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alwong 08-17-1999 02:17 PM

I drove my wife's 300sd for the first time in months, and noticed the rpms drop from around 750 to 500 rpms witht he A/C turned on. Is this normal? I don't recall this ever happening, and it doesn't happen on my 190E.

When the rpms drop, it idles a bit rough, and appears to be lacking power when accelerating from a complete stop.

Any suggestions?

Al Wong
83 300SD
85 190E

Benzmac 08-17-1999 10:27 PM

Make sure the aux fan is working. If the fuse is blown and the aux fan does not come on, the pressures can be way high and cause the sluggish engine.


Joe-1 08-18-1999 07:39 AM

I have a simialar problem with my 134 converted 190 2.3-16. When I start the car for the first time and run the a/c, the idle is just fine until I shut off the engine. The next time I start the car, it idles really rough and seems to hunt. I asked about the pressures for high and low side and few weeks ago. The aux fan comes on and the compressor engages. The car has no problem cooling when the outside temp is 95 degrees but once it climbs to maybe 100, it cant keep up the cooling. The car was filled to the 2.2 lb capacity. I think it is overfilled. SHould I release a little bit? I have heard that the system should only be filled to 85% capacity or 1.85 lbs.
Thanks, Joe

Tobias MB 08-18-1999 11:10 PM

You are correct in that the pressure is probably too high. Most shops and a dealer in my area only fill to suggested capacity LESS .3 lb. as the Nippondenso compressors are fragile to say the least, and too much will kill one and too little only cools a little less! Your choice, be cool or pull out your wallet!

Try the WaterWetter product in your radiator also to cool it better as the AC condensor
(in front of the radiator) will feel the
radiance given off.


Joe-1 08-19-1999 07:12 AM

I did a high and low pressure check and the readings were as follows: about 225 on the high side at idle and 80 degrees outside air and between 35 and 45 on the low side. Does this indicate an overfilled system??
Tobias, thanks for suggestion.

alwong 08-19-1999 06:04 PM

Okay, here is a dumb question....

Where is the aux fan?

I only see the fan on the water pump.

Al Wong
85 190E
83 300SD

metricman 08-19-1999 09:23 PM

Your auxiliary fan is in front of the condensor and radiator. It should spin freely by hand, and is turned on by a switch on the receiver drier (a tank with 2 switches). The switch with the pigtails controls the fan, the other is the low freon cut off switch. On older cars this fan does not come on as often as newer cars.


Tobias MB 09-29-1999 02:03 AM

The reason the old cars don't come on as soon as the newer is because the pressure switch on the old ones are green in color and come on around 290-300#. The new cars are red in color and come on around 250-260#
which is a lot better. If your car runs hot then replace the switch with the two pigtails on it at the top of your drier. You will have to pump out and recharge the system to do it.

300SE, 300CE,190/5.0

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