'91 240, 171k. R12 A/C system. Compressor sometimes fast cycles (5 sec on, 5 sec off) especially under a load, like climing a hill or accelerating from a stop. I know this can be a symptom of low coolant. However, it still gets very cold in 90+ weather--too cold, in fact (I know I need to repalce the heater control valve.) Could it get so cold if the coolant is actually low? Is there long-term harm to the compressor to just let it go for a while? Money is extremely tight right now and I have no place to do my own work.

Some older cars have a capilary tube in the evaporator that, when it gets cold, shuts off the compressor for a bit. Then as it starts to warm up, it clicks the compressor back on. The click-on / click-off points depend on where you set the little thermostadt knob. If that's what's going on with your car, it's a sign that things are working good.

If the pressure is low enough the low pressure switch may be the cause. If you wanted to verify that just add a jumper between the two switch terminals and take her for a drive. If you do not feel the compressor switching in and out then there is your source.

A system low on coolant will blow very cold air and if you went by air temp alone is would seem to be working just fine. At the same time the compressor is working much harder than it should be. You should really be able to feel that at idle the vibration from the compressor is very evident. When the system is charged back up the load the compressor puts on the engine decreases and the idle should have less vibration.

Most likely you will need 18 to 24 Oz to get back to a full charge if your system is that low. If the system is tight, worth the money.

Regards,

PT