Another AC thread; when the AC is on, the compressor is on all the time. unlike other Japanese car that the AC compressor will cycle on and off. Is that normal?
Thanks.
91 S4

 

Normal for the 928. It will cycle on and off with the freeze switch or low pressure.

 

My AC never cycles. When AC button is on, the compressor is on all the time.
Does it mean that my AC charge is low?
Cheers.

 

could be, or your freeze switch is not working right

 

Where is the freeze switch located? Thks.

 

hooked up to the evaporator core under the cowel cover.

 

You can try to look at the sight glass in the drier. When clutch on comp is engaged with a full charge you should see a clear fluid stream if you have R12, milky with R134a. Properly operating the line between drier and expansion valve will be warm to hot and line out of evap to comp will be cold and sweating. Or you can put on a set of gauges.

WWW.griffiths.com

 

Kevin with the system filled correctly the compressor will run all the time when AC is engaged. It will only stop if the evaporator gets too cold (near freezing) and will start again once the temperature rises - the freeze switch does this for you.
It will also stop if the pressure is too low.
On your car the AC will run if the HVAC control slider is in the recirculation position automatically overriding the AC on/off button. Same for defrost IIRC - does not happen very often in TX or HK.

 

Thanks Roger, it makes all sense now. Basically, the compressor will run all the time when AC is selected and my car does exactly like that.
I did checked the high/low pressure tonight as shown in pic. Any commeny please. Cheers.

Ambient temp is 80F (converted to R134a already)

 

Those reading look typical to a car with a slight undercharge and high ambient conditions >80 degF. If there was adequate charge, the low-pressure reading would be higher and the freeze switch would be cycling the compressor. As Griffin suggested, you'll find a stream of bubbles in the sight glass.

 

Thanks Bob for your reply. Appreciated.

I tried to add a bit more AC gas in the system, to the low became 45psi and high became 275psi and both cooling fans ran like crazy. Very loud and quite scary.

So I re-adjusted the AC charge to a bit lower (as shown in the above pic) and the fans ran a bit slower as it used to be.

My 91 S4 was converted to R134a by the local Porsche dealer before and I don't have the sight glass in the drier. It has a solid head without any glass.

Cheers.

 

If you have air or moisture in the system that will really mess up the pressures, you will be chasing an albatross. Recover, evacuate, recharge and test.

 

The fans need to come on as high-side pressure increases, so they were working as planned when you added more gas. 275 on the high side is about the max you'd like to see, but since the car is sitting still with engine idling that's probably OK for now.

The system is normally charged by weight, and pressures are read with engine at 1500 RPM to simulate driving conditions. Low side no higher than 30 PSI or so, preferably lower. You'll need a good thermometer to watch the center vent temperature, doors closed and fan in second speed position. As you very slowly add puffs of gas, you'll reach a point where vent temperature suddenly drops hard a couple degrees Fahrenheight a minute after you add a short puff of gas. Stop adding gas, since at that point you have just enough to have liquid at the expansion valve, the perfect condition for coldest temperatures. You'll gain a slight amount of headroom while driving, since the condenser will do more cooling with more airflow. Resist the urge to add more refrigerant gas, since raising pressures raises temperatures and buys you nothing. Remember, max of 30 PSI on the low side as you charge.

For those watching at home, note that picture of the manifold gauges shows temperatures in Celcius, so be sure to adjust for that when you try to correlate pressures with R-134a temperature. Took me a few looks to figure out why the temp numbers were so far off.

 

Looks like the gauges call you a retard when you get to high.

Doesn't seem very PC.