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Island300D 07-19-2007 11:53 PM

78 300D Climate Control question-only defrost works.
Well, I've searched long and hard and can't find a thread with the answers to my question. Like many, I am having a problem with my climate control. Previous owner has had it fixed before, including a new controller unit for like $1,300 or such. AC is newly recharged and all. PO tells me everything was working fine until very recently.

Only defrost works. Pumps out hot air no problem. No other buttons on my controller do anything. No fan, no anything. No matter the temp setting on the dial.

I know many, many people have faced this problem and someone must have guidance for me!!!

Thanks in advance. :confused:

Island300D 07-20-2007 01:58 PM

Any climate control help out there? I know a bunch of people know a whole lot more about it than I do!

TX76513 07-20-2007 02:18 PM

Unfortunately your problem may be a little hard to diagnose with the limited information you supplied. At the top of your screen there is a tab that says DIY Articles. Click on that then scroll down to AC Diagnosis - this will allow you to systematically test some components. Other than that we can just guess at things like:

Fuse is blown - CCU defaults to heat
Low on Freon -Compressor won't engage
Mono Valve Stuck Open.

The list goes on, but I think you get the idea.

rs899 07-20-2007 02:20 PM

I haven't messed with my old Chrysler unit in the 300SD, but if the servo is new and functioning properly, the next most likely culprit is the board in the dash behind the glovebox. You may need to consult the manuals or go looking at some Chrysler Imperial website for ca 1971 CC troubleshooting data. It's basically voodoo and witchcraft....

edit- tx's info is not geared toward your unit- you do not have a monovalve as you have the earlier Chrysler designed pos.


TX76513 07-20-2007 02:25 PM

Sorry you are correct on the mono missed the '78 in the post - had 126 on the brain.

Island300D 07-20-2007 06:20 PM

Sorry if I did not provide enough info. What is that about Chrysler components?

I'm new to MB, but were they sourcing American Chrysler parts back in 1978? That voodoo and magic part is exactly what I have hoped to avoid with this car. I'm just a guy at home with tools and some synapses firing. No shop and no fancy testing equipment! Maybe buying a 1978 was too recent!

My dad has a beautifully pristine 1956 Chevy with 56,000 original miles. Not a single thing on it that we can't fix blindfolded. Want to get there with my 300D, and hope this heater/AC not working problem does not require a trip to the dreaded Indy.

rs899 07-21-2007 06:46 AM

Unfortunately, you found your car before you found us.

Yes, they did use a Chrysler control system on 300Ds from 77 to 80 and 78-80 W116 300SDs (as well as gassers like the SLs in the 70s). They went to an MB made system in 1981 that is somewhat better, but still irksome. The 240D and the euro cars have the most DIY - friendly system. People have fixed a lot of these units by themselves, so it can be done. There is a new electronic version for $600 or so that replaces it.

But here- look at the Auto-temp stuff at the Imperial site to get an idea of what you have.


anarchy 07-21-2007 11:58 AM

I have the same exact problem with my 80 300SD, I replaced the "voodoo board" but it didnt fix anything so i dont know what to say i think its a vaccume problem

Island300D 07-21-2007 02:15 PM

Geez, another case of the folly of making something more complicated than it need be. I'm reminded of the famous case of NASA spending countless dollars and time developing a pen that would write in zero gravity, while, the Russians, used a pencil...

I'm becoming resolved to the notion that my defrost only system may have to suffice. We live in a cool place anyway, and can't imagine using AC but for a few days a year. At least the defrost does blow through the side panel vents. So my feet stay cold a little longer, oh well.

I think I'll troubleshoot all the simple and obvious stuff, and then call the Indy who worked on it last to see if they will check their recent work for free, and then maybe chalk it up to a 30 year old car with a silly system that isn't worth fixing...

Hey "anarchy", did you replace your voodoo board with the $600 unit mentioned here?

slarson80 07-21-2007 02:58 PM

It could be a vacuum leak, the push buttons control which vents open or close, and those are vacuum controled. Do a search for center vents. A lot of CCU info has been posted under that heading. You also might want to check some of your vacuum lines for oil. You may have a leak in the vac pump that will allow oil into the vac lines, and that can cause diaphragms and such to fail.

