A/C Troubleshooting

[The Critic]

During hot weather (90F+), my A/C compressor has begun to rapidly engage/disengage every few seconds, resulting in the car constantly jerking. I suspect this is due to a system overcharge that is exacerbated by high ambient temperatures, as the system works fine in cooler weather. I personally recharged the system recently, and I'm almost certain I put too much in.

In order to correct the condition, I should probably have another evac and recharge done, but that option isn't on the table right now. However, I have ordered a set of A/C manifold gauges and they should be here tomorrow. What pressure readings show I expect to see on the low and high pressure sides?

I remember Chris142 posting that for the low pressure, I should see between 35-50 psi. For the high pressure, the pressure reading should be about 2.25x the ambient temperature or 150-200psi. All readings at 1500 rpm?? Is this correct?

Thanks.

 

[JHZR2]

On hot days you can see higher than 150-200psi even with low side down in the 30-40 range. If you dont have manifold gauges, you cant really recharge - how did you do the last one properly? Obviously you recharged close, but did you just not watch high side?

I had a rapidly cycling compressor like that and it was due to undercharge. I guess there was enough there to just get to max pressure, then it would disengage and the system would destabilize. Get those gauges on there!

 

[The Critic]

First time I recharged, I used a machine and evac/recharged it to the correct amount. This was in June 2009.

Then back in April, even in 65F weather, the A/C was mediocre, so I dumped in 1/2 can and it worked better. Added the rest of the can and it worked slightly better. Drove it in 80-85F heat without any issues at all.

Two weekends ago, I went home to 95F heat and the A/C starts cycling on/off every 5 seconds, causing the car to jerk. This only happens in hot (90+) weather.

Anyway, it's about 65-70F out here so of course it is working fine right now. The gauges will be here this afternoon so we'll see.

 

[JHZR2]

keep us posted! Sounds like you may have a leak given the first point you made. If you are too low, I guess the underpressure switch could stop the cycling, and you just dont have enough working fluid on the really hot days to move the heat around.

 

[The Critic]

Originally Posted By: JHZR2
keep us posted! Sounds like you may have a leak given the first point you made. If you are too low, I guess the underpressure switch could stop the cycling, and you just dont have enough working fluid on the really hot days to move the heat around.

Or if it's too high, the high-pressure cut-off?

 

[JHZR2]

Absolutely. But Im hearing charged, then didnt work, needed to add some - tells me slow leak. There may be a low pressure switch that gets invoked because the compressor sucks down too much without enough return - like youre on the cusp of being non-functional or working. In cooler weather you arent evaporating as fast, so maybe things stay more constant through the system, whereas when it is real hot, you need to move that refrigerant through FAST to pick up all the heat.

Gauges will be very telling, could be either way.

Do you have the right taps to add refrigerant through the gauges? Be sure to connect everything up so as to not get air in the hoses...

 

[TooManyWheels]

Originally Posted By: JHZR2


I had a rapidly cycling compressor like that and it was due to undercharge. I guess there was enough there to just get to max pressure, then it would disengage and the system would destabilize. Get those gauges on there!


This is the correct answer.

 

[SrDriver]

Did you purge the hose? If you get any air into the system, it will cause it to trip on high head pressure.

Either high head or low suction pressure will cause it to trip.

 

[The Critic]

Originally Posted By: TooManyWheels
Originally Posted By: JHZR2


I had a rapidly cycling compressor like that and it was due to undercharge. I guess there was enough there to just get to max pressure, then it would disengage and the system would destabilize. Get those gauges on there!


This is the correct answer.

Then why will it only rapidly cycle in hot weather? The system works great when it's 65F out.

 

[scurvy]

Originally Posted By: The Critic
Then why will it only rapidly cycle in hot weather? The system works great when it's 65F out.

Why are you running the AC in 65F weather anyway?

You need to find out the system state. GET A SET OF GAUGES ON IT. Everything else is just farts in the wind at this point.

 

[JHZR2]

Originally Posted By: The Critic
Originally Posted By: TooManyWheels
Originally Posted By: JHZR2


I had a rapidly cycling compressor like that and it was due to undercharge. I guess there was enough there to just get to max pressure, then it would disengage and the system would destabilize. Get those gauges on there!


This is the correct answer.

Then why will it only rapidly cycle in hot weather? The system works great when it's 65F out.

Likely due to quantity/availability of working fluid at the various conditions. Also remember that you can measure static pressure. the higher the temperature, the more refrigerant will be in the vapor phase in an ambient static system. The more that is a vapor, the mess you can cool... so youre starting off with even less refrigerant at a higher ambient temp, before you even start to cycle.

Get yourself a static R134a chart to look at ambient temp versus sitting system pressure.

 

[Cutehumor]

I bet you have a leak in the a/c system. I took my 96 sunfire to the shop, they said it was just low on refrigerant, they filled the refrigerant to full. it blew cold for another two weeks and wasn't working again. After searching hard, I found the a/c compressor was leaking.

 

[The Critic]

Disclosure: Since I thought I had too much refrigerant, I held down the schraeder valve several days ago for about 5-10 seconds and let a bunch out.

I just took some readings. Ambient temperature is around 55F.

At about 2000rpm.

Low pressure is reading about 24 psi
High Pressure is reading about 135 psi

At idle:

Not sure why there's such a big difference. All readings were taken on a cold engine after about 7 minutes of idling with the A/C on.

I haven't been able to recheck the car to see if the compressor cycles on/off as the ambient temperature is not high enough to duplicate such a situation.

Thoughts on the readings?

Thanks.

 

[JHZR2]

held down the valve? naughty, naughty, naughty...

I dont see that as a very big difference. The compressor is working at different rates. Sucking down more and compressing better at 2000 RPM.

Seems low to me, though it is cold out.

Did you get that static reading? Should be fully cooled at ambient temperatures, read hot and cold side, which should be the same.

 

[RH+G]

Its cool out so ur readings are gonna be low. But u are definetly low on refrigerant. The only proper wayt to charge an ac system is to evacuate vacumm and weigh in the amount u put back in.
But i know most people dont do that

 

[mechtech2]

Outside/ambient temperature has a direct effect on the high side.
The low side should actually cycle, and not be at a static pressure.
Measure the temperature a couple of inches in front of the condenser, and multiply it by 2.2 for R134a high side pressures.

So for 80 degrees F , about 176 on the high side.
Note that the temp will be warmer right in front of the condenser than the actual ambient temperature. You want THAT temp, not he ambient one.

 

[JHZR2]

good tip, thanks, mechtech2!

 

[The Critic]

Update:

Drove the car around in 80-ish heat yesterday. A/C worked fantastic. It was blowing ice cubes in comparison to the Mazda A/C I was complaining about.

So I guess I should leave it alone for now?

 

[Vikas]

What is the size of those ice cubes? Are they at least one inch by one inch? If not, go ahead and tinker more

Seriously, was it over charged a little? In retrospect, what would be your advice? I purchased a kit couple of years ago but so far I have not found any particular need to add it to any of my vehicles.

- Vikas