My daughters 1988 190e temp gauge hit 120C and the aux fan didn't come on. After doing some research, it appears that there is no aux fan turn on for hi temps, only A/C cutoff at 130C???
isn't 130C a little above the red mark on my temp gauge? Why on earth would mercedes design a A/C cutoff switch that engages after the temp indicator hits 120C???
Curious and Confused in Cleveland.

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CCC,

That sensor is listed as a/c cut-off, but it is also the 105C high fan cut-in .. It is actually the thermistor that feeds the ac panel and that contains the switching temps interpretation.

Make sure you have that sw, b/c '88 models used two different types in that year [ it will be a blu/2 wire sensor]..if it is green, it is a mechanical type sensor and the test below does not apply.

Anyway, if it is the blu one, unplugging it will default the fan into high fan..
That is first High Fan circuit integrity test........

first High Fan circuit integrity test........ FAIL fan aux fan in front of condensor did not turn on. The fan will go on when the A/C hi-press sensor tells it to turn on,,, that I know,,, Wait,,,, I have both a blue and a green,,, along with a red. I unplugged the blue (as seen in the photo) is that the test???

disregard the unplugged red, as I just did that so you can see the red sensor,,,

Attached Thumbnails

 

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Okm
Next test is aux fan fuse ..
Next is bring 12v over to the big Resistor behind the left headlamp with a jumper wire from Battery. Jumper from bat + to the 2 wire terminal on the Resistor..[ looki for burnt wires here at same time]

Okm
Next test is aux fan fuse ..
Next is bring 12v over to the big Resistor behind the left headlamp with a jumper wire from Battery. Jumper from bat + to the 2 wire terminal on the Resistor..[ look for burnt wires here at same time]
Does that test get you high fan????????

Arthur shouldn't he jumper the connector for the blue switch to get the aux fan to kick in? Does it work like the red switch for the big fan? Just curious, is it a different type of switch?

No..........b/c it is not a switch..it is a Negetive Coeficient Thermister..which is just tech lingo for a resistor that changes value with temp change.
So, if the CC sees a high resistance value in this sensor , it does not trigger the fan circuit..but if it sees an OPEN circuit [ as in unplugged] , the CC defaults to FAN ON automatically. [ High temp protection in case of sensor/wiring fault] That is why we use the unplug sensor test to check the high fan circuit..we are simply putting the CC into DEFAULT funtion..simplest test there is for high fan circuit integrity..if that test fails , we start looking at fuses/relay/ resistor/etc..but we START right there w/that sensor....
If it were a regular temp sensor , [mechanical type sw ] you would then have to jumper it to turn on the fan.
This type circuit is just the opposite. It triggers a transistor in the CC at 285 ohms [ which eguates to 105C ] OR open circuit.

If it's a negative coefficient thermistor, then the resistance DROPS with higher temps. Unplugging it would increase the resistance to infinity, so maybe i need a 100 ohm resistor across the leads,,,

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Go ahead ..that will also trigger the CC ..but not needed with the default function.. The infinity reading of an unplugged sensor is an OPEN CIRCUIT..and an OPEN CIRCUIT is detected by the CC default circuitry.
If the fan does not come on w/default , you problem is fuse/relay , or wiring to the fan..

No dice,,,, when the two leads on the evaporator/drier are shorted, the aux fan turns on, when +12V is applied to the resistor, the fan turns on,,, BUT when the blue sensor in the picture is unplugged, nothing happens, when the blue sensor is shorted out, no aux fan,,, I did notice some clicking when the blue sensor was hooked up or disconnected or shorted,,, so something is happening

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You don't get it..

When you jumper the leads on the drier, you are jumping the LOW SPEED fan circuit..NOTHING to do with the coolant temp circuit. All you have done is verify the LOW FAN circuit.

..and what does evap/drier mean??? the evap in in the cabin/dash and the dries is at the condenser in front of the car..
I assume you mean reciever/drier.
Now, lets talk HIGH FAN,,OK?
You have a relay clicking when disconnecting the blu sensor..that means the coil side of the circuit is fine , but the contactor side of the relay has NO POWER ..you have a BAD HIGH SPEED FAN fuse or wires going to the RESISTOR.
THe Battery 12v jumper to R15 has verified FAN MOTORS and the clicking relay with sensor has verified HIGH FAN relay activation..so, there is NO POWER going thru the relays LOAD side b/c you have none. FEED FUSE is where to look. If that is OK , then you could have bad contacts in relay.

So,

Thank you Arthur, with your help, I was able to figure out that the relay with the blue painted top, which sits in front of the fusebox, was bad. Guess having a stuck aux fan caused the fuse inside it to melt. Interestingly, the plastic of the fusebox by the number 3 is a little glassy, as if the plastic got a little hot, wondering if it took that much power to fry the relay.

Well anyway, for now, I just cut pin 87A off of an "ice cube" relay, using that, the fan works. I kinda figure that the blue painted top relay is a little beefier to handle the current of the fan in high speed. On a side note, the aux fan relay listed here at mercedesshop isn't the correct one for my car,,, according to the picture. The blue top relay that my car has, does not have a pin at 87A AND there is no space for a pin to go into on the relay holder.

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Very Good..
Any relay that will handle 30A is fine .
I have had good luck with a standard 30A from RS that is short $$$ that I use on different access. stuff and have never had one go bad...
Just make sure the main contacts [ switch section of relay] are rated 30a and the coil [ solinoid section] is rated 12v ..
Remember , a relay is a 'Switched Switch'....it allows a low currect sw to switch On a high current sw. That's it's main function...allowing a large load to be comtrolled ON/OFF with a low load sensor/switch of some sort...