I've resisted posting this problem until I felt like it had gone where no man has gone before, and I've hit that point, I think.

I bought my 1984 300D when the weather was warm last Summer. It has a low mileage non-turbo engine replaced by a dealership for the PO. Some bits and pieces were obviously never hooked up right after the swap. I have no temp guage or tach working. The engine shutoff was changed from vacuum to a manual pull, which I don't mind in the least.

When the weather changed, I blew fuses when I turned on heat. Swapped the fan motor with one from a parts car in the woods I located with a buddy, after doing a brush replacement. Swapped the damaged climate control unit, and re-soldered the original where it had burned from the motor problem (ouch). I discovered that the aux. water pump was bad, and swapped it too. I have replaced the monovalve with one from the parts box I got from the basement of a house scheduled for demolition (I've been on a salvage mission). I have verified that the pump, fan action and monovalve are all working. The whole system holds vacuum. If I park it for a couple days then pull a vacuum line, I hear suction. I've burped and purged and topped off coolant multiple times, and have warm heater hoses on both sides of the heater core.

Here's the deal now. If I start the car and let it idle, I get heat and can defrost my windshield. However as soon as I take off, within a few seconds, the temperature from the vents drops and I go from lukewarm to cold air. I basically have no heat at all while I drive. The really interesting thing is that if I cut the ignition, then turn it back on, the air goes almost immediately to hot, then cools back off. This leads me to believe I'm looking at an electrical issue, or the climate control #2, rather than anything clogged or not flowing.

My question is simply, has somebody got a theory? This one baffles me. I tore apart the old monovalve today, and it looks good, so I'm taking that out of the equation. Are there any thermostats or temp guages that might be shutting off heat flow inside the cabin, or electrically is the only variable the control unit itself? I've read a million posts and can't quite get a handle on this. Good news is spring is almost here.

Ben

The inside of the monovalve is good, what about the electrical portion?

__________________
1981 300D 147k
1998 VW Jetta Tdi 320k
2001 Dodge Ram 2500 141k
1979 300D 234k (sold)
1984 300D "Astor" 262k(sold)
Mercedes How-To and Repair Pictorials
I love the smell of diesel smoke in my hair

You may want to check and see if the engine has a thermostat, or if the thermostat is stuck open. With the weather in VA you could easily run cooler while in motion that while stopped.

Dig through skinnerbox. There should be a diagram that will help you wire the temp gauge. The tach might be a problem. The non-turbo engine may or may not have the sender for it.

-Tad

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1995 G320
2003 C230 Kompressor Coupe
2005 ML500
2007 ML350

The engine is heating fine; the thermostat opens as evidenced by hot hoses to and from radiator. But regardless, that is on a separate loop altogether.

The monovalve makes an obvious thunk when the heat is turned on or off and I don't have the engine running. I've also run it with the the electrical connection off, which keeps it in the open position. I verified it's operation before installing it.

Another thing that puzzles me though, is that after replacing various parts, I did have heat for about a week before it went dead again. I guess the monovalve could be suspect, but why would it work while idling?

I just downloaded the climate control manual from skinnerbox, and it looks like the interior temperature sensors, of which there are several, in fact CAN turn your heat, monovalve, and aux pump all off. But if that were the issue, why does the air delivery still work?

Ben

Can you kindly set the temperature dial all the way up.........maximum temperature........until you hear a click from the temperature wheel.......as far as it will possibly go?

Then repeat your tests..........exactly as you have done........and post your results.

It is the same, on full hot or whatever the thermostat calls for.... I can hear something moving dialing up or down. Also, the fan will cut off if the cabin is quite warm, so at least one of the cabin temp sensors is good. It looks like there might be one behind the cluster, and another near the fan?? I'm not sure how to troubleshoot it further. I am thinking a temporary bypass of the monovalve with a length of heater hose would eliminate the possiblility of failure of valve #2, since it could be in question. It had been in a box for a long time, used, so could have failed after a week of work.
Ben

I had this same issue years ago. I also found that if I shut off the ignition, waited about 30 seconds, ran the thumbwheel to "MAX" and then started back up, I'd get full floor heat UNTIL I moved the thumbwheel. I had a new monovalve put in (by somebody else! ) and problem solved.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.

Monovalve #2 diaphram was torn. Fixed
ben

That will sure do it!

Glad you got it fixed.

__________________
1981 300D 147k
1998 VW Jetta Tdi 320k
2001 Dodge Ram 2500 141k
1979 300D 234k (sold)
1984 300D "Astor" 262k(sold)
Mercedes How-To and Repair Pictorials
I love the smell of diesel smoke in my hair