When I first start the car cold everything seems to work fine. After engine warms up (about half a mile), heater kicks in and puts out heat. After about a half hour or so of driving the air gets cold but continues blowing. If I turn off the heater and turn it back on again, I get hot air again for another half hour or so. It makes no difference if the temperature wheel is on warm or maximum heat. I can't get hot air continually for sustained periods of time.
It seems to me that the system works until the valve allowing hot coolant to circulate thru the heater core is told to shut down since the car is warm enough. Then, it wont open again until the system is shut down.
I replaced the push button unit with one from Pick and Pull. Has made no difference.
Can anyone help me avoid spending a long time diagnosing this?

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I was going to suggest the climate control unit but you already replaced that. Could it be both were bad?

Kinda doubt it although possible so it must be the circulator pump [sorry... can't remember the correct name] that is going bad.

I assume this is a 123 we're talking about, right?

Cheers,

Bill

I think you're referring to the auxiliary water pump. That will really only affect heat at low rpm. Mine didn't work for a while (due to a failing ACC unit) and I always had heat while driving, it would only drop off a little when I stopped. The folks who rebuild ACC units will usually test your unit before selling you another one.

Thanks Craig.

Now, if its not that then what the heck would cause his problem? It has to be the CC unit.

Cheers,

Bill

I agree, the auxilliary pump seems like an unlikely culprit.
I've considered that I might have 2 bad control units and I agree it's possible but if there is something else that could go wrong, it seems that problem would be more likely.
Where is the valve that controls the amount of coolant flowing into the heater core? I suppose I could test the voltage, or is it vacuum, that is going to that valve when it's working/not working.
Can anyone think of what the problem might be other than inadquate hot coolant flow thru the core?

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

The valve that controls the coolant flow is the monovalve, located under the hood, on the firewall, near the battery. It is an electrically operated valve that opens with 0V and closes with 12V. It's located in the heater hose line. If you unplug the electrical connector the valve, it should open and supply hot water to the heater core. You can usually hear it click open and closed. You can also use a volt meter to make sure the valve is getting a signal. Do a search on monovalve and you should find a bunch of info.

Thanks. I'll disconnect the monovalve and drive for a while. If that solves the problem, I'll investigate further.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Did it work?

Cheers,

Bill

It sure sounds like a monovalve problem to me because I had the exact same symptoms on my '85 and fixed it with a new monovalve cartridge. Just unplugging the monovalve may not tell you anything. Take the top off the monovalve (four slotted screws) and pull out the brass cylindrical inset. Look at the rubber on the end very carefully. If it's split you need a new cartridge (available from Phil). Replace the cartridge and the problem should be fixed.

__________________
LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold

Have your checked your Auxillary water pump to see if its working.....it pushes water to the heater core.. maybe what water is getting through is being cooled off....and giving these symptoms...its on the right fender between the wheel and the headlight....easy to confirm if its working by hooking direct to a 12 volt source.

__________________
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

I haven't had an opportunity to experiment yet, but I'm inclined to go with the monovalve problem diagnosis. I don't think it's the auxillary pump because that pump is most essential at low rpm's but the problem is not correlated with engine speed, only with reaching proper inside temperature and turning the climate control on/off.
But I do have a question about the monovalve. Why would it work ok the first time the climate control calls for it to open but fail each additional time?

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Conducted a preliminary test with the multimeter. After starting the car and pushing in the defrost button, voltage at monovalve dropped to zero. This seems correct. Pushed the EC button which should provide heat (if the engine is warm enough--I'm not sure it was) and the voltage at the monovalve was jumping all over the place, 13, 12, 7, 4, 3, 0, etc, back and forth between these numbers. This continued for about 3 or 4 minutes and then the voltage dropped to 0 or .3 or so and stayed there. That seems correct since it should have been calling for heat.
But, why was the voltage jumping all over the place for a period of time?

I haven't driven it far enough to test the voltage after the heat has been on for long enough to cut out. The multimeter's in the car and the next time it happens, I'll test it.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Hi all,

I have an 83 300SD and I'm getting a similar problem. I can get relatively continuous hot air using the Defrost, but when I try any of the heater options it blows hot air for a couple of minutes (not 30 minutes) and then starts blowing cold air. If you guys come up with a solution here I'd be so grateful!!

Also (may be totally unrelated) I sometimes get what smells like exhaust when turning on the heat or defrost. This problem appears worse when I try the defrost when the car is not fully warmed up, and when I try heat or defrost when I'm stopped at a light. Any ideas here?

Thanks a million.
-Jeremy

I would bet you have an exhaust leak someplace under the hood (possibly the flexible part of the exhaust pipe) that is causing exhaust fumes to be sucked into the air inlet grills. It will be more noticeable when the car is cold because the fumes a worse.

I disconnected the monovalve, and now have persistent heat. Have to open the windows now!

So the issue appears to be the monovalve, or perhaps the voltage going there. I don't think I have a multimeter so I'm going to pose a few more questions.

I pulled out the monovalve unit, cleaned and dried it. I must be a total idiot, because I did this while the car was still warm and some coolant came out. Not a big deal (I hope!) but I sure felt stupid, wasn't expecting that. Duh.

I cleaned off and dried the monovalve unit and it's component parts. The two rubber seals look good to me... not cracked or worn. There's a little cylindrical wire-mesh screen that surrounds the neck of the bottom of the metal cylinder, between the two rubber seals. The wire mesh doesn't look compromised, although it curves inward a bit (not a perfect cylinder).

I'm putting the unit back in, finger tight on the 4 screws. I hope it still holds the seal!

What should I be looking for besides wear on the rubber seals? And if it's not the monovalve, then I have to get a multimeter, right?

thanks. sorry I'm so green at this stuff.
-Jeremy