Question on how to troubleshoot a minor irritation with my A/C. Car is a 99 E300DT. During daytime operation on a sunny day with windows up and the thermostat(s) set at 75, the car pumps out TONS of cold air. It's definitely way colder than 75. At night, temperature controls appear to be more accurate - producing warm airflow. Is there a sun sensor or something that might affect the day vs. night control of temperature? I'm baffled as to why there would be a difference if the system works correctly at night? I know how to access the diagnostics on the climate control, is there something specific I can look for that would indicate a failure somewhere?

Yes, the climate control system most definitely considers the outside temperature. You may have to change the desired temp a few degrees under varying conditions. That is if the system is properly charged and working well.

If the system is a little low on refrigerant it will also cool better at night or in cooler weather.

Good luck,

Mine does the same thing, whereby I usually change the temperature upwards and it seems better. Do these go out of whack over time? How often should one check the load of coolant?

Yes, there is a sunlight sensor. It's the little trapezoidal thing between the defroster vents.

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Rick Miley
2014 Tesla Model S
2018 Tesla Model 3
2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
Chain Elongation References

DO NOT check the coolant pressures as a preventive maintenance step. Do not connect gauges unless you suspect a real problem. Every time you connect the gauges to ANY refrigeration system, you take the chance of introducing air or moisture. Moisture combines with the refrigerant to make an acid that will eat everything up from the inside out.

A better way to do a preliminary check of the refrigeration system is to use a thermometer. You can get a stem/dial thermometer at any auto supply for less than $10.

Measure the air temperature at the condensor and again at the vent. With the a/c running max and after about five minutes to stabilize temps, you should have at LEAST a 30 degree F lower temp at the vent than at the condensor. These are measured at virtually the same time. If you do then consider it okay and leave the gauges in the tool box.

Good luck,

Thanks for the info. In my particular case, I'm not suspecting any issues with the evaporator system since it can get plenty cold, so I won't be mucking with the refrigerant. I'm suspecting a sensor. These two links might help me (and others):
http://www.edsoto.com/w210/climcontrol_sunsensor.htm
and
http://www.continentalimports.com/ser_ic40142.html

I will be checking the diagnostic codes on the sun sensor this weekend to see if I have a failure code there. It seems that the sun sensor can influence not only blower speed but also temperature control??

Lot's of good information available directly from the control head on the 210 cars. You can directly read the outside temp, in car temp, evaporator temp, left and right heater core temp, refrigerant high side pressure and temp.

My understanding is the sun sensor has very little influence over the operation of the climate system. I would not expect it to cause your problem.

I would pay close attention to the in car temp sensor. Your symptoms are similar to a failed aspirator blower motor. That is the small fan that draws air over the in car temp sensor. The sensor itself is located near the dome light, behind those little grilles cast into the plastic. On the 210 I don't know exactly where the aspirator blower is; on earlier models it was tucked in behind the glove box. I suspect it is as well on the 210.

Good luck.

- JimY

Jim, thanks.
I had previously made sure the vent holes were clean, and I read somewhere to try a test of placing a small piece of tissue paper over the vent holes to see if it would "stick" from the suction action of the apirator blower and though it wasn't a strong suction, it did appear to stick. I'll check the car temp itself and if it seems way off. If so, would you suspect the temp sensor itself to be faulty?

It's always a possibility the in car temp sensor has gone a bit flaky on you. It should be easy to tell - call up the diagnostic mode when the a/c is freezing you out, see what it reads. If it's an unreasonably high number, you know where to look.

- JimY

Many thanks for all the material above. It is highly illuminating.

I'm still fighting with my 210 climate control.. out of nowhere it will blow full hot air, or full cold air at total randomness. Very annoying!!

i have a good sun sensor that i will loan to anyone who wants to use it for testing. send email

Update on the control head diagnostics.. I entered the "Failure Code" mode on the A/C control, it immediately displayed Eb1234 (exactly like the article link in my previous post). Does this mean my sun sensor is giving an error code? When I entered the mode to display various sensor values, the internal car temperature registered 105 degrees. It was a sunny, but cool day, and the car was parked in the parking lot at work. No way it was even close to 105 in the car. So perhaps my temp sensor is also flaky? Doing the same reading at night, parked in my garage, the temp returned 70 degrees.