Converting Klimaautomatik to dual manual HVAC control

[500AMM]

Anyone ever seen this done on a 124 or any other '90-era MB?

I have the klimaautomatik in my '92 and I can't say I'm overly enthusiastic about its function. To be more specific - the fan speed blows either full storm or too low due to slow response time to cabin temp variations. It is an air suction inlet up in the panel with sunroof switch, but I'm not sure the temp.sensor sits there as well. If not the sensor must be somewhere in the dash, which means the cabin air is sucked all the way down to reach the sensor and adjust the fan speed to bring the temp up/down relative to the control panel setting. It worked the same way in my previous '94 Limited so I don't think it's any malfunction with the system.

I know the trick with pointing one of the dash vents upwards to blow more directly on the air suction inlet which reduces the response time, but I am really missing the smooth function on the manual dual HVAC system. One way could be to adapt a variabel switch on the wiring to the fan, but I'm not sure how the fan speed resistor handles that. The safest way would be replacing the needed units.

 

[gerryvz]

I believe there are a couple of temperature sensors in the system, not just one. One is at the air sampler fan in the overhead control pod, as you mention. There is another air temp sensor at the blower motor underhood (below the cabin air filters on facelift cars).

The ACC system fan speed relies on several factors, but it is hard-programmed and not flexible. Several parameters go into the fan speed, including the mode (push-button), temperature, whether A/C is on or not on, etc.

I think this idea of replacing the ACC system with the factory "manual" system has been discussed here on the forum and on the 500Ecstasy at various points over the years, and likely on other forums as well. However, it would be a very difficult transformation simply because all of the parts and the wiring involved. I don't know that anyone has ever actually DONE it.

More realistically, as you say, would be to wire in a switch with set speeds to the fan control circuit. But even that (though I recall on other forums that has been done) would be a difficult task.

I will say, I enjoy the "manual" system as used on my G-wagen -- having control of the four fan speeds is a big bonus.

 

[gsxr]

I have the klimaautomatik in my 92 and I cant say Im overly enthusiastic about its function. To be more specific - the fan speed blows either full storm or too low due to slow response time to cabin temp variations. ... It worked the same way in my previous 94 Limited so I dont think its any malfunction with the system.

Arnt, it's not a system malfunction. I have the same complaint about the automatic climate control. I'd be happy with a "medium" fan speed setting.

I know the trick with pointing one of the dash vents upwards to blow directly on the air suction inlet which reduces the response time, but I am really missing the smooth function on the manual dual HVAC system. One way could be to adapt a variabel switch on the wiring to the fan, but Im not sure how the fan speed resistor handles that. The safest way is to replace the needed units.

I recall someone (@LWB250? @sixto? @Jeremy5848 at PeachParts?) investigating this but can't recall details. From memory, the fan speed controller uses a signal between 0.x-6.0v from the pushbutton unit to adjust speed, near zero is low speed, around 6v is max speed. In theory a variable signal would control fan speed to your preference. It would take some experimenting to find an appropriate signal for a fixed "medium" speed.

Anyone ever seen this done on a 124 or any other 90-era MB?

As Gerry noted above - I don't think I've ever seen it done. There are a huge amount of parts needed for this swap, including the entire heater box which contains the evaporator & heater core, converting from monovalve to duovalve, replacing or modifying the wire harness, replacing the automatic fan speed controller with the manual resistor pack, different control panel & wood, different heater hoses in the engine compartment, etc etc. I'm not sure I'd attempt this without a complete donor car.

 

[kwontumspeed]

I feels your pain. When I was installing my dash cam I took time to clean out the sensor at the overhead panel along with blowing compressed air to clean out any debris from the attached line and the response time improved.

 

[gsxr]

On a related note... if you have a "CoolHarness" in your car, which fools the HVAC system into thinking the engine is hotter than actual temp: With the fan on "Auto" speed, after a cold start, the blower will start ramping up speed in winter when calling for heat, before there is any heat available. So you get a nice blast of cold air. The fix is to leave the fan speed on Low until you feel heat available, or remove the stupid CoolHarness.

 

[LWB250]

Besides cleaning the sensor (and I highly recommend removing it from the panel to get it really clean) have you verified that the little blower at the base of the passenger side "A" pillar is working? It's responsible for pulling air into that opening/grille, so if it's not working your temperature sensor will function but will not be very responsive because ambient cabin air isn't being drawn across it.

They are typically dead by this time in the car's life, so it's not surprising to go in and find them non-functional.

Dan

 

[500AMM]

Good advice about cleaning up things!
I haven't checked anything yet on the HVAC system yet, and that small blower at the A-pillar could be dead of course. The car has now sat in storage for years so I guess I have some unexpected fun ahead of me.

I recall someone (@LWB250? @sixto? @Jeremy5848 at PeachParts?) investigating this but cant recall details. From memory, the fan speed controller uses a signal between 0.x-6.0v from the pushbutton unit to adjust speed, near zero is low speed, around 6v is max speed. In theory a variable signal would control fan speed to your preference. It would take some experimenting to find an appropriate signal for a fixed medium speed.

As Gerry noted above - I dont think Ive ever seen it done. There are a huge amount of parts needed for this swap, including the entire heater box which contains the evaporator & heater core, converting from monovalve to duovalve, replacing or modifying the wire harness, replacing the automatic fan speed controller with the manual resistor pack, different control panel & wood, different heater hoses in the engine compartment, etc etc. Im not sure Id attempt this without a complete donor car.

A complete transformation may be a long shot, and the fan speed is actually the annoying part in it. I searched here on the forum before posting yesterday but couldn't find anything. But today I came across a thread at PeachParts which you probably had in mind Dave, you have posted on it:
W124 Manual Blower Control DIY - PeachParts Mercedes-Benz Forum

I'm not sure what @Jeremy5848 ended up using, the OE fader control W or WO an external pot.meter? However, the idea of wiring in a separate pot.meter seems to be the solution.

 

[500AMM]

Based on the wiring diagram in post #1 at the PeachPart thread; is the PBU N22 and the Electronic Blower Control Unit N29 the only control modules for the ACC? Which means no control module sitting in the CAN box?

 

[gsxr]

Based on the wiring diagram in post #1 at the PeachPart thread; is the PBU N22 and the Electronic Blower Control Unit N29 the only control modules for the ACC? Which means no control module sitting in the CAN box?

Correct. PBU N22 sends a control voltage signal (roughly 0-6v) to regulator N29 (aka 'porcupine') which resides underneath the blower motor. There is NO other control for the blower motor. Nothing through the CAN box.

The only HVAC control in the CAN box is the A/C compressor control. The BM/GM power supply module replaces the KLIMA relay used in non-V8 models. The BM/GM and KLIMA compare compressor speed to engine RPM and open the clutch if the RPM's don't match, as a safety.