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500Eboard Management
A/C temperature at the vent at coldest setting?
- Thread starter Kyiv
- Start date
Good question! I've never tried a laser / IR thermometer in the vents. I use a metal-probe style, similar to what you see below.
With the probe-style thermometer, in 100F / 30% humidity ambients, a top-notch system will be 42-44°F at max fan speed, [and recirculation mode on], cruising at 30-40mph or faster. Sitting at idle RPM for 5-10 minutes and the vent temps will increase to 50-52°F or so; again with max fan speed. This data is from multiple 124.034/.036 cars with stock R134a systems.
Vent temps may be lower with a lower fan speed; and if the system has enough capacity for conditions, the compressor will cycle based on readings from the evap temp sensor. When this happens you'll see the thermometer go back and forth between roughly 40-45°F, and you can sometimes hear the refrigerant flowing through the evaporator when the compressor cycles.
What I don't know is... how the above data may vary for high humidity areas. If you're dealing with 60-80% humidity (or worse), the temps may not be as cold. Anyone on the east coast (or Houston, @nocfn) have data to share?
https://www.amazon.com/gp/product/B00009WE45/
With the probe-style thermometer, in 100F / 30% humidity ambients, a top-notch system will be 42-44°F at max fan speed, [and recirculation mode on], cruising at 30-40mph or faster. Sitting at idle RPM for 5-10 minutes and the vent temps will increase to 50-52°F or so; again with max fan speed. This data is from multiple 124.034/.036 cars with stock R134a systems.
Vent temps may be lower with a lower fan speed; and if the system has enough capacity for conditions, the compressor will cycle based on readings from the evap temp sensor. When this happens you'll see the thermometer go back and forth between roughly 40-45°F, and you can sometimes hear the refrigerant flowing through the evaporator when the compressor cycles.
What I don't know is... how the above data may vary for high humidity areas. If you're dealing with 60-80% humidity (or worse), the temps may not be as cold. Anyone on the east coast (or Houston, @nocfn) have data to share?
https://www.amazon.com/gp/product/B00009WE45/
Dave, thanks. I was wondering if what I saw with below thermometer, which is a cheap Amazon item was correct. I drove for an hour in 30+C outside ambient and close to 80% humidity. Before switching the engine off, idling, I measured temperature of all 4 vents and saw -0.5C in each... Slightly surprised by the reading, as I expected 10-14C. Does this imply that something is wrong? Later today I saw very slight condensation around the central vents...
-0.5°C / 31°F is likely an inaccurate reading. At those temps, the evaporator would ice up and restrict airflow.
The evaporator temp sensor is not supposed to allow temps under something like 5°C (exact number is in the FSM somewhere). Years ago there was discussion on other forums about how to use a resistor to "fool" the sensor and get colder air. I tried this and ended up with an iced evaporator (on an older W124 with R12), along with vent temps somewhere around 2-3°C / 36-38°F. I removed that modification and definitely do NOT recommend it. Just mentioning in case you've read about it elsewhere... or if a previous owner if your car may have done it, which I seriously doubt.
I'd invest the $10-$12 in a probe-type thermometer and see what readings you get. Next time I have a car out on a warm day - which could be a while, with COVID resurging in our area - I'll try to remember to use my IR gun and see if it reads similar to my probeometer.
The evaporator temp sensor is not supposed to allow temps under something like 5°C (exact number is in the FSM somewhere). Years ago there was discussion on other forums about how to use a resistor to "fool" the sensor and get colder air. I tried this and ended up with an iced evaporator (on an older W124 with R12), along with vent temps somewhere around 2-3°C / 36-38°F. I removed that modification and definitely do NOT recommend it. Just mentioning in case you've read about it elsewhere... or if a previous owner if your car may have done it, which I seriously doubt.
I'd invest the $10-$12 in a probe-type thermometer and see what readings you get. Next time I have a car out on a warm day - which could be a while, with COVID resurging in our area - I'll try to remember to use my IR gun and see if it reads similar to my probeometer.
Usually you test the outlet temperature not at highest fan speed. engine in idle soeed, fan speed recommended to be in stage 2 and ac in max cold setting. Best measuring method is the metal probe type unit which can be placed properly in the vents air stream short before exiting the vent. With the car cooled down already (possible heated ducting under the dashboard already cooled) you should see exit air temps around 5-8 degrees commonly.
Max fan speed is worst-case scenario. I like to use that to have peak load on the system. 
Also forgot to mention, my data above was with recirculation mode on... however this requires the main air flap vacuum pods to both be working properly, or the main air flap won't fully close. If your AC system is working ok but not great, that's another item to check.
Also forgot to mention, my data above was with recirculation mode on... however this requires the main air flap vacuum pods to both be working properly, or the main air flap won't fully close. If your AC system is working ok but not great, that's another item to check.
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