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Is it by design or there is something wrong?

abbouddi on Sat September 12, 2009 3:02 AM User is offline

Year: 2007
Make: Mazda
Model: 6
Engine Size: 2000
Refrigerant Type: R134a
Ambient Temp: 95
Pressure Low: 18 psi
Pressure High: 250 psi
Country of Origin: Saudi Arabia

I had this issue since I bought my mazda 6, 8 monthes ago, the A/C when idling: it is not blowing cold air (compared to other cars when idling) , and blowed air temperature may reach 63 deg. where it is not effective in our hot climate, so I have to press the accelerator to 1200 rpm to get some cold air.
Note that I supposed there is not exact quantity of refrigerant in my system, I did vacuum, recharge to exact amount 16.6 oz, and checked low and high sides 18 psi and 250 psi (as per manufacturer recommendation). Both fans are working when compressor is engaged. The compressor is cycling even at idle where the blown air is not cold enough. While when pressing the accelerator to 1500 rpm air blown is cold enough(around 42 deg).

Is it something by design to save fuel? or something wrong? and is there a way to decrease the cycle period of compressor to have more cold air at idle?


mk378 on Sat September 12, 2009 7:29 AM User is offline

Compressor should not cycle unless the air is fully cold. Perhaps the engine is idling too slow, causing the computer to turn off the compressor to prevent the engine from stalling out.

Don't expect peak performance at idle, but on a TXV system (which I think this is) the compressor should stay on constantly until the evaporator becomes fully cold.

94RX-7 on Sat September 12, 2009 11:29 AM User is offline

18 psi on the low side? That sounds way too low, and would put the evaporator temperature below freezing which will cause the computer to cycle the compressor off to prevent freezing. You sure it was that low?

squeege on Sat September 12, 2009 9:47 PM User is offline

I agree the low side is to low, but with the high side at 250 sounds like might have some sort of restriction, maybe faulty txv valve? what are the pressures at 1500 rpm?

Edited: Sat September 12, 2009 at 9:51 PM by squeege

abbouddi on Sun September 13, 2009 12:27 AM User is offline

The above mentioned pressures are at 1500 rpm and it is according to vehicle manual, at idle the low pressure is 22 psi and high pressure 220 psi

It is mentioned in the manual:

1. Install the SSTs (gas charging set).
2. Warm up the engine and run it at a constant 1,500 rpm.
3. Set the fan speed to MAX HI.
4. Turn the A/C switch on.
5. Set the RECIRCULATE mode.
6. Set the temperature control to MAX COLD.
7. Set the VENT mode.
8. Close all the doors and all the windows.
9. Measure the ambient temperature and the high- and low-pressure side reading of the SST (49 C061 001).

abbouddi on Sun September 13, 2009 12:30 AM User is offline

Mr mk378, the engine is idling at normal speed (around 900 rpm)
It is TXV system

squeege on Sun September 13, 2009 7:21 AM User is offline

Well according to the chart it is right. Have you asked the mazda service dept? The chart seems like it's not right as far as low side numbers.

Edited: Sun September 13, 2009 at 7:55 AM by squeege

pettaw on Sun September 13, 2009 5:16 PM User is offline

Depends on where the cycling control for the evaporator is. If its a thermostat based on temperature, with a TXV system can tend to get lower suction pressures because you get a higher superheat compared to orifice tube systems. You might be expecting too much from your A/C system. At those high ambients you're going to struggle to reject enough heat from the condensor to get the optimum vent temps at idle. Only slight strange thing is that you're getting cycling which you shouldn't be getting at all so possibly you could be undercharged.

Did you empty your gauge lines when you charged the system. After you charge, a good quantity of liquid refrigerant sits in the high side line. probably not far off 100g. You can drain it down by shutting off the tank and the high side coupler without disconnecting them. Then slowly open the high and low side valves and let the compressor draw the liquid into the low side.

NickD on Sun September 13, 2009 7:58 PM User is offline

At 95* F, low side range is 0.06 to 0.145 Mpa or 8.7 to 21.03047 PSI.

High side is 1.45 to 1.8 Mpa or 210 to 261 PSI.

Pressures are normally shown at 1,500 rpm to 2,000 rpm, AC on, blower at max, a door open. Looking at your PT chart, it's almost like you have an R-12 system that was converted to R-134a with doing nothing else but using an R-134a compatible receiver assuming it is a TXV system and compatible oil.

Do you have a parallel flow condenser? For good performance your high side pressures appear to be about 10% too high and your low side about 6 psi too low. Know when I was down in Venezuela, the imports to the USA were nothing like the stuff they jammed together for Venezuela, looks like Toyota, Honda, Nissan, and others used whatever was left over. Wonder if they did that to you? Doesn't appear to me you have air in your system, low side is way too low for that.

mk378 on Sun September 13, 2009 11:39 PM User is offline

It's a 2007 model. Should be a factory R-134a system. Everything concludes that the refrigeration system is working properly, as long as the compressor is running. Full cooling potential is not being realized because the compressor is dropping out.

You need to find out why the compressor doesn't stay on constantly at idle. It's not because of excessive pressure or low evaporator temperature. Late model cars have all kinds of strange "features" and someone with model-specific experience may be needed to figure it out.

NickD on Mon September 14, 2009 8:55 AM User is offline

Country of Origin: Saudi Arabia, just thought I would toss in the possibility of what I found in Venezuela, no OBD II there, that was great for working on vehicles, but get choked to death locked behind a smoker wasn't so great. Lot's of smokers in that country.

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