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Climate control mod for 87 300TD

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We recently bought a 1987 300TD. We love the car, but are not in love with the "automatic" climate control system. Is it possible to override the built-in controller and set the fan speed, temperature, and vent choice manually via separately installed switches, or even rewiring the existing controls to do what I want instead of what the computer "thinks" is best?

-Zeb
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1976 BMW 2002. 1991 250TD. 1995 E320. 2018 Honda Africa Twin
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You do know that you CAN set the temperature yourself, and that you can choose from three (high, auto, low) fan speeds, right?

As far as I know, unless you delve deep into the HVAC system, you can't pick the vents the system uses.
 

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'87 300TD Myna 7mm pump and HX30 /'89 Vanagon TDI 12mm pump and GT2052
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It's kind of annoying that you can't infinitely set the fan speeds, and the center vents are only for AC cooled or outside air, and never heated air. I suppose it would be possible to use a separate vacuum switchover valve teed into the line, in order to to send vacuum to the center vent pod as a manual override.
 

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1990 300E 2.6
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182 Posts
Took me a while to get used to it as this was my first car with climate control. I used to play with the dial all the time, turning it to max to heat the car up faster. I now think it heats up slower if you put it on "max". I've learned to leave it at 70-ish and let it do it's thing with fan speed on auto.
 

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1987 300TD
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58 Posts
Discussion Starter · #5 ·
It's not just that heat doesn't come out of the center dash (the manual says that it will come out intermittently), it's that in heating mode, it's always blowing on the feet. For me, that's a big problem... for my wife, not so much.

The way I look at it, there are three components to the climate control system and modifying any one of them will result in a better experience.

Fan
It seems to me that there is a fan (or fans?) that simply need to be powered to run. Why not rewire them to the three fan buttons on the dash and alter the function of those three buttons to Low, Medium (replaces "Auto"), and High? Have the off switch on the right of the function selector turn off power to the fans (if it doesn't already). Find the fans, disconnect whatever wires go to them (cap them for reversing the mod later), and run new wires to your dash switches while introducing some sort of voltage regulator to control speed - depending on the type of fan.

Vent choice
Perhaps this is the hardest thing to modify, but can't the mechanisms that direct the flow of air to the feet, dash, or windshield be manually powered and wired to the function selector? The function switches could be changed to direct air to (from left to right):
  • the windshield and outer dash vents
  • feet and dash
  • just feet
  • just dash
  • off
I'm guessing that there is some central IC somewhere controlling what vents are open... could it really be that hard to bypass it?

Temp
I'm not sure exactly how the temp selector works, but there must be at least one blending door to adjust the mix of hot and cold air. Can't the temperature dial be wired to control this blending process without the car's computer frequently changing the temp to "maintain" the selected temp?

I will admit that I know nothing about working on cars, let alone old MBs. I guess I'm looking for someone with knowledge of these components who might know if this is at all possible.
 

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1990 300E 2.6
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182 Posts
It's not just that heat doesn't come out of the center dash (the manual says that it will come out intermittently), it's that in heating mode, it's always blowing on the feet. For me, that's a big problem... for my wife, not so much.

The way I look at it, there are three components to the climate control system and modifying any one of them will result in a better experience.

Fan
It seems to me that there is a fan (or fans?) that simply need to be powered to run. Why not rewire them to the three fan buttons on the dash and alter the function of those three buttons to Low, Medium (replaces "Auto"), and High? Have the off switch on the right of the function selector turn off power to the fans (if it doesn't already). Find the fans, disconnect whatever wires go to them (cap them for reversing the mod later), and run new wires to your dash switches while introducing some sort of voltage regulator to control speed - depending on the type of fan.

Vent choice
Perhaps this is the hardest thing to modify, but can't the mechanisms that direct the flow of air to the feet, dash, or windshield be manually powered and wired to the function selector? The function switches could be changed to direct air to (from left to right):
  • the windshield and outer dash vents
  • feet and dash
  • just feet
  • just dash
  • off
I'm guessing that there is some central IC somewhere controlling what vents are open... could it really be that hard to bypass it?

Temp
I'm not sure exactly how the temp selector works, but there must be at least one blending door to adjust the mix of hot and cold air. Can't the temperature dial be wired to control this blending process without the car's computer frequently changing the temp to "maintain" the selected temp?

I will admit that I know nothing about working on cars, let alone old MBs. I guess I'm looking for someone with knowledge of these components who might know if this is at all possible.
Something tells me you should attempt to clean the cabin temperature sensor. I'm a smoker and cracking the window doesn't affect the fan speed or temperature much.
 

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01 G500, 82 300TD, etc
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it is possible.

but you'd be better off importing a euro car with the manual climate control. would probably be cheaper too!
 

