86 300E (W124)

Greetings from the Valley of the Sun!
Warning: This message may self destruct from the repugnant heat and excessive sunlight shortly after viewing.
Some of you may recall that I purchased an '86 300E back in December last year. Over the past few months I've been curing some minor ills it had at purchase time. To review; I've replaced the fuel injectors, O2 sensor, fuel tank sending unit, flex fuel hose (pump pressure side), right inner tierod, cruise control module, windshield, radiator expansion tank, valve cover gasket, spark plugs, fan clutch, plus all the necessary adjustments, filter and fluid changes. Additionally, I had the A/C recharged with some R12 at a local shop and recently removed the blower fan motor and cleaned the noticeably plugged evaporator. This leads me to the issue at hand. Let me preface it with a reminder of the daytime temperatures here in Phoenix (113-116 degrees F).
That said, on to the problem. Directly after the R12 was added to the system, I've been experiencing compressor "cut out" at idle or in traffic. I'm surmising it's because the head pressure is too high and the switch is doing it's job. Though I ask myself and you all; why? What is causing the compressor to "cut out" in traffic or while stopped? If I turn off the A/C in traffic and wait until I'm moving again to turn it back on, it performs normally. When the compressor cuts out, it will not come back on until the engine is turned off and restarted. If I do this in traffic, the compressor repeats the "cut out", if I do this at the on ramp to the highway, it remains on. During all of this, the engine temperature remains at about 85-90 degrees C and does not fluctuate greatly. Recently, I drove from Phoenix to Denver and back (1922 miles) and had no problems with the A/C with one exception which I'll mention later.
Being a man of reason, I'm thinking I may have two problems; one being the fan switch, which is mounted on the "high side" is inoperative. I have shorted the two wires from this switch to run the fan at all times. If the fan is off in this heat, the compressor "cut out" problem increases it's frequency. The problem does seem to be mildly better (less frequent cut outs) since I cleaned the evaporator. The second probable cause could be the engine "under cover" which is missing. I believe this is allowing the reflective heat (145 degrees F) from the asphalt to mix with the incoming air across the condenser, thus exacerbating the problem with an influx of heated air. I'm now on the prowl for an engine "under cover" in hopes this will add another mend to the process. Any thoughts to this complicated issue would be greatly appreciated.
Another concern I have is the vents. I have no center vent air volume, only the side, floor and defrost vents. This I believe to be the vacuum servos, but I'm not sure as I haven't delved into the "under dash" area of this car. Only a short time ago (before the heat arrived) I used to hear one of the plenum doors slam shut or open (whichever the case) a few seconds after starting the vehicle. This noise has stopped. Another noise that comes and goes is a "whistling" sound coming from the glove box area as soon as the key is turned on. The engine doesn't have to be running and the climate control can be either off or on. If the ignition key is turned on and back off, then on again, the "whistle" will sometimes go away. It does however always go away after a short run time, say from the driveway to the street corner. Come to think of it though, the "whistling" has disappeared recently. Possibly because I cleaned the evaporator? I don't know. Anyone versed in the air distribution of this W124 that can offer some assistance would be greatly appreciated.
Thank you for your time it took to read this and thank you in advance for your response.
Blessings, Michael Bonville '86 300E
"The best, the most exquisite automobile is a walking stick; and one of the finest things in life is going on a journey with it" ~ Robert Coates Holliday
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I would first suspect R12 overcharge. Slower moving makes shedding the heat in condenser a little bit difficult. That's why the front fan is important to the AC function. Higher temperature means higher pressure. However, I thought the high pressure cut-off is enough for blockage, not for high temperature. The usual outcome from high temp is not enough cooling.
Another thing you want to check is the belt and pulley. The system compares engine speed and compressor speed. If the difference is something like 20-30%, the AC relay assumes the compressor is seized. To protect others on the same belt (for example, alternator, water pump, etc, I am not familiar with this engine though), the relay will disengage the compressor clutch. Engine must be turned off to reset the AC relay. For MB cars in 80s and early 90s (I am not familiar with newer cars), if you MUST turn off engine to regain AC, #1 reason is slippery or loose/worn belt.
As to the noise cover under engine, I think its effect to heat is minimum. MB designs it for noise, not for aerodynamic nor cooling.
Michael wrote:

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First thing I would check is your aux fan... is it working when your car is idling? Does it come on at all? If not, I would check the fuse for it... along with the fuse on the relays... you will have to remove the fuse cover to access that relays... there should be two of them. If you do not have the relays, the I would suspect the thermoswitch that controls those fans.
As for center vent, is all the air going to defrost vent? if so, yes, then it is the servo that controls it... should be accessible by the glove box... once removed.
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