Hi, I notice this couple days that my auxilary fan is running all the time on my 88 560 sel. And I also notice my AC compressor doesn't engage (run) when the fan is running. However if I pull the plug connecting to the auxilary fan switch on top of the thermostat housing the fan would stop and compressor would engage. I am wondering should I change the auxilary fan switch or something further up stream thanks.

Rick

The fan is controlled by the Pushbutton controller. It looks to engine temp and shuts down compressor in a severe overheat and turns on the fan. Looks like the sensor is bad.

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

Hi, thanks for the reply. It makes sense. I am not familiar with cooling/ac systems. The sensor you are talking about is it the water temp gauge sensor that is right next to the fan switch that also on the thermostat housing? Can I use a voltmeter to test if the sensor is bad? If yes how?

Lastly, just for the knowledge sake if the auxiliary fan is running that means the auxiliary switch itself is ok and vice versa right?

Thank for helping me learning AC/Cooling systems now.

Rick

There are a number of temp sensors and temp switches. It is a two pin sensor. The sensor is a variable resistor. The fact that the fan runs doesn't condemn the sensor, but it sure doesn't make any case for it being good.

It is NOT a temp switch! It is an input to the pushbutton controller that bases its control strategy for both fan and compressor on the sensors interpreted input.

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

On your 1988 chassis , there was a mid-year change on what sensor type was used. The thermistor type SB mentions replaced the earlier 3 prong temp switch, [ 105/115 sw]..which is just a temp sw [ not a thermistor type]

You can check your engine number to see which system you have , but the easy way is to just see if your car has a 2wire/BLUE sensor or a 3 wire/green sensor..

Here is the change engine #
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=2AO123CS42AV18CUHG&year=1988&make=MB&model=560-SEL-001&category=R&part=A%2FC+ACC+Temp+Sensor
My bet would be that you have the earlier temp sensor and it is shorted, thereby giving you full fan and no ac.. and by you removing the connector , the fan stops b/c you have interupted the fan circuit..whereas , if you had the later thermistor circuit, removal of the connector would nor stop the fan b/c the CC panel has a full fan default circuit if the sensor is bad and connector removal would energize the fan.
Check and see which you have...........

If one has the blue thermistor type, you can unplug the connector and bridge the connector with a 1K ohm resistor ..that will give the CC Panel the required input to stop the fan activation circuit..which would indicate a bad/open sensor/thermistor....

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A Dalton

Thanks for the replies. I am sorry that I mistakenly wrote the wrong year for my 87 560sel. 88 is for my 300ce. So here is the green sensor in my car:

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=2AV19LFDZ2AV19LKHY&year=1987&make=MB&model=560-SEL-001&category=G&part=Engine+Temp.+Sensor

In my first post I said I unplugged the auxiliary fan switch. Now I realized it was the green sensor that I unplugged. So Arthur Dalton you nailed it right on the head that the sensor is shorted and unplugging it stops the auxiliary fan and allows AC compressor to start. A new green sensor is what I need.

Thanks for your swift informative replies.

Rick

Very Good...

et al,

This post is a perfect example of the importance of correct model/year in order to get a good diagnosis...one year off and the diagnosis is different just b/c of a production change in something as simple as a factory aux fan circuit sensor modification ..same model, different years..different diagnosis.

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A Dalton