A/C compressor not cycling just staying on

[precopster]

Hi I've finally had a moment to check my A/C system as it has been driving me a little batty of late. At highway speeds it tend to work fine but at low speeds say under 50kph it goes warm/hot. I managed to check the low speed fan and it kicks in with the compressor just fine. The A/C is cold enough when it's working so no complaints there at all.

Now here comes the interesting bit. The compressor is no longer cycling every 15-20 secs as it used to. It just stays on all the time. Mind you this is on a 35degC day (95 degF) today.

The weather has been so hot here that I haven't had a moment to check it on a mild day to see if the compressor cycles . The problem only seems to surface above 79 degF ambient temperature. Below this temp all is good. I assume the higher temps will create excess pressure and have the pressure switch turn off the system.

Once I leave it off for 3-4 minutes then turn it on as long as I travel at more than 50kph then all is fine (this is on over 79 degF days. On cooler days it is perfect.

[abscate]

We don’t get that nasty heat up here but 100% duty cycle at that heat sounds right. The boys from TX will know for certain.

Dropping at low speed is more puzzling. Compressor clutch is the common problem but that won’t get fixed at speed.

Maybe it is slightly overcharged? I believe Volvo recommends 10% undercharge for hot climates

[vtl]

My bet it is overcharged. A/C system works by creating pressure difference. If low pressure contour is already close to a critical value (when pressure relief valve is about to trigger) then the system shuts off.

[oragex]

Have you had the car for long time? Was the a/c already serviced? A previous owner may have brought it to a mechanic who overcharged it.

There is also the evaporator temp sensor but when this goes bad it will keep the clutch running at all times.

[jimmy57]

If it doesn't cycle then the evaporator will freeze if it is doing enough to cool the car. This will explain the stops cooling and works again after an off period. I assume cabin filter is clean so that leaves temp probe as the highest likelihood. I also assume that the charge isn't high as it was cooling fine before and there is low likelihood of errant pranksters pumping extra refrigerant into systems (I hope. Maybe there really is such a thing as aircon terrorists).

Temp probe doesn't explain poor cooling at low speed. Prop accel pedal to get 1200 rpm +/- and then feel the suction line back to compressor. It should be very chilly if the system has been on for 5 min with windows up even at 35C. If it is not then first verify clear air path through condenser. Roo and wombat poo and HUGE Aussie bugs may have harmed air flow. If clear then try adding 1/2 can R134a or 6 oz/175g if using other charge equipment and see if it cools down to verify it is low. If that doesn't change the performance it is time for expansion valve and compressor diag.

[precopster]

Thanks for all the responses.

This car has been in city driving for 2 years and condenser is clean. Last Summer it worked fine.

I just went outside to see if compressor is cycling at lower temps (morning) and it is (woopee!!)

So that leaves temp probe and low charge.

I will check the a/c pressure level through Vida and report in. Last time I checked it I'm pretty sure it was over 1,200kpa with compressor running but we'll see.

[vtl]

For +35C meltdown high-pressure side should read about 2000 kPa.

[Rattnalle]

Low charge means it has to work much harder to stay cool so it sounds reasonable.

I topped mine up this summer and then went for a drive that included 35-38 C temperatures and I believe it still cycled although it didn't stay off for long.