Island300D 07-21-2007 04:48 PM

When I say only the defrost works I mean that none of the other buttons engage the fan or do anything. Absolutely nothing happens when any other button is pushed, regardless of the temperature setting, except defrost, which blows very hard, hot air.

For this reason I surmise my heater blower works fine and the heat exchanger is getting hot coolant fine.

If it was just a problem with my fan blowing but the vents not opening properly or the air being cold, I would be in different shoes. At least this is what I surmise.

slarson80 07-21-2007 04:53 PM

Right, if you have vac leaks, that is excatly what happens. Really, do a search on center vents, or vacuum pods. There is a lot of info on these topics.

Island300D 07-21-2007 08:06 PM

Thanks slarson80,
I will look into it. I see you have virtually the same MB. I sure love, love mine, and this climate problem does little to dampen my enthusiasm. How is the weather in the Bay? We're getting a massive storm in the NW.

slarson80 07-21-2007 09:18 PM

Sunny, warm, beautiful! Everything is very nice except for the earthquake that nearly pitched out of my bed the other night.

Yeah, this older CCU is the biggest draw back to these earlier 300Ds. My heat/ac does not work, but all my fan settings do. For a while only my defrost setting worked. I removed the glove box and reset some of the vacuum connectors, after that my fan settings worked (no air through the center vents though). Since I didn't really replace anything I suspect the problem will return. Right now it's pretty low priority. You really should check some of your other vac lines for oil. You may have a leaking vac pump. If oil gets in the vac lines it can cause a lot of problems. (locks, shut off). It's also a pain to clean them up too.

kernelpanic 07-22-2007 08:46 AM

This may help

copied from a newsgroup:

First, the dash does not need to be removed to get to the temperature
control resistor. Your problem is not likely the temperature wheel.
Follow the steps below. The parts for this system are too exensive to
throw parts at it.

The temperature is sensed/controlled by a sensor chain. The sensor
chain consists of a series of resisters; the ambient air temperature
sensor (located on the firewall just behind the engine), the
temperature dial, the in-car temperature sensor (for the 107 chassis
the in-dash sensor is located in the center of the dash) and the
resister in the servo.

The first step in checking the servo is to verify the resister chain
and amplifier.

To check out the resister chain,
1. Remove the ACC amplifier (located to the right of the glove box)
and connect a ground to pin 3 of the connector plug.

2. At the servo electrical connector, connect an OHM meter between
pins 1 and pin 2 on the left of the connector.

3. Place the temperature wheel at max (85 degrees) and the resistance
should be between 2.2K ohms, and 4.7K ohms.

4. Place the temperature wheel at 65 degrees, and the resistance
should be between 1K ohm and 3.5 K ohms.

When you finish, re-install the amplifier.

If the circuit is open at any place in the resistor chain, the result
is heat. If the chain is OK, then the problem might be the amp.

To check the amplifier, connect a voltmeter between pins 1 and 7. With
the ignition switch in the on position, the voltage should be about a
volt. Move the temperature wheel from cold to hot. The voltage should
increase to about 5 volts, and when the servo finds it home, the
voltage will drop back to about a volt. If it does not vary, then the
amp is bad.

The servo (usually big black contraption under the hood) responds to
the temperature control resister chain. It has a small electric motor
that routes vacuum to control flaps, moves variable resister to send a
signal to the amplifier, and opens and closes the hot water valve.

The motor in the servo receives a voltage signal from the amplifier
that is located behind the glove box. If you want to test the servo
motor, remove the vacuum lines and the 10 pin electrical connector.

Pin one is on the right of the connector looking at the servo.
Connect a ground to pin 1 and +12 volts to pin 5. The motor should
Reverse the wire and the motor runs in the other direction.
If the motor does not run (you can hear it), the servo is bad.

You might also have a shorted aux water pump. This electric pump is
located just in front of the servo. Disconnect the pump and connect it
to 12 volts to see if it runs. The brown wire is ground.

Hope this gets you started.

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