AC Fan Speed Starts/Stops - Mercedes-Benz Forum
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post #1 of 32 (permalink) Old 07-03-2019, 05:58 PM Thread Starter
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Vehicle: 1999 E320 Wagon: 130,000 miles
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AC Fan Speed Starts/Stops

The air conditioner fan on my 1999 E320 Wagon (base, not 4Matic) runs at high speed when car is idling or the power load is reduced. Once I accelerate or if a slight positive incline puts a load on the engine, the two side vents seem to not be affected, however air volume from the two central vents is greatly diminished, nearly to no air. The fan noise stops as well. Once I start to slow down or load is reduced by road conditions, the fan returns to full speed. Is this normal; does the AC fan stop when the motor load is increased? Ive added the information from the AC codes in case they may be useful. I followed directions as below:
Preparation for Test

2. Ignition: ON
3. Press Auto .
4. Set both temperature selectors to 72F.
5. Press Rest for > 5 secs.
6. The left display will alternately show the number 1 and the in-car temperature (e.g. 72F)
7. By pressing Auto the next highest test step is displayed (see table).
8. Press Rest to end test program.

And copied the sensor readings as shown in attachment. Conditions for readings are below:
1. Before the car was started; has not been started in 2-3 days; key in position 2.
2. About 1-2 minutes after number 1 above; car idling.
3. After driving car 10-15 minutes in city traffic; car idling;
4. Turned car off.
5. After sitting in hot parking lot for 10-15 minutes; key in position 2.
6. After driving 10-15 minutes, car traveling around 40 mph.

Thanks for your help.
Attached Files
File Type: pdf Mercedes AC data 1.pdf (109.2 KB, 16 views)
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post #2 of 32 (permalink) Old 07-03-2019, 06:18 PM
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What a/c fan are you talking about ? The electric fan under the hood or the blower fan ?

The airflow change is due to a bad vacuum supply somewhere. It appears that you either have a crack in the vacuum line somewhere between the engine intake manifold and the climate control switchover valve, or a bad vacuum reservoir, or a bad check valve.

When the engine has load like when you accelerate, the vacuum is reduced at the intake so there is insufficient vacuum for the climate control airflow control mechanism to operate when you have a leak. To address the changes in vacuum levels your car has a vacuum reservoir which supplies the vacuum needed when the engine is under load.
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post #3 of 32 (permalink) Old 07-03-2019, 07:33 PM Thread Starter
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It is the blower fan. The electric fan under the hood works fine. Would you think I would be getting "check engine" light on and codes when scanned from a vacuum leak?
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post #4 of 32 (permalink) Old 07-03-2019, 08:32 PM
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Not really. Not when the vacuum leak is small. It is possible that the vacuum lines all the way to the vacuum reservoir is fine, but you have a leak between the reservoir and the climate vacuum switchover valve. You need a hand held vacuum pump to isolate the segment.

STAR TekInfo

In the above link, figure 1 (applicable to your car), the vacuum lines for the airflow control start from the intake manifold port at the back of the engine, and at "7", it is connected to the switchover valve which is located behind the glove box, beyond cabin air filters.
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post #5 of 32 (permalink) Old 07-04-2019, 07:20 AM Thread Starter
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MRBOCA. Thank you for all of your help. I have a hand held vacuum pump. Would you please let me know a few descriptive steps to start for the search for this leak? Also, are there any concerns with the sensor data in the attached file?
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post #6 of 32 (permalink) Old 07-04-2019, 08:02 AM
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No concerns. Looks good. Note the change in the blower motor control current in Item 10, see how it changes. The blower motor speed is adjusted based on the current conditions of outside / inside temperature readings, setting of target temperature etc. ONLY in AUTO mode. If you set the manual mode with -/+ blower control settings, the value should be more or less the same. The blower motor always sounds louder when in recirculate mode.

Regarding the vacuum leak, have a look at a recent thread in identifying where the problem is. The guy did a great job in diagnosis.

https://www.benzworld.org/forums/w21...ac-heater.html

BTW. Your compilation of the sensor values is also excellent, and well detailed.
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post #7 of 32 (permalink) Old 07-04-2019, 09:16 AM Thread Starter
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Thanks for the help. Enjoy the 4th.
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post #8 of 32 (permalink) Old 07-13-2019, 03:10 PM Thread Starter
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I am trying to determine where the vacuum leak is for the a/c vent controls. I removed the glove box and plastic covering under the glove box and do not see any thing accessible to check for the leak. I looked at a 1999 E320 Sedan at a junkyard to see if I might be missing something and the valves/controls appeared to be under the center of the dash with removal of the dash and passenger air bag required. I also removed the passenger fender cover on my vehicle and checked the vacuum reservoir and it is fine. I checked the lines from the reservoir to the engine manifold and they all are good and will hold a vacuum from hose at engine manifold to the reservoir outlet (going to cabin). I tried to pull a vacuum on the line from the reservoir heading into the cabin and no pressure could be obtained, appeared to be completely open. My question does anyone have a suggestion to find leak. Is the switchover valve beyond the cabin air filters accessible without removing dash (a huge job with a certainty of many broken (very difficult to replace) plastic parts)? Any help/guidance appreciated.
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post #9 of 32 (permalink) Old 07-13-2019, 04:13 PM
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post #10 of 32 (permalink) Old 07-15-2019, 04:54 AM
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first thing to check is to make sure the whole system is not leaking through the main tube.
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