I have a 1983 300D turbo (285,000 miles)- the climate control unit on the car works well.....but when I put the heat on, the heat blows full hot, regardless of th setting. I've reviewed many threads and re-did the form tubing under the dash, got a used mono valve from the junk yard (that turned out to be in worst shape than mine - that resulted in not heat at times) so I replaced it with my mono valve. I can not think of anything other than rebuilding the mono valve. .....do you have any other suggestions?
I have also tried several different control units from the junk yard .....but the heat still blows full heat regardless of the heat setting.

Thanks,

Ron

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Those units fail routinely, or perhaps your plug has a problem.

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83 SD

84 CD

There are two parts to the mono valve - the plunger and the solenoid. From what I can tell the plunger moves up and down based on the voltage applied to the coil.

If the plunger is ruptured (common) you get no heat, or weak heat. If electrical portion is messed up (not as common), you will get full heat since the default is state is full ('inferno').

Im fairly sure you need to replace the electrical part of the monvalve. This you can get at a junkyard. You may need to buy a replacement plunger ('monovalve rebuild kit') if yours is ruptured.

Someone who knows the specifics can walk you through, but you can certainly use a meter to test the voltage coming from the the CCU to the monovalve to make sure that portion is OK. Its probably fine since you swapped the CCU and had the same results.

dd

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

when you say blows full heat...do you mean fan on full and ducts spew hot air?? If the fan is going to full youve got a cotroller problem.

I had the exact same symptoms on my 83' 300D. Cleaning the electrical connections to the monovalve and using some electrical grease solved my problems.

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1983 300D 280,000
1983 300TD 125,000 Killed by oak tree.
1983 240D Sold
1986 Ford F250 Diesel 6.9l 169,000

The controller units themselves fail. Internal cracked solder joints is a typical cause, and easy to fix.

There are vacuum accuators under the dash that also age and fail and cause the flaps to not open or close as they should.

Apparently, sometimes the connector plugs fail -- I've hear numerous odd and interesting way they can fail, many that would affect any CCU unit installed.

The system is separated into electrical and vacuum subsystems. Vacuum controls ducting and electrical controls heat transfer (pumps, valves & fans).

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'83 300DTurbo http://badges.fuelly.com/images/smallsig-us/318559.png

Broadband: more lies faster.

Thanks for the responses,

when you say blows full heat...do you mean fan on full and ducts spew hot air?? If the fan is going to full you've got a controller problem.

The heat blows from the duct at inferno hot heat. The fan operates as it should based on the control speed.

Apparently, sometimes the connector plugs fail -- I've hear numerous odd and interesting way they can fail, many that would affect any CCU unit installed.
The system is separated into electrical and vacuum subsystems. Vacuum controls ducting and electrical controls heat transfer (pumps, valves & fans) .

How can I repair the electrical connector? I'm not having any problems with the control unit itself.....I have tried several others with the same results.

I had the exact same symptoms on my 83' 300D. Cleaning the electrical connections to the monovalve and using some electrical grease solved my problems.

What did you use to clean the electrical connector? What type of grease? Are you referring to the electrical connector under the hood and at the top of the mono valve?

Thanks,

Ron

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Ron,

I cleaned the contacts at the monovalve. I used emery cloth and electrical contact grease, available at any mega parts store.

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1983 300D 280,000
1983 300TD 125,000 Killed by oak tree.
1983 240D Sold
1986 Ford F250 Diesel 6.9l 169,000

to me it sounds like you have two mvs with different problems try to fix the plunger if you can in the other one, then replace it.
i have a servo issue so i could be wrong but it might be worth a try.

if the plunger is ruptured (common) you get no heat, or weak heat. If electrical portion is messed up (not as common), you will get full heat since the default is state is full ('inferno').
dieseldan44

If the heat "turns off" when the "off" mode is selected on the CCU, the basic electrical connections have to be good. The problem sounds more like a control issue than a problem with the monovalve or physical electrical connections.

Which vents does the heat come out of? It is supposed to switch around in the different modes.

