Mazda MX-6 Forum banner

1 - 6 of 6 Posts

·
Registered
Joined
·
90 Posts
Discussion Starter #1
My ac works fine with no leaks. The problem is when driving on the highway, after about 2 hours, the
blower air gets restricted. The air is still cold, but very little can pass. It is something to do with condensation or ice building up. The only way to get it to work normally again is turn off the ac for a while, and let the ice melt off.
Is there any other solution to this issue ?
 

·
Registered
Joined
·
238 Posts
Your AC needs re-gassing, icing up is a symptom of low gas pressure.
 

·
Registered
Joined
·
90 Posts
Discussion Starter #3
Not really, the meter pressure is fine in the normal range, no leaks. I had read somewhere in the past that if you live in a climate with high humidity, this is a common issue.
 

·
Registered
Joined
·
161 Posts
Not really, the meter pressure is fine in the normal range, no leaks. I had read somewhere in the past that if you live in a climate with high humidity, this is a common issue.
There is a thermocouple attached to the evaporator fins inside the dashboard. It's purpose is to turn off the compressor automatically when the evaporator get near 32F (freezing). It is a simple resistance thermocouple whose resistance INCREASES with INCREASING temperature. I needed to bias mine because if was turning the compressor off way too early (57F). So, I calibrated the resistance vs temperature for my thermocouple. The resistances is very linear with temperature. The relationship is Temperature = 25.876 * Resistance - 59.5 where temperature is in Fahrenheit and resistance is in Ohms. This is the line of best fit to my measured data. The correlation coefficient is 0.9926 (1.000 is the theoretically perfect fit). In other words, this equation is very accurate.

If you evaporator is icing up, check to see if you compressor ever shuts off. My guess is that your thermocouple has failed to a very high resistance or even infinite resistance. This would tell the computer that the evaporator was very hot all the time and your compressor would not shut off in order to prevent icing.

Final note: These thermocouples are known to shut off the compressor too early like mine did. The problem is that the sensor is telling the computer it is near 32F when actually it is at 57F. That's a 25F error which greatly reduces the capacity of the A/C to cool. I built a very simple biasing circuit that compensates for this sensor's error and tells the computer the actual temperature.

The evaporator with the sensor attached is located behind the glove compartment. If you pull the glove compartment out, you will see a large black plastic box. Inside this box is the evaporator which is what cools the air. There should be two wire coming out of it that plug in to the wiring harness. This is the thermocouple sensor. If you can locate these two wires, measure the resistance across them. If it infinite, then it is failed. Resistance at 80F - 100F should be: 4.2 ohms at 80F and 6.2 ohms at 100F, for example. So if you are measuring the resistance on a hot day without the A/C running at all, the reading should be somewhere between 4 - 6 ohms.

Once I corrected this sensor's error, my A/C worked extremely well. It has been 15 years since I did this and my A/C still cools well. The air exiting the vents is roughly 35F and my compressor cycles when it needs to. Hope this helps.

Jim Post
94 Mazda MX6, LS - V6, 5 speed.
 

·
Registered
Joined
·
90 Posts
Discussion Starter #5
That is interesting. I will check or change it out. Thankfully I have cold air blowing most of the time as summer starts in earnest.
 

·
Registered
Joined
·
90 Posts
Discussion Starter #6
This what I am getting when outside temp is 95. I can feel the load on the engine changing as the compressor cycles on and off.
13037
 
1 - 6 of 6 Posts
Top