1984 300TD Stubborn Climate Control Problem...

Hi,
I have a 1984 300TD station wagon that belonged to my mother. I got the car four years ago. Overall the car is in great shape, 105,000 miles, practically everything works, etc. The only major problem was no cooling. It
took only $100 in parts to make it pass the Maryland vehicle inspection. My wife and I decided, rather than getting an SUV or van for our newish family, to get the AC fixed and use this one.
The climate control controls all seemed to work fine, i.e., the air came from the right vents at the right time, and the heating and cooling (without chilled air, of course) came on at appropriate times. In the winter, there was no need to touch the controls. All this car needed was the means to chill the air. So we took it to a garage and they basically renewed the heat and AC.
They replaced the evaporator, heater core (since they were already in that compartment), AC hoses, expansion valve, filter drier, and belt. The compressor seems fine. In fact, the chilled air is now all you could ask for.
The problem is that now the cold air comes only when the temperature setting knob in the control cluster is set to maximum cooling (or "Min"). Whenever the knob is moved away--however slightly--from "Min" the heat comes on full tilt, the vents change from cooling (all four dash vents) to heating mode (side vents only) and the AC compressor continues to run. It appears that both heating and cooling are going full steam and the heating is winning. From the feel of the temperature control knob as it moves, the "Min" (and "Max") settings seem to trip some sort of override circuit that forces maximum output. Evidently, the "Min" setting forces the heating to shut down. When the control knob is set to the extremes, the fan speed usually increases if it wasn't already running at that speed. I don't know if the "Max" setting increases the fan speed now. It did in the past.
The mechanic has been scratching his head for over a week now. He has replaced the climate control control cluster, the thermostat (whose sensor is positioned between the evaporator and heater core), the auxiliary pump in the heater hose under the hydraulic leveling fluid reservoir under the hood and has verified that the heater control valve attached to the firewall under the hood is opening and closing as it should. Nothing seems to make the slightest difference and he has reinstalled the original equipment. He has accepted that the heating functioned correctly when I brought the car to him and that something must have gone wrong during the repairs so I can't complain and I'm not. In checking the car after getting it back the first time, I discovered that the fuse for the climate control, et al, had been replaced. I suspect that he didn't disconnect the battery at first and shorted something. The Mercedes garages around Baltimore would probably have disconnected the battery but they would have charged me 5 times as much and are clearly too snobby to have been willing to accept any responsibility for a snafu like this if they had caused it.
I keep thinking that the problem is with the thermostat in the plenum housing of the evaporator and heater core--it's not positioned correctly--but I have no practical way to prove anything.
The point is (thanks for staying with the narrative) that there seems to be nothing left to account for this malfunction. I am hoping for someone who will read this and say, "No problem. There's a blue and green control module just behind the glovebox that has gone bad and you just replace it. Happens all the time." or something like that. :-)
Thanks very much in advance for any light you can shed on this problem. If there is any further info you might need or want please don't hesitate to say so. Also, if you respond to the list would you please cc to me personally?
Thanks again,
Ken
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I just finished chasing a problem with my Climate control. It took me a week of digging to decide to finally take it to the dealer. The dealer spent 4.5 hours just to find out that the coolant recirc pump (that ran fine) was shorting intermittantly and causing the climate control system to shut down. The part was 90 bucks (aftermarket) and the 4.5 hours of searching was 400 dollars. Anyhow the moral of this story is that those climate control systems are an sob to fix. Isnt there a mixture control valve or something that controls when hot water comes through the core or not. (besides the heater control valve) KH

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The heater's control valve "monovalve" defaults to full heat if it's worn out or damaged. Your post states that this "monovalve" IS functioning - suggest you check how WELL it's functioning ON and OFF at various settings before tearing into the control systems etc.
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Thanks very much for the info. The "fail state" of the "monovalve" was something I had wondered about. But the air vents change over to heat mode as well--and back to cooling mode, depending on the setting of the temperature control knob--even during the few minutes after the car is first started and is still cold. If the changeover in the vents depended on the function of the "monovalve" it seems they would be changing almost constantly. It _appears_ that the two separate systems (heating and cooling) are functioning correctly but they are not working well _together_, which suggests to me that it's a control somewhere that is screwing things up. I just don't have a freakin' clue anymore which one it might be.

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The old Chrysler system in my '80 300SD (terribly complicated) has outside and inside temperature sensors as part of a circuit. Your car must also have these. If one or the other were disconnected the circuit would be incomplete and strange things would happen. The sensors are "solid state" so they, themselves, rarely fail so don't automatically replace them.
Another thought, back to my old system which has a logic board called an "amplifier". On my '80 two or three of these have broken. The signs are that the system operates perfectly for 10 or 20 minutes and then suddenly veers to maximum heat. Apparently the little solder paths on the circuit board develop tiny cracks over time and the circuit becomes intermittent. Someone with great soldering skills can make a DIY repair, the rest of us buy a replacement.
I suggest you approach the problem from the simplest possibilities to the most complex. The DEF mode will simultaneously run the compressor AND maximum heat, perhaps it's in DEF or stuck in DEF.
Good luck.
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In twenty-five years of servicing the 123- chassis cars, I have yet to replace an evaporator OR a heater core. As for the rest of his guesswork- you may well run out of money before the dealer runs out of parts.
Get it to someone who will test and diagnose it befopre fixing it.
Bill Ditmire Ditmire Motorworks,Inc. 425 White Horse Pike Absecon,NJ 08201 http://www.ditmire.com 609-641-3392
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Amen to that. My 4.5 hours of labor that I paid the dealer to Dx my Climate control system was well spent. I actually sat and spoke with the mechanic for 10 minutes after he spent four and a half hours testing and swapping parts to find the problem. No small local repair shop would have the resources to do that. I didn't pay for any of the parts. He then told me where to go buy parts locally for the car at better than dealer prices. It turned out that the coolant recirc pump was intermittently shorting out and causing the system to shut down. Recycling the key would fix it temporarily. Just unplugging it (pump) fixed the problem and made everything work. I then spent 90 bucks on a new pump and put it in myself. I just redid my Fords entire ac system and converted it to 134, that whole system was a mess and full of black death crap. It didn't even get a new evaporator, just a thorough flushing. KH

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