From what I've read, the CCU units tend to fail internally with the same result, which is usually that it only works on defrost mode.

There's also a foam rubber tube that ducts air from the cabin to the air temp sensor. If it disintegrates, then the air temp sensor starts reading the air under the dash and heating/cooling the cabin accordingly.

If you suspect the conenctors to the CCU, you have to pull the CCU and disassemble the cable connectors to check them. I'd read about cracked solder joints where the pins connect to the wires in the cable and loose pins that wouldn't make contact. Loose pins are pretty obvious, but the only way to know about cracked solder joints is to open them up and check. FYI, the pins on the CCU can have cracked solder joints too. On mine, the cracked solder joints inside the CCU were the connections where the pin-boards for the connectors fastened, so to the CCU it looked like some of the pins in the cable weren't connecting. BTW, there's similar pin connectors for the temp wheel and fan switches.

The grease you're looking for is called dielectric grease. On 25 year old cars, the contacts in the connectors all over the car are suspect, so your problem might even be somewhere in the wiring harness where the wires route through a connector. I fixed one such issue on my volvo last summer that was causing the front turn signal to intermittantly participate in the blinking. Ended up being oxidation on the metal surfaces of the connector where the turn signal bulb holder plugged into the wiring harness. That was so much fun to find that my general policy is now to clean and dielectric every connector I need to pull when working on the older cars.

__________________
'83 300DTurbo http://badges.fuelly.com/images/smallsig-us/318559.png

Broadband: more lies faster.

Which vents does the heat come out of? It is supposed to switch around in the different modes.

From what I've read, the CCU units tend to fail internally with the same result, which is usually that it only works on defrost mode.

There's also a foam rubber tube that ducts air from the cabin to the air temp sensor. If it disintegrates, then the air temp sensor starts reading the air under the dash and heating/cooling the cabin accordingly.

If you suspect the conenctors to the CCU, you have to pull the CCU and disassemble the cable connectors to check them. I'd read about cracked solder joints where the pins connect to the wires in the cable and loose pins that wouldn't make contact. Loose pins are pretty obvious, but the only way to know about cracked solder joints is to open them up and check. FYI, the pins on the CCU can have cracked solder joints too. On mine, the cracked solder joints inside the CCU were the connections where the pin-boards for the connectors fastened, so to the CCU it looked like some of the pins in the cable weren't connecting. BTW, there's similar pin connectors for the temp wheel and fan switches.

The grease you're looking for is called dielectric grease. On 25 year old cars, the contacts in the connectors all over the car are suspect, so your problem might even be somewhere in the wiring harness where the wires route through a connector. I fixed one such issue on my volvo last summer that was causing the front turn signal to intermittantly participate in the blinking. Ended up being oxidation on the metal surfaces of the connector where the turn signal bulb holder plugged into the wiring harness. That was so much fun to find that my general policy is now to clean and dielectric every connector I need to pull when working on the older cars.


1. The heat comes out of the 4 front dash vents .....not the defrost vents. It does switch around depending on the mode.

2. I thought it wasn't the CCU because when trying other units the problem remains.

3. I have replaced the foam on the rubber tube under the dash.....

Thanks,

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If you have a spare temp control module available, I would suggest that you try swapping it out.

Not familiar with the temp control module....please explain and tell me the location of this part...(help me find it) : )

What is its function?

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The temp controller is the brains of the climate control system. It controls the monovalve, controls the temp according the the setting on the temp wheel, controls blower speed, operates the center flaps, etc. It is located in front of the glove box.

Looks like this:
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=28D0KHXF229S19LOIZ&year=1983&make=MB&model=300-DT-001&category=R&part=Temp+Control+Regulator

Not to be confused with the blower controller:
http://catalog.worldpac.com/mercedesshop/sophio/quote.jsp?clientid=catalog.mercedesshop&cookieid=28D0KHXF229S19LOIZ&baseurl=http://catalog.peachparts.com/&partner=mercedesshop&year=1983&product=R2022-11843&application=000359957