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1990 300E 2.6
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182 Posts
where is that sensor? beside the moon-roof switch? any suggestions on what to clean it with?
I've not torn it apart on mine, only read of others cleaning it. It is above the dome and map light assembly which also has the roof buttons. I think I saw someone state using windex??? Try a forum search and you should find the discussions.
 

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'87 300TD, '85 300SD, '84 190D, '81 VW 1.9TD, '93 Chevy 4x4 5sp 5.9L Cummins. '68 Bronco w/4BTA
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108 Posts
Disabled Climate Control

Be careful what you ask for.
I just bought a 87 300TD with disabled climate control. The vacuum line to vents are plugged from engine. Looked, seems the 12v vacuum pump quit and the connection from the engine is plugged at firewall. They just capped the feed line to vent plumbing. No doorlocks. Otherwise car is imacculate.

This car will run you out with HEAT. :eek: I hate it. :surrender:
You have to continue to switch the system off or roll window down.

Do what you want. I am fixing mine.

Wayne
 

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2000 ML 430, 2014 MLBluetec
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307 Posts
Cleaning the sampler air area is easy. The right side of the assembly up there is held in with two springs. Pry them down with plastic wedges and slide assembly to the right-you will see two tabs that stick to the left. Disassemble the big tube, clean it out and then remove the small housing that the hose was attached to. There is a small circle of wire that must be what samples the air. I sprayed electronic cleaner on a q tip and cleaned all the dust and stuff aound the wire and reassembled. I think the whole system is much more responsive. It was obvious it had never been done. The small sampler wire looks fragil so be careful, but it is very simple to do.
 

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About a dozen 1988, 1989, 1990, and 1991 sedans, wagons, 4Matics and 1 coupe
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To the original poster: what you're asking about can, of course, be done but the time and effort would be substantial. Even more so for someone not already familiar with auto temp control systems and their repair.

As the owner of 124s since 1992, I've been pretty happy with the automatic climate control system operation. It's pretty well thought out and works well. If you're unhappy with the system in your TD it may not be operating properly. If that's the case you'd be better off fixing it than trying to go around it.

The ACC system has some weird features (as compared to American cars) that make it harder to do what you're proposing.

For the blower control portion, well, that's not a simple switch, wiring, resistor pack, and motor setup. Instead, blower speed control is accomplished using a transistorized voltage regulator that is mounted directly under the blower motor itself. The ACC brain (which is all inside the control button assembly) looks at outside temp, cabin temp, and the desired temp and then decides how fast the blower should run. The brain then sends a voltage signal over 1 wire to the transistorized voltage regulator.

Discharge air temp is even more unconventional. There is no blend door at all. Instead, the ACC brain monitors heater core temp using 2 sensors and then controls the flow of coolant through the heater core. BMW uses this method too (and probably others). The flow is controlled using the monovalve, an on/off solenoid in the coolant hose to the heater core. The monovalve is located just ahead of the battery. Disconnect it and you get max coolant flow all the time. Try that and see if you like it better.

There's also the auxiliary coolant pump, the fist-sized thing mounted on top of the frame rail just under the coolant surge tank. There's another thread going on this part right now. At idle there isn't enough coolant flow provided by the engine coolant pump (at least on gas engine cars) so the aux pump runs to help out.

And as far as aiir discharge location, well that is handled by vacuum servos (aka pods). The pods have vacuum hoses connecting them to a small bank of solenoids behind the dash just to the left of the glovebox. The solenoids are controlled by the ACC brain. It's fairly common for the pods to develop leaks (they're 20 years old) and then air discharge location isn't what it should be. Ths could also be making your car's system a poor performer.

Lastly, the cabin temp sensor is behind the little grill next to the sunroof switch. It's a thermistor which just means a resistor whose resistance depends on temperature. Being dirty isn't a problem until so much dust prevents airflow past the sensor. Use your vacuum cleaner to suck air past the sensor to clean it.

This brings up the fact that a tube runs from that sensor across the top of the windshield and then down the RH a-pillar to a little fan below the glove box. The fan sucks air past the sensor. At 20+ years old your fan may have quit and that affect the systems ability to get an accurate cabin temp reading.

All these things may sound complicated or exotic but they're not. ACC systems on US cars have had some or all of these features in some form since the mid-60s.

If you PM your snail mail address to me I'll mail you the MBZ schematics for your car's
ACC system.
 

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1987 300TD
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58 Posts
Discussion Starter · #14 ·
Thanks augapfel for the detailed info. I've had a vacuum hose replaced and the system is working a little better now. I'll try cleaning the cabin sensor to see if it gets any more responsive. But regardless of the system's performance, I would still like more control. Perhaps I'll consider Zeitgeist's suggestion for some manual overrides on the vacuum lines to the feet and the center dash. That might be all I really need. I can probably live with the fan speeds and the temperature.

I suppose it would be possible to use a separate vacuum switchover valve teed into the line, in order to to send vacuum to the center vent pod as a manual override.
What would be involved in adding a separate vacuum switchover valve? Would I also have to install separate vacuum pods or solenoids?
 

