My AC compressor cycles. It cycles all the time. Whether it is cold outside or warm. I purchased a gauge for checking the pressure. It reads 25psi when the compressor is not running. Then when the compressor goes on it ramps up to 45 psi in about 5 seconds. Then it stays there for about 5 seconds then the compressor clutch dissengages. Then about 15 seconds later the compressor comes on again and the cycle repeats. The manual that comes with the gauge says that is the proper pressure for a fully charged AC system.

The FAQs say that pin number 1 on plug B should reveal the ECC trouble codes. My plug B does not have a wire connected to pin 1 so I get no codes.

I do not know where to look to troubleshoot this problem. My Chilton's book says see an expert for any AC repairs.

Someone on EBAY sells a kit that has all of the AC pressure sensors for about $30. However, I am sure I have to drain the system to install the sensors. I do not want to do that unless I am sure the sensors are bad.

Can anyone help me on how to diagnose my system problems?

A compressor will cycle whether it is cold or warm outside. Are you saying that the compressor cycles with the ac switch "off" (lit) in the console and the air selector to any position other than defrost? Then you may have a problem with that switch. The comp will cycle with the defrost selection and the ac switch turned off. Some more info on your part would help.
--
92 244 lowered, Bilsteins, IPD sways, Unitek cam/header, TSW Revo 18x8's - 94 965 Niiice MONEY PIT! - 76 Datsun 280Z lowered, modified head, cam, headers, intake

The compressor quits cycling when I depress the AC-OFF switch (lit). Which is how I drive it now because I get annoyed with the AC switching on and off.

A little more history. I have not had the car very long. At one point I took it to the dealer to have him do a vehicle inspection. On item stated that the compressor cycles on and off and therefore the AC need servicing/charging. I had not really noticed that before he mentioned it. Since then I noticed that it is true the AC cycles on and off all the time. I normally like to drive with the Fan and Position switch in the Auto mode. I set the temperature at 72 degrees. With the car in these positions the AC continuously cycles on and off. Since this has gotten annoying I now drive with the AC-Off button depressed. This has stopped my AC from cycling.

Normal day would be that I start the car in the morning cold (cold here is about 45deg F). All ECC controls set auto, auto, 72 deg. When the car is started the air blows gently through the system. When the engine begins to warm up, the Fan on the ECC goes to high and starts blowing warm air throughout the car. When the air in the car reaches the proper temperature the fan slows back down to have a nice gentle breeze of warm air blowing through the vents. During all of this time the AC cycles on and off, unless I depress the AC off button.

The A/C cycles on and off because it is removing humidity from the air, not for A/C purposes.

On some cars, when you select the "auto" setting, the CAR determines whether to use the A/C or not. The only way you can deselect the air conditioning is either to turn the heat on (selecting "heat") or to just turn the temp all the way up.

The reason the compressor cycles on and off the way it does, is because the A/C isn't transferring a lot of heat out of the cabin of the vehicle. Therefore the pressure remains low most of the time, just switching on periodically. If the A/C is removing little heat, the system will cycle frequently.

This does not harm the compressor or the system, and it is not necessarily indicative of a low system charge. Again, if the interior is cool and the outside air is rather dry, the A/C will cycle on and off a lot.

Turning the A/C off effectively stops this condition, but allows moisture to return to the climate system. If you want a true "auto" feature, just set the dial and allow the A/C to do it's thing. If it is too cold outside, the A/c will not come into use. But it is not unusual in an ECC vehicle like the 960, for the A/C to be used periodically by the system.

In a few short words: Sounds normal.

However, if the cycling persists when it is very warm outside, and you run the fan speed at max, have the temp set to coldest, and have the windows open, then there is a low charge condition. If the air remains cold, don't worry about the charge level.
--
chris herbst, near chicago

Thanks a lot for your answer. Once again it sounds like the dealer was just trying to make some money on me.

Sure; it's my pleasure.

You'll be able to better gauge the system condition when the outdoor temperatures rise a bit. Then go from there.
--
chris herbst, near chicago

The low side pressure doesn't tell you anything except that the compressor is shutting on and off at the correct low side pressures.

If this is happening with the fan set to "high" with the car sitting in the hot sun, on a hot, humid day with the windows open, you have a problem.

The quick cycling can also be indicative of a cool interior, low charge, plugged orifice, low ambient temp, low outside humidity, or a few other things. If the output temp from the vent is cool and doesn't vary radically, it might be behaving normally given the outside conditions.

Usually people who notice this kind of thing notice because their system behaves differently all of a sudden. If this is the case, you really need to measure the charge in the system, or measure the high side system pressure. A lot of newer R134 a/c systems don't have high side pressure ports (and you'd need a gauge manifold set) so you would have to actually have a shop measure the charge in the system and top off if necessary.
--
chris herbst, near chicago