1986 300SDL runs hot, air conditioner intermittent

My 300SDL is running at 100 most of the time - even a little hotter when climbing. There was a new thermostat installed, but the car continues to run at 100. The overflow radiator cap was replaced because the old cap had
broken apart - rubber hard, spring broken. The new cap, purchased at Autozone, is supposedly rated at the same pressure as the original. The overflow was originally filled too high and I have been losing antifreeze out the overflow tube, but the coolant level is now where it should be. Three years ago, both the radiator and auxillary fan were replaced.
The air conditioner is fully charged with freon but only works intermittently. I was told I needed to replace the kick down relay and that would fix the problem. I purchased and installed a new relay, but it does not act any different. The compressor comes on for maybe a minute or two then switches off.
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These engines' cooling systems are quite sensitive and one can not skimp on their maintenance. Overheating this motor leads to blown cylinderhead gaskets and possibly worse, a cracked cylinder head.
You say the radiator and auxiliary fan were replaced three years ago. Does the auxiliary (electric) fan work now? (It should run at 100+ degrees C. engine temp.) Has anyone checked the engine's belt driven fan's viscous clutch?
Given that the engine's temp. tops at about 100 degrees C. I suspect the electric fan is the one providing the cooling and the belt driven fan is lame.
I believe the A/C compressor has a low refrigerant pressure switch cut-out, others here may know more detail of that. You say the A/C system is fully charged - how do you know that? Was this A/C system ever converted? If so, what hardware was converted along with the "new" refrigerant gas?
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The fan does run now - it had a bad switch which was replaced; however, the temperature still is the same. No, the viscous clutch was not been looked at. What exactly am I suppoed to look for?
My husband has a gauge he hooked up to the high pressure side to see if the system needed charging. The A/C was converted several years ago. There were old connectors on the hoses. My husband replaced them the other day in order to connect the gauge to see how much freon was in the system. The gauge indicated it was okay.
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The belt driven engine fan's viscous clutch engages the fan as the engine temp rises or allows it to idle when the motor is cold. That's the principle but I, alas, can't tell you how it should be tested. Someone else needs to help you on that one.
My limited understanding of A/C systems charge is the high pressure vs. low pressure sides of the A/C compressor, not just one side.
An A/C conversion should have included changing the systems access valves from R-12 to the new ones. Conversions often also involve changing the receiver/dryer to an accumulator and changing a safety switch thereon. I'm also aware that R134a charges differ in quantity from R-12 charges. There are some web sites that explain conversions from R-12.
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The viscous fan senses the temp OF the radiator. Poor water flow thru the center of the radiator will never cause the fan hub to work.
In reality, the viscous fan is only needed at idle and low speeds.
Does the engine run at 100C + going down the highway? If yes, it is not a fan problem. You have one or more problems: A plugged radiator [inside], does not matter that it may only be 3 years old. A plugged A/C condensor and/or plugged radiator with bugs [blocked air flow]. Or a cracked head/bad head gasket. [water coming out the overflow with a new cap is a sign of this]
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When it's cold it's floppy and will spin easily. When it's hot it should be noticably harder to turn.
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Alot of time on our old W126, the temperature display are faulty... that they are not accurate anymore. At one time, I switched instrument cluster to find that the newer one shows temperature normal... whereas my old one shows high.
Changing the temperature sensor for instrument cluster will help a bit too as they do go bad too. Best bet is to use a infrared temperature sensor and measure the engine temperature at the thermostat housing for accurate reading.
As for the compressor... you probably need to change out the pressure sensor... which requires evacuating the freon.
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Good points.
Also, the obvious - the thermostat! It should open at 80 degrees C.
Was it changed with the radiator?
Generally, the fans push air through the radiator when the car is moving slowly or stopped. So if it heats up then the fans are suspect. However, at speed there's plenty of air flow so the fans are moot and poor coolant flow is shown by a hot motor.
All that said, the engine's temp. WILL climb when hauling up a long grade - in any car - due the high fuel burn and subsequent BTUs that need to be shed. Normal.
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This poster changed everything and yet still show high than normal temp. I been through this already... changed three thermostats of three different brands and same darn thing.... changed the radiator... same darned thing.... changed temp sensor... same darned thing... changed fan clutch to newer version... same DARNED thing!
One other thing you can do is back wash the radiator and ac condenser with water or air to push out the bugs.
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This is a long shot ,there is a coil inside your bottom radiater hose thet keeps it open so the water pumps suction won't close it . it may have fallen out when you worked on the radiater.
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Nope... new hose too... no kink at all.
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You probably didn't see my earlier post... I stated that when I switched instrument cluster... the temperature gauge on that one shows normal temp. Almost like 8 degree difference.
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