My 1991 300E (3.0) has this A/C problem. The condensor engages. The freon has been checked and is sufficient. The high and low side pressure is ideal. My aux fans do not go on when the a/c is turned on, so I do probably need a new temp sensor for the aux fan.

When I bypassed the sensor and got the aux fans to work, there was still no cold air at all! What part or parts should I be replacing???

I want do make this repair myself and just need to be pointed in the right direction. Thanks in advance for any direction anyone can provide. This site is great!!

Zander300E

To begin with the aux fans are not turned on for a/c purposes by a temperature sensor. They are turned on by a high pressure switch. They are not turned on for the purpose of making the a/c perform better. They are turned on to bring down the high side pressure to prevent system damage.

It sounds as if the refrigeration portion of your air conditioner is working correctly. The cabin temperature is regulated by the introduction of heat. This is done by regulating the heater valve near the battery.

The pushbutton control unit supplies a signal to that heater valve to adjust cabin temperature. If the pushbutton unit sends no signal to the heater valve it is open allowing heated coolant to circulate through the heater core.

That said, don't rush out and buy a heater valve or pushbutton unit without doing some investigation. There are a number of elements involved in the overall ATC system. There are temperature sensors, a fan to draw sample air over the temp sensor, there is a measurement sensor to measure air across the evaporator, the valve, the pushbutton unit and associated wiring.

I think I would start by somehow plugging the coolant line leading to the heater valve. After plugged, drive the car to see if you then get cooling. If so, then it is time to go down this road to determine what is causing the heater valve to open.

Once you determine that this is your problem provide smoke at the little grill near the rear view mirror with the ignition on. The smoke should be drawn into the grill. If not, then find and check the little fan, it will probably be located behind the glovebox.

With those two simple things out of the way, see if there is a signal to the heater valve. You will need to research and see what to expect. I think it is a duty cycle type signal, but I expect that a voltmeter will show up SOMETHING if it is giving a signal. If it is not, the problem is likely to be the pushbutton unit, but investigate as best you can before shotgun troubleshooting and replacing it as an experiment. Also before replacing it, take the back off and heat up the solder joints. There will sometimes be a cold solder joint that is the problem.

Also when replacing the pushbutton control unit check aux water pump current draw. If it is 1.3 Amps or more, replace it, if you don't you will knock out the new PBU.

Good luck,

Thanks for the comprehensive response. Oddly enough, after driving the car a bit (after testing and recharging system) the climate controll seems to work. The real test will be July and August!!

So the aux fans are not supposed go on immediately when the A/C is turned on??

No, the aux fans will come on once the high side pressure reaches a critical point. If you will watch the gauges on a properly or slightly over charged system. you will see the high side pressure decrease rapidly once these fans kick in.

Have a great day,

Well, it was warmer here in Chicago today and I turned on the A/C and, well, not much!! The other day it was about 56 degrees outside and I swear I felt very cool air coming from the vents. Today was 80 degrees and the a/c did not have a chill to it.

I have read much on the board about monovalves. Could this be sticking in my situation?

The 124 chassis cars such as yours do not have the problematic monovalve. The valve itself is different.

There is a little grill next to the rear view mirror. With the ignition on, see if smoke will draw into that grill. There is a little fan that draws air temperature over the sensor. When it quits, the a/c will do all sorts of strange things.

If it does draw smoke, after running feel of both heater hoses. If they are both the same temperature after running the car from cold for five minutes, then the pushbutton control unit may be opening the heater valve. This would be a possibility if the a/c is putting out HOT air, not just less than cold air.

Good luck,

Lots of things to defeat good A/C.

To decide whether one has good A/C get a digital thermometer and place it in the center duct. Set the blower to max and drive a couple miles at 50mph.

Good A/C starts at duct temperatures about 40 degrees below ambient in this test. Now try the test at low blower speed. It will quickly be evident why I choose high speed. Any A/C worth being called A/C will cycle the compressor under these conditions at low blow. Even on the hottest days.

So, now take your experience. You are judging A/C on a 56 deg day. Well the system should cycle before you get out of the parking lot. No way to test as the cycling point is probably 40-45 degrees. So if you did my test and the A/C was working you would only get 10-15 degrees off Ambient. On the 80 degree day you still couldn't be sure as minimal A/C (40 degree drop) will get you to the cycle point.

Now if its 100 out and you only get 40 degrees off of ambient duct temp will be 60 and there will be no cycling as the A/C is not getting to that point. To determine the cycling point at that point one can reduce the blower speed to "Min". Decent A/C will cycle at low blow during my test.

If you use a digital thermometer for this testing the cycling will be easily viewed as a rise and fall of center duct temps by about 2-5 degrees F. other thermometers aren't fast enough to view this.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Here is more direct info to shed some light on my situation...

Today the temp was around 80 degrees with 60% humidity. On hifh, the a/c blowing was around 70 degrees ambient. The compressor is engaging when turning on a/c. The high side pressure was between 150 & 200 on the gauge, with the low side at about 35.

However, when turning the a/c on, the low side hose does not get cold, but the high side is hot. Could it be overcharged?

Any ideas???

Sounds low on refrigerant to me..
You are not getting low fan b/c the trip pressure on that is 20 bar [ 290 psi].. high side
At 80 ambient , you should be close to fan trip..and you know fan circuit is healthy when you jumpered the high pressure sw.
Check the eye site on the reciever/drier... slightly low freon will result in some cooling , but not enough for higher ambient temp and high side will not get to fan trip pressures...
One of the first indicators /complaints of low freon is no aux fan
and some cooling..the fan circuit gets the blame , when in reality it is a low freon charge that is not allowing system to reach the fan trip spec....

After posting many questions on the forum, flirting with using commercial refrigerant (and thankfully NOT damaging my system), I evacuated the system and checked for leaks. After finding none, I charged with r-12 (not cheap) and it is COLD now!!

I am finding though that when needed, the two aux fans DO NOT come on to cool off the system. What are the likely culprits of this predicament???

Again, unless you have excessive high side pressure the aux fans will not come on. You need manifold gauges so you can monitor high side pressure.

If it is cooling well and the high side pressure is not reaching this point, that is a good thing.

Have a great day,

We only see cool times part of the year but it is my experience that the fans come on low speed (due to A/C pressure) very shortly after the A/C comes on. Usually before the engine warms up. This doesn't happen in the winter but when its 90 out the fans should spend most of the time on.

To verify the fan function go to the reciever-drier and view the two switches. One has short wire pigtails and the other doesn't. The one with the pigtails is the pressure switch. Separate the switch from the wires and put the wires together (electrically). The fan should run on low speed. If not check the fuse the ballast resistor and the relay in that order. Actually by=passing the relay to verify thatthe fans work is a good idea unless they have been seen to work on hi speed.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician