This will be my first cold weather with my 81 300SD so I decided to check the heater. I worked OK (driving at 60mph) the 2/3 times I tested it so I was happy. Sunday night, drove at 70mph and here is what happens: Works OK first 2 or 3 mins then it starts blowing cool (not cold) air. Air through vents is not as cold as outside air temperature but it's still on the cool side. I do not get the problem when driving slower (below 60 mph) or maybe I have not driven far enough at low speeds for the problem to show up. I drive 17 miles to work, stop and go and fastest I go is 60 mph, no problem.

Anyone with hints on what I need to look at.

Is your ausillary water pump working? I bet its not. Monovalve can be sticking too.

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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)
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Section 609 MVAC Certified
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"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

Most likely a torn diaphragm in the mono valve. With this condition, the heater will work fine at low rpm, but blow cold air at highway speeds. Come to a stop light, hot air returns. There is a repair kit that will fix the problem. Simple to install. Fastlane part #R3023-15174. $41.14.

Disassemble the mono valve and you will be able to see the condition of the diaphragm.

Thank you. I know where the Monovalve is but am not sure about the Auxiliary Water Pump. Let me search before I ask the obvious. I like searching, I always read more information than I need and I've found that information to become handy when I run into other problems.

Thanks again!

The aux water pump mainly assists coolant flow during low engine rpm conditions. The main water pump provides adequate flow at above-idle operation. A failed aux pump should produce symptoms opposite to what you described. Poor heat at idle, good heat at highway speed. To check the mono valve, disconnect the electrical connector at top. Remove 4 screws at the top of valve, remove valve. There is a black rubber seal near the end of valve. Check it for tears.

It certainly sounds like a torn monovalve diaphragm. Extremely common 126 item, and just as easy to DIY. Here's a writeup with a few pictures that will help you check, and if necessary replace yours. http://mbdieseldiy.tripod.com/monovalve.htm

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Tjohn

82 300 SD
77 450 SL (gone)