1996 Jimmy a/c cycling on/off every seconds

[jdmccright]

I have a 1996 Jimmy 4x4 in which the air conditioner will cycle on for 5 seconds then off for 10 secs. I hear two relays clicking while this is happening, one mounted to the firewall (far right one of the three) and another I can't locate but near the ignition coil. Of course, no cold air comes from the registers. None of the components feel cool or cold. Hooking up the manifold gauges I get the following readings:

Cycle on
Low side - 60 psi
High side - 95 psi

Cycle off
Low side - 15 psi
High side - 85-90psi

Please help diagnose...thanks!

[old_master]

When the low side reaches 22 psi the clutch should disengage, (turn off). When low side pressure rises to 42 psi the clutch should engage, (turn on). This is controlled by a pressure switch near or at the accumulator. The switch can not be properly diagnosed for correct switching points unless the system is fully charged. With R134a refrigerant and R12 refrigerant, in this pressure range, the pressure and temperature are approximately equal. With that in mind, the AVERAGE pressure/temperature of the evaporator should not be lower than 32F. This keeps the evaporator above freezing so it will not ice up. While checking pressures, the windows should be up, doors closed, engine rpm around 1500, and adequate air flow across the radiator. There are variables involved with A/C performance such as ambient temperature and humidity. Both affect high side pressure readings and register temperature output. The high side pressure at 70F ambient should not exceed 225psi. The readings you have posted indicate the system may be low on refrigerant. Unfortunately, with a cycling clutch system such as yours, there is no way of knowing how much refrigerant is in the system which makes "toping off" dangerous to both the technician doing the work and to the components in the system. If the refrigerant charge is in fact low, the system should be leak tested and repaired as necessary before performing any service work. The proper way to charge the system is to drain, (recover) and evacuate at 29.9 inches of vacuum for a minimum of one hour, then recharge with the proper amount of the correct refrigerant and oil. More questions? Just ask. Hope this helps.

[jdmccright]

Would the low refrigerant also be the reason I can't select which vents to blow from (seems stuck on the windshield registers) or is that a vacuum problem?

[old_master]

The HVAC system incorporates vacuum actuators, sometimes referred to as vacuum motors, to direct air flow to the floor, dash registers or windshield. Temperature is controlled by an electric motor that moves a blend door. The blend door mixes outside air with air heated by the heater core to regulate temperature output. The HVAC system will default to defrost if there is no vacuum supplied to the control head. Vacuum is supplied from a fitting on the intake manifold of the engine through a hose to the control head in the dash. On the rear of the control head there are several hoses that run to the various vacuum actuators.

[jdmccright]

I was able to charge it enough to stop it from cycling. Now I get 31 psi on the low side and 160-165psi on the high side (the needle was bouncing between the two gauge marks). Air is being chilled to about 25 degrees below the blower air temp. Shut off, the system shows ~90psi. I noticed that the compressor makes a clacktey sound when engaged and shows some slight oil staining along the part lines of the case. The belt tensioner pulley is also moving to keep constant tension on the serpentine when the AC is on but stops when the AC is shut off. I'm no expert, but seems to me it's about time for the compressor to go.
As for the climate control selector, would I be able to clear any blockage in the vacuum tubes by using compressed air or is it more likely the selector body itself? I did have a problem with transmission fluid in the vacuum lines due to a bad transfer case switch but have since fixed that. However, there could be fluid in there not letting the door/vacuum motor do its thing. Where is the vacuum hose that leads to the AC unit in the cab? I can see one that is labeled "Evap service port - 1psi max pres...etc" with a green cap located on the passeneger side near the firewall atop the valve cover. There is another thin vac line entering through the firewall in the very corner of the driver's side. Your help has been great so far! Thanks!

[old_master]

It sounds as though the system is overcharged. The clutch is supposed to cycle. The compressor noise may be due to being overcharged also. Cycle times will vary with ambient temperature and because of this, it is impossible to properly diagnose without the proper amount of refrigerant. If the system is overcharged, the compressor is unable to drop low side pressure far enough to satisfy the cycling switch at 22 psi. The compressor will run continuously until the pressure is relieved. The excess pressure WILL be relieved one way or another! One or more of several things might happen. The compressor might blow a gasket out or lock up. A refrigerant line might rupture. It might blow a hole in the condenser or evaporator. It will relieve the pressure somewhere, and you might not like the result. Did you follow the instructions for charging in post #2? The system MUST be evacuated to remove ALL old refrigerant. The theory and operation of an A/C system is actually quite simple however, consequences can be costly and or dangerous if service procedures are not followed correctly.

