1984 300SD a/c fan speeds / operation - PeachParts Mercedes-Benz Forum

300SDiesel · #1 09-15-2018, 01:36 PM

I am looking for guidance on how the cc system should work. I have had all three of my cc components rebuilt by “James dean”, great guy, easy to work with.

I have two issues that I am attempting to figure out.

1. The speed of the blower seems to be low when the system is sending air through the center vents. The blower is changing speeds, just not very high, a gentle breeze is what it feels like. Should the system in Auto mode kick the fan to “high” or near to it when the interior is hot? It has been really hot and humid in Alabama this summer! If this is how the Germans designed the system, then I will live with it.

2. When the system senses the interior is reaching the temperature set on the temperature wheel, the system closes the flow to the center vents(with a clunk) and then directs the air flow to the defrost. An interesting side note, when the air is coming through the defrost, the fan speed will increase to a higher speed than when the system is using the center vents. When it is humid and cool in the morning, the windshield will fog up from the cold air the ac is putting out. Every now and then the system will switch back to the center vents.

The only system in the car not looked at is the solenoid group of 6, could that be the issue?

Thanks for the assistance, I really appreciate it.

Clemson88 · #2 09-15-2018, 06:43 PM

Seems like the center vents are fresh air only.

I wonder if there's any chance the vacuum pods are not properly functioning and putting the main flow of air to the wrong locations. Sometimes those things get so bad that you can only get a defrost air to blow.

Diseasel300 · #3 09-15-2018, 11:14 PM

The center vents in the dash are conditioned air. Not sure where the idea came from that they were fresh air only.

When in cooling mode, the center vents should always be open, even if the fan is ramped all the way down and the monovalve is cycling.

When in "Auto", the fan should ramp to the highest setting if you have the temp wheel turned to "Max" cold. If you're in the white area, you can expect various different behaviors depending on the temperature in the cabin and the exterior temperature.

The "clunk" you hear when the temp is being reached is probably the "diverter". When the temp is being reached in the cabin, the diverter is moved so that more air can flow through the heater core. If you have a vacuum leak or a bad vent pod somewhere, you can expect some bizarre behavior. The default with no vacuum is to go to defrost.

Make sure your aspirator fan motor works and that the tube to the temperature sensor is intact. If either one is kaput you'll have poor temperature regulation in the cabin.

Clemson88 · #4 09-16-2018, 12:30 PM

Quote:

Originally Posted by Diseasel300

The center vents in the dash are conditioned air. Not sure where the idea came from that they were fresh air only.

When in cooling mode, the center vents should always be open, even if the fan is ramped all the way down and the monovalve is cycling.

When in "Auto", the fan should ramp to the highest setting if you have the temp wheel turned to "Max" cold. If you're in the white area, you can expect various different behaviors depending on the temperature in the cabin and the exterior temperature.

The "clunk" you hear when the temp is being reached is probably the "diverter". When the temp is being reached in the cabin, the diverter is moved so that more air can flow through the heater core. If you have a vacuum leak or a bad vent pod somewhere, you can expect some bizarre behavior. The default with no vacuum is to go to defrost.

Make sure your aspirator fan motor works and that the tube to the temperature sensor is intact. If either one is kaput you'll have poor temperature regulation in the cabin.

I've owned only three MBs, a 82 240 and a couple gen 1 w126s. Neither had working air so from my narrow perspective only fresh air came through the center vents. I didn't mean to mislead anyone.

Diseasel300 · #5 09-16-2018, 01:08 PM

Fresh air will come in the center vents when the flaps are open, but when the A/C is running, the air will be conditioned. It would be counterproductive to have conditioned air blowing out the side and ambient outside air blowing through the center vents.

When in heating mode, the center vents will tend to close with most of the air going through the side vents and partially through the defrost nozzle.

300SDiesel · #6 09-16-2018, 03:02 PM

Thanks for the information. I will check the aspiration fan and the gang of 6 to make sure there are no vacuum leaks.

On the blower speed, even in the MAX cold setting, the blower takes its time to get to a gentle breeze, never high. Unless I force the fan to high via the switch.

Thanks again for the insight on how all of this works.

vwnate1 · #7 09-16-2018, 03:07 PM

"aspirator fan motor"

? Is this different than the HVAC motor ? .

Diseasel300 · #8 09-16-2018, 03:14 PM

Not sure if the pre-86 cars have it or not, the '86+ cars do. It's a little plastic fan motor with a tube on it that connects to the temperature sensor. Lives behind the glovebox.

vwnate1 · #9 09-16-2018, 03:22 PM

Thanx ! .

My Klima II equipped cars don't have this .

resto108 · #10 09-18-2018, 01:05 PM

The free online factory service manual has a whole section about how to ops check the climate control system.....

SD Blue · #11 09-18-2018, 04:03 PM

Here's a link to the FSM:
https://www.startekinfo.com/StarTek/outside/12253/program/climate_300SD.htm
https://www.startekinfo.com/StarTek/outside/12253/program/ETM_300SD.htm

300SDiesel · #12 09-18-2018, 10:29 PM

Thanks for the links, however they show the server is down at the moment.

For a test I unplugged the auxiliary water pump. Well I must have unplugged the line going to the mono valve as I unleashed the fires of hell heat in the vehicle. So now I have a/c coming from the center vents and lead melting heat coming from the defrost and the small side vents. I will reconnect the line that I disconnected and will see what happens.

I will test the gang of 7 this weekend and see what works and what doesn’t.

Thanks again for the assistance.