I have an 84 300SD. I have been slowly chipping away at some problems since I purchased it about 6 months ago.

Originally only heat worked with no outside air what so ever. I found that the monovalve was unplugged. Then it still was only kicking heat so I tried resoldering the CCU and ruined it. Bought one off of EBAY and all the functions seem to work (although I don't know for sure since I don't have air conditioning-haven't checked into that- if I get all this stuff figured out I will check into that for next spring as I live in MN and won't need much air in the winter!). I was still getting a lot of heat so I bought the monovalve replacement. Now it seems that the air does not get as hot ever as it used to. I do get what seems to be 'cooler' air. What I mean by cooler is not full heat air not that it is actually cool.

I also have always had the system 'default' to max heat or defrost mode when I am accelerating hard or when I have the system above about 3500 RPMs.

What does this mean? Any one have any good ideas?

I guess the next 2 thoughts I have is I will check the interior temp sensor to see how that is working (any suggestions as what to look for?) and look at the foam tubing (any suggestions as to which ones and where to check?)

Thanks!

I have been having similar issues with the heat in my 1985 300td. Where in MN are you? I am in Mankato.

Hi Karl,
I currently live in Anoka (just northwest of Minneapolis about 15 miles for the non-MN people). I went to school in Mankato about 10 years ago.

Have you done anything to your car to try to fix the heat?

Anyone else have any input?

Thanks again!

how is the monovalve controlled? if it is vacuum i would look at the lines as being suspect. Sometimes the old rubber will have almost glued itself to a component and once you break that to replace the component it won't seal worth a damn and cause it to leak. Perhaps under high revs the engine produces more vacuum and pulls it open further?

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2003 Dodge Ram 2500 Cummins - Georend fully built trans, Banks Big Hoss Bundle, Smarty Tuner, 5" turbo back exhaust. 500 HP and 1100 ft/lbs to the ground.
1979 Mercedes-Benz 300CD - Lost to a four car pile up on 275, Thank You Michigan Drivers.
1986 Mercedes-Benz 300D - Sold, Euro non-turbo car.

Monovalve is electrically controlled. Full 12 volts, it's closed. 0 volts, it is open. I do not know if it gets cycled with 0 then 12, then zero, or if it is variable, like 6 volts and it half way opens.

I do know that if the moving parts get crudded up, it can hamper movement resulting in either no heat, weak heat, or full heat. Also check it's diaphragm.

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RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 210K

90 Dodge D250 5.9 Cummins/5 speed. 400K

I just put a NEW monovalve repair kit in about two weeks ago so the monovalve itself should be working. I guess I should take a multimeter to it. Will check that as well but that doesn't answer my main question as to the heat kicking on under high RPMs.

Thanks guys!

Check your aux water pump. It may be unplugged. Before you plug it in, splice in a 1 amp fuse to it's power lead. If the aux waterpump is locked up, it will fry the CCU, as it gets it's power from the CCU.

I have not had to work on the monovalve on my car yet, so I have no idea what parts are replaced.

__________________
RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 210K

90 Dodge D250 5.9 Cummins/5 speed. 400K

I have just ordered a monovalve rebuild kit. I am hoping that this will help moderate the fluctuations in the heat output. I am currently getting super hot at max and the red zone and when I turn the wheel out of the red zone it goes straight to cold air. There seems to be no middle. Does this sound like a monovalve problem? I am concerned that it might be the CCU.

Where is the aux water pump?

Could be mono valve, could also be that the foam tube in the dash is gone, so the temp sensor can't get a good reading from it's sample air drawn in from the little vent on top of the dash.

The aux water pump is behind the alternator.

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RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 210K

90 Dodge D250 5.9 Cummins/5 speed. 400K

Thanks for the replies.

My aux water pump is unplugged. I unplugged it at the beginning of summer thinking that the hot coolant wouldn't flow through as much and thus it would not get as hot. I was debating whether or not to plug it in now that it is colder out again.

When I put the multimeter to it, how should I connect it to check the voltage? Should I plug one side of the plug in and the jump the other side with the meter? Or plug both sides in and stick the positive pin in the connection and ground the negative?

Also I don't think the aux water pump (or lack there of) is what is causing the kicking in off heat under high RPMs as when it was plugged in it did the same thing (which is one of the reasons why I unplugged it in the first place).

I will look into it tonight as I may have some time after the little guy goes to bed.

Thanks again!

Check the foam tube that is behind the glove box. Also check the temp regulator.

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RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 210K

90 Dodge D250 5.9 Cummins/5 speed. 400K

I looked behind the glovebox and the foam tube was gone. I replaced the missing piece with a rubber hose. i will see if that help regulate the temp.

thanks for the help.

rrgrassi,
I don't believe the 126s have a foam tube after doing some research on here. I am not positive though.

After doing some more research I am leaning towards the vaccuum pods behind the CCU. Perhaps the flaps in there are not opening or closing correctly and when I hit the pedal hard it causes one of the vaccuum components to 'default' to defrost?? Just a thought right now. I will try to look more into it this weekend.

I get heat no my 300sd, but think I should be getting more from the "feet" area.... might be a vacuum pod. I tested the those little crossover switches not too long ago when I got my center vents working again.

Keep us posted on your "heat"... love to pick up some tips on what to check (I have replaced monovalve etc).

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~shell
As of 2/2010:
2001 CLK55 0o\=*=/o0
13.6 @ 106mph
10K mi
1984 300SD
260K mi and going and going...
97 S600
46K miles
1991 Sentra SE-R (extremely dorked with)
www.se-r.net

I definitely will.

Anyone have any advice on how to check the vacuum pods that control the vents??