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1987 300TD
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58 Posts
Discussion Starter · #15 · (Edited)
I'm revisiting this project. I no longer want to completely change the ACC, all I really want is for the center vents to operate anytime the side vents do. Can I tee into the center vent vacuum line with some sort of switched valve that would give me the ability to choose between using the vacuum from the current center vacuum line and the side vent vacuum line? Controlled by a switch on the dash? Something like in the attached diagram.

Would this work, and do the parts exist?
 

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2001 R129 SL500, 2002 W163 ML500
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405 Posts
IMHO: DON'T

Start off by getting the manual on CD then read some of the section I mention in my fun thread '1995 E420 Climate Control problems'.

http://www.benzworld.org/forums/w124-e-ce-d-td-class/1563163-1995-e420-climate-control-problems.html

Specifically, you are interested in:


Section 68 will help when you pull the console..
Section 83 is climate control. Now your fun begins. You will want to look at any wiring diagram for this - there's a quick one at 83-606, but better ones in the ETM.

See that bit labelled Y7 - that's your new best friend. Seven little computer-controlled valves. that use vacuum to open and close your flaps. Pull your glovebox, and it's hiding right there on the left of the hole. Good time to inspect - those rubber vacuum elbows and fittings are known to age. How you decide to re-cotrol is your problem.

The fan is easier - you already have a high and low speed button. So now you just need to get the center button to be 'medium'. Blower speed is controlled by (wait for it) the computer called the Climate Control Module. There is a yellow wire to the blower regulator that is the control voltage for fan speed. 1 volt = low. 11V = high.
You get to pick your medium voltage...and how you deliver it.

The temp dial is a rheostat, and it controls not a flap for air, but a valve for coolant. It fails open (is open without current applied). The computer uses 5 temp sensors to decide what to do.

So, to my mind, you want to junk the Climate control module, reuse the buttons, and make your own computer. Cause you won't do it via cables like the controls of old...

I think I would buy another car before I messed with this. But YMMV, and it could provide valuable info for us.

Good luck!
 

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1987 300TD
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58 Posts
Discussion Starter · #17 ·
betapi, I'm no longer interested in changing the whole system (as I said in my last post). I am only interested in getting heat from the center vents... preferably at the same time as the side vents.

BTW, I've read over section 83, and I actually have the service manuals. In fact, I'm kinda confused because there isn't any mention of the side vents. I'm not sure which vacuum line from the "switchover valve block" (Y7) controls the side vents. Either there isn't one (and they're always open), or they're tied into the defrost vents.

Assuming there exists a "Y" or "T" valve that allows me to switch between two different vacuum lines, then I could see a few different options:
  • I could tap into one of the defrost vacuum lines (5 or 7).
  • Or I suppose I could tee into another vacuum line that always has a vacuum present because the center pod opens when a vacuum is present.
Then I could use a custom switch to switch between utilizing one of these "other" vacuum sources and the intended vacuum source (which is line 4 on Y7).

(I changed the attached image in my last post to better reflect what I'm suggesting.)
 

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2001 R129 SL500, 2002 W163 ML500
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405 Posts
Carry on, you might yet succeed. Many people would like an easy cure for what you ask.

The center outlet flaps are controlled by the y4 part of the Y7 switchover valve block accorting to my ETM 124.83-6.00 wiring diagram. looking at mine from the hole where the airbag was (your glovebox?) there's a handy label - numbered top to bottom 1 to I can't see the bottom. Anyway fourth from the top of the vacuum lines facing rearward is 4.

Sounds like you want them always open, except in defrost. Not sure how to do that yet... what does defrost pull closed, if any?
 

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1987 300TD
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58 Posts
Discussion Starter · #19 ·
I just figured out what I'm looking for. It's called a Vacuum Change-Over Valve. One of these will work. They're used elsewhere in many Mercedes models, but it would be perfect for this mod. An electrically controlled solenoid changes the vacuum from one valve to another. I could run a wire from the contacts on the change-over valve to a button somewhere on the dash or console. When activated, the valve could either use vacuum from the intended source (line 4 on Y7), i.e. no change in function, or use vacuum from another source. The only other parts I would need are a small length of vacuum hose and a tee to tap into another line.

Does anyone know if there would be a problem with tapping into a vacuum line? Would there be enough vacuum available to operate another pod? Are these vacuum lines designed to supply only enough "suction" for their intended purpose, or is there plenty of "suction" available?
 

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93 300E 2.8
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101 Posts
The lack of hot air from the center vents is a safety feature common to many older German cars. My older Audis did the same thing, although my 1995 BMW does allow hot air from the center vents.

Anyway, the theory is having cooler air (or no air) blow on you from the center vents keeps you from falling asleep at the wheel, while having warm air blow on your face can make you drowsy.
 
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