AC clutch cycling

[c9zx]

I will related the short version about trying to get the AC air colder. It is a 134a conversion, was only getting a 21 degree drop from ambient, the pressures looked good (low side 30), added 3 ounces, no change. From cold start, I used hose clamp pliers to make sure no hot coolant was in the heater core, no change. Removed the clamps, no change (heater control valve works). New symptom, the clutch is cycling. The only thing I know of that will cause these old car to cycle is the evaporator temperature switch. Either the switch is displaying an odd failure mode or, the evaporator is trying to freeze up. If anything the system may be overcharged not under charged. I've never seen one freeze up from being overcharged. So, has anyone seen this or have any ideas about what is causing this? All responses will be appreciated. Chuck

 

[Steve McMahan]

When it runs, are you getting condensation out of the drain? I guess you could have a defective icing switch. Are both of the blend air doors moving when you move the levers?

 

[Mike King]

You can bridge the A/C thermostat to test if this stops the clutch cycling out early. (Iis located on the evaporator box). It is not uncommon for a pre-set thermostat to fail. I am installing a manually adjustable thermostat in mine with the knob hidden under the dash. Also check to see if a pressure cut out switch has been installed on the high pressure side of the sustem.

 

[c9zx]

We started over yesterday. Evacuate, vacuum, recharge. Pretty much the same result (20 degrees below ambient) only no cycling. All the doors operate normally, no high pressure switch installed. The TXV seems VERY active, high pressure bouncing rapidly from 220 to 250 on the high side. Low side is lower at 26. It has an original style and size condenser. I may try a larger condenser. Thanks for the responses. Chuck

 

[Mike King]

We started over yesterday. Evacuate, vacuum, recharge. Pretty much the same result (20 degrees below ambient) only no cycling. All the doors operate normally, no high pressure switch installed. The TXV seems VERY active, high pressure bouncing rapidly from 220 to 250 on the high side. Low side is lower at 26. It has an original style and size condenser. I may try a larger condenser. Thanks for the responses. Chuck

Just for your interest, a charge of R134a should be about 80% of the original R12 charge. Pressure readings are relative to the ambient temperature. Your pressure readings suggest an ambient temperature of between 95 and 105 degrees fahrenheit. Has the system ever been flushed?

 

[c9zx]

Once again, I think the system is overcharged. The ambient temp was 78. The system was flushed during restoration. With the exception of the evaporator and TXV everything was replaced. Chuck