The vacuum lines and actuators need to be clean and free of any contaminant such as transmission fluid. Compressed air however is definitely not the way to do it unless you are absolutely certain that all of the lines are disconnected from every connection. Compressed air will destroy the diaphragms in the vacuum actuators. The best to way to be assured the lines are clear is to replace them. Cleaning the actuators would involve removing each one of them. The vacuum line for the HVAC system is usually black and approximately 3/16” in diameter and enters the passenger compartment near the center, or slightly to the passenger side of the firewall.

The Evap service port with the green cap is exactly that, a test port for the evaporative emission system, something totally separate from the HVAC system.

[jdmccright]

Welp, I guess in my haste I've revealed my true ineptitude. I started this believeing I had a major leak, thus saw little reason to pay for someone to evacuate & recharge the system only be done again to fix whatever was wrong. I first I used a can of the red leak detection dye in hopes of showing where the problem was but no luck. The system took ~8ozs of refrigerant to be where it is now...still no dye. In my meager defense, the compressor was making the clattering noise long before I began.
I will note that I have noticed that when I remove the low side dust cap, I can hear a slight release of pressure, then silent. Perhaps that connection valve has a leak. Also, while I was adding refrigerant, I was watching the gauges as it was cycling and did notice that it would cycle off at around 22psi and on at around 68-70psi. I have the Haynes manual but it says nothing about the proper amount of refrigerant, the type or amount of oil or the proper pressures for a given temperature.

Regarding the actuator(s?), thank you for the sage advice. Again, the manual says nothing about where these are located or how to check/replace them. If I am correct, there are two vacuum actuators to move the mode doors. Where are these located? I've removed the control valve from the dashboard selector and found no trans fluid there, so perhaps the actuators are still fluid free, but just not working.

I truly appreciate your patience and help...only with wisdom shall I conquer!

[old_master]

There is no substitute for knowledge, wisdom, and experience. The trio can be quite time consuming and expensive to acquire. The service port valves have been known to leak; perhaps that is your only leak. If that's the case, it will take a considerable amount of time for the red dye to show up to indicate such a small leak. A better type of leak detection is available. A florescent dye is installed with the refrigerant, the system is operated for a day or two, then, while using a black light, the leak(s) leave a tell tale bright green area indicating where the leak is. Works every time!

Charging A/C is always done via the low side service port and the refrigerant is installed as a liquid, (bottom of can facing up). While charging yours, you noted that low side pressure was 68 to 70 psi. Knowing what you do about refrigerant temperature & pressure relationships, you know that the refrigerant you were installing was approximately 68 to 70 degrees! Until the charging process is complete, you're guaranteed an inaccurate reading on low the side. This is because the pressure in the can and the pressure in the system need to stabilize, (equalize).

The specifications for refrigerant and oil are on a decal that should be attached to the evaporator housing under the hood. It will tell what type and the amount of refrigerant the system uses and what type of oil.

The actuators are mounted on the heater case behind the dash. It usually involves removing parts of the dash to access some of them. I believe there are three: Recirculation/fresh air, which is located near the top of the passenger side kick panel. Floor/dash/defrost, which is located on the driver side of the duct work above the transmission tunnel, (hump). This actuator may be a dual diaphragm style, meaning there are two vacuum hoses that route to it. The third is the temp blend which is electrically operated. There is a vacuum reservoir mounted in the engine compartment and a vacuum check valve installed inline to the intake manifold. From the reservoir, the line is routed to the interior, and eventually to the rear of the climate control mode switch. The first step in diagnosis is to make sure there is vacuum available at the mode switch, with the engine idling. Then trace the vacuum lines to each of the actuators and check their operation.

While they are still better than nothing, the aftermarket manuals don't publish everything. If you want everything, you have to spring for the GM shop manual to the tune of $135.00USD.