Hi There,

I posted a question yesterday regarding an electrical fault with my car, an 04/1986 Mercedes W124 300E, but it seems to have gone astray, (or perhaps I didn't press the right buttons and it didn't go anywhere) as there is no recognition in the 'My Questions section that I have posted anything.

I don't have time at the moment to go into the background detail I posted yesterday again, so I'll be brief.

Would a faulty temperature sensor cause the automatic temperature controller in my car's Air Con system to not produce the correct voltage, to send to the compressor cut-out controller, via the pressure switch, to activate the relay?  I'm only getting about 80 mV at the pressure switch (open circuit).  What voltage should I be getting?

Thank You

Best Regards,

Gary Hope


A fault in the AC system or any of the many sensors in that car can cause the AC system to turn off. I don't know the exact voltage by 80mV is not correct. There is no power going to the pressure switch.

What you should do is read the codes via the instrument cluster.

See instructions here: http://www.mercedesmedic.com/ac-air-conditioner-diagnostics-with-trouble-codes-list/

Once you read the codes, clear them and see if your AC system comes back to life.

Thank you for your comments above.  Really appreciated.

Trouble is I'm pretty certain that my car, being a 1986 vintage W124, (not a 1996 E Class) doesn't have a code type diagnostic system.  (I had to enter something in the entry fields of the 'Question' page, in order to allow my question to be posted, (discovered that was why I lost the original), so I entered the earliest model I could find on the available list, which was the 1996 E Class, just to get in - perhaps a little bit of deceit there - as a means to an end.)

As far as I'm aware, the 300E only sends 'diagnostic codes' to the diagnostic terminal in the form of a duty cycle, and this only for fuel/air ratio (lambda).  Other pins on the diagnostic socket give the TD signal frequency, (which can be converted to RPM), and O2 sensor voltage output.  (Please correct me if I'm wrong, as this is only my very limited understanding at this point.)

Since posting my question, I have managed to track more ac circuits and have taken some resistance and voltage readings on the 5 temperature sensors present on my system.  These readings were taken at the Temp Controller terminals.  Sensor supply voltage was around 7.0V.

Each sensor, bar one, delivered a resistance reading of between 5 and 7 kOhm and 2.0 to 3.2V.  Ambient temperature at the time was around 35 deg C (95F).  The odd one out was the outside air temp sensor, which returned readings of 1.8 kOhms and 1.6V, well out of the ball park of the other 4.  The car was outside, but in probably 80% shade at the time, and I thought the ambient temperature at each sensor should have been roughly the same.  I imagine each sensor would be a NTC type, with roughly the same temp vs resistance characteristic, so I'm wondering if this sensor is faulty, and if this is causing the controller to shut down the compressor.  Your comments would be most welcomed, and appreciated.

It would be handy if I had a characteristic graph for the sensors, so I could see how they should be performing.  It may be that they are quite sensitive, and present a fairly high variation in resistance for only a small change in temperature,and that the sensor is ok.  (This sensor is the 'outside' air temp sensor, - don't even know where it is located at this stage - so it is quite probable that the outside air temp could have been a little warmer than the inside temp, although it didn't seem so at the time.) 

I would really appreciate any advice as to where I could get some info on these sensors, if you are able, thanks.

Really appreciate the advice so far, thank you.


Best Regards,


Gary H.

Plain text

  • Allowed HTML tags: <a href hreflang> <em> <strong> <cite> <blockquote cite> <code> <ul type> <ol start type='1 A I'> <li> <dl> <dt> <dd> <h2 id='jump-*'> <h3 id> <h4 id> <h5 id> <h6 id>
  • No HTML tags allowed.
  • Lines and paragraphs break automatically.
  • Only images hosted on this site may be used in <img> tags.
The content of this field is kept private and will not be shown publicly.

Disclaimer: Youcanic has made every effort to assure the accuracy of the information on this page. Neither Youcanic nor its affiliates assume responsibility for any loss or damage due to any errors, omissions, lack of information. You agree to hold Youcanic free from any liability arising out of the use of any information contained within. All pictures and references to trademarks and car names are for reference only and do not imply an association with the manufacturer. Consult your owner's manual or authorized factory manuals when performing repair procedures. By entering this site, you agree to our Terms & Conditions and Privacy Polity.