
43 Posts
Discussion Starter · #3 ·
Thanks for the FAST response Martin!
I just got back from Autozone. Getting fuses. I checked all the heater/AC fuses, but forgot to check the door assist one in the trunk! It wasn't burned out, but it wasn't working. Door assist works now, so subtract it from the Symptom List. The car was at operating temp, but not hot. Autozone is about 4 miles. Ambient on the dash was 55 degrees. Climate control for the ride was set at 72 on both sides from this AM's test. When I got home here are the new test numbers:
1. 89
2. 55
3. 79
4. 79
5. 53
6. 169
7. 5
8. 64
9. 17
10. 0.8
11. 3.4
12. 3.9
20. 0
21. 7
22. 0
23. 0
24. 14.2
40. 5
41. 3
42. 73
43. 136

· Registered
1998 S320 W140 120K+
43 Posts
Discussion Starter · #5 · (Edited)
I will thanks! When i replaced the door assist fuse, I started getting heat, but when I turn up the blower, the heat fades out to cool (not cold). g
Update: I went and ran the test again. Cold engine. #1 reading started at 86. I just left it on to see what it would do. It dropped to 76 pretty rapidly. Since the test is run at 72/72 I am guessing the interior temp is working. I also pulled the overhead consule and I could hear the little fan and the connections are good. The filter is clean as well. I am back to: Heat That Fades Out as I turn up the blower!?! Thanks again everyone!

· Registered
1998 S320 W140 120K+
43 Posts
Discussion Starter · #6 ·
The valves were shot and the pump was fried. What I didn't know until I got it apart was that the rubber washers from the valve side had crumbled and were blocking the pump side in the lower reservoir. Also the inside of the return to the water pump hose had collapsed and I had no flow back to the engine.
Lessons Learned:
1. If your duo valve dies, look for clogs under the pump and in the return hose. Especially if you only get cold air in the cabin or warm air that fades to cold or only warm air when the blower is on low.
2. You can test to see if there is a clog in the heater cores by pulling the pump, starting the car, putting on the heat and revving the engine a few times. If the cores are clear, you will get coolant shooting out the bottom two hoses. Then put it on AC and rev the engine and you should get coolant shooting out the top large hose, the heater core bypass hose.
3. Pull the duovalve WITH the small hoses on the back attached, much easier to get to the hose clamps.
4. You can make a bypass for about $20 in hose, clamps and a "Y" fitting. BUT you have to leave it on heat. I was worried about back pressure, but with the main return line blocked, it didn't seem to cause any problem. You can also make an even cheaper bypass without the "Y" from the heater core bypass hose to the main return hose if you want AC only. I am just going to have to leave it on heat until I get to a junk yard in Phoenix. Shouldn't be a problem at 7000 feet in the winter in Flagstaff!
5. This DuoValve is about 5 years old and from E-Bay. Bench tested it when I got it and it worked fine. What I should have done is IMMEDIATELY get the gasket rebuild kit and rebuild it after testing. Also, given my return hose collapsed on the inside, I would replace the hoses with the pump. There is a good YouTube video on the gasket rebuild kit, many have posted it on this site.
1 - 7 of 7 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.