View Full Version : Automotive A/C Vent Temp Range vs System

Don Browney
01-06-2001, 02:07 PM
I have a question about the normal air vent temperature ranges that can be expected from the 3 different automotive A/C systems I have read about.
1.Receiver Drier (Filter Drier) - Expansion Valve System
2.Accumulator - Orifice Tube System
3.Suction Throttling Valve System
I ask this because I recently purchased a foreign vehicle that uses the Exp valve type system and the vent temperature varies 12degF(34F-46F) within 15 seconds at 2k rpm(all windows and doors open,A/C set to max). This seems to me to be a bit wide of a temp range. The low pressure side measures between 11psi and 34psi during this time with the compressor cutting off at 11psi.
This has the effect of making the air out of the vent warm and humid before the compressor kicks on again.
I have been in 2001 Nissan Frontier that has a 10degF range under the same
test conditions. (TXV system)
I have 2 other vehicles with the SVT system and they don't vary more than 3degF.

My basic question is this vent temp variation normal for the different system types?
And which of these three normally holds the tightest vent temp variation.

I have posted this on aircondition.com with no exact answer yet.

Vehicle System Specs:(From shop manual)
Evaporator thermistor- Off at 34F, On at 38F
Pressure Switch- Low Pressure Cutoff= 25psi, ON at 36psi. High Pressure Cutoff= 484psi, On at 398psi

15-05-2002, 06:19 AM
If memory serve me correctly (going back 15 years), the vent temperature should be 40 to 45 deg F with the engine running at 2000 rpm (or 35MPH) when it's 80 deg F outside. 37 is optimal.

IE a 40deg F drop in temp from outside (ambient) to inside (vent) is a good rule of thumb. More than 40 deg F drop and it freezes up below 80 deg F, less than 40 it doesn't cool the passenger compartment adequately. Again, this is @ 2000 rpm or 35 MPH.

I have seen temps go as high as 55 deg F at idle.
Usually, the older R12 systems were 5 deg F colder than a new or retrofitted 134a system.

Another consideration is at what point is the thermostadt for the engine coolant opening? Is it opening during the test releasing extra heat in the engine compartment?

You might consider driving the vehicle at normal city speeds (25 to 35 MPH) to see how the a/c feels.