Your customer comes in with a concern that the “A/C isn’t blowing cold.” How do you check the validity of that concern? When I’m faced with this concern, the first thing I do as I bring the vehicle into the shop is verify the operation of the various controls:
- Is the blower operating properly?
- Do the mode doors direct airflow where I want it?
Then I set the system for maximum cooling (max A/C, blower on high speed, temperature full cold). I place my hand in front of the center duct to get a quick gauge of how cold — or how warm — the air coming out is. Once inside the shop, I take a quick look under the hood to see if the compressor is engaged. If so, I’ll shut down the system and install my manifold gauges to the service ports, and next I’ll use a tool to monitor ambient temperature and humidity as well as the temperature coming from the center duct (Figure 1).
What can the temperature delta tell us?
How much of a difference should you see on a properly operating system, between ambient and duct temperatures? Most of us are used to seeing 30+ degrees difference but according to most OEMs, today’s cars may only have reached a 20-degree difference between the two. Testing specifications vary between OE manufacturers and vehicle model. For this reason, it is always important to check service information for the vehicle you are servicing. Humidity may also have an effect.
How you obtain your readings is also important. When we measure the ambient temperature from in front of the condenser and center duct (with the doors open or closed), the position of the recirculation door can may make a huge difference between the readings. For example, measuring the temperature on our test vehicle, we kept the doors closed for the purpose of this comparison (Figure 2).
When the recirculation door is open, the air entering the system has the full heat load to dissipate. If we compare the temperatures with the ambient reading taken at the condenser, it should the same as the temperature we measure at the fresh air inlet. If there is a 20-30+ degree temperature change measured, the A/C system is doing a pretty good job of taking on that heat load (Figure 3).
But what happens if I we close the recirculation door? Compared to the ambient reading, the difference is significant! Now there is a 40-50 degree drop in temperature! Does that mean the system is working better? The air is now being passed through the evaporator core is gradually experiencing a lowering heat load, as the system continues to adsorb a portion of that load on each pass. To judge the system performance more accurately, you should reference your ambient reading to the air entering the evaporator. In this case, now it's from its inside the car.
If we then compare the actual difference between the air temperature going in versus the air coming out, the difference is close to the same as we saw when the RECIRC door was open (when we referenced outside air temperature). The key is that both temperature readings are showing us how well the system is actually doing when it comes to removing the heat load. In the case of this vehicle, the temperature drop is less than what I would expect, justifying the need for further diagnosis. I could then compare the diagnostic readings in service information to the OEM pressure/temperature charts for direction or use an enthalpy chart to get a feel for the cause — but that’s the topic for another time.
