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Intermittent A/C with Electronic Climate Control

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I have a 1990 Buick Riviera with a cold A/C but it will shut down the cooling for no obvious reason. When the problem started it was more on than off so was tolerable, even in the desert heat of Arizona. The situation has become worse and I get very little cool time before it shuts down. I tried to correlate the switching with something going on with the car. The best I could come up with was it liked higher RPM’s i.e. when idling at stop light the cooling would go off.

My mechanic did not have a clue and sent me to A/C shop he uses. The shop confirmed that the cooling unit was working correctly and concluded the problem was in the computer control. He admitted he had seen the problem before on another Riviera and was not able to fix it. He offered to keep it in the shop for a few days to see if he could find the cause, but was still going to charge by the hour and would not give me a limit on his charges. I was not comfortable with that so have not gotten it fixed. I would like to have a clue as to what a technician should be looking for before working on the problem.

Anyone have any similar experience? I am new to the forum but my quick reading of some of threads show some real experts on A/C. Anyway would welcome any comments or suggestions.

My Riviera is a beautiful automobile, but not pleasant trucking in this wasteland called Arizona.

Thank, Jack

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In all of the automatic hvac systems, there is an "Ambient Temperature" sensor located somewhere in the instrument panel area. Usually near the ignition key, behind an array of small holes punched in the instrument panel cover or trim. In it, there usually is a small fan that moves the ambient air from the interior over an electronic sensor that then sends its signal to the a/c programmer/controller to change modes of where the air comes out and at what temperature (with respect to the temp setting on the instrument panel control).

There can also be a "sun load" sensor that is usually mounted in the center of the upper instrument panel area, just behind the windshield. It's function, IF equipped, is to add in some more cooling capacity when it detect sunny weather. In some cases, it can vary the fan speed when passing under a cloud (lowering it "for no reason", automatically, then resuming the higher fan speed when the cloud's shadow has been traversed).

The OTHER thing is to make sure the outside condenser is not clogged with debris (bugs, weed blooms, etc.) AND that the fan clutch (or electric fans) are working as designed at the proper engine temperatures. Plus that all plastic/rubberized baffling around the radiator and condensor AND lower air dams are in place and in good condition. Lowering the effective heat transfer capabilities of the system will result in lower cooling performance AND higher high-side pressures (which might override the cycling switch to keep the compressor from running the length of time it would need to for desired levels of cooling performance).

And this would then move to the cycling switch itself. It might not be letting the compressor run long enough in its "run" cycles for adequate cooling. It's a simple and somewhat inexpensive thing to try first . . . plus making sure the system is adequately charged and the condensor is not externally clogged up too.

There might also be a "WOT Cut-out" in the system too. This will cut off the compressor during "power demand times" for better acceleration performance. It might be run by the Throttle Position Sensor or by a vacuum switch, but as soon as normal throttle conditions return, so should the cooling. I believe this would be an ECM-controlled function, but I'm not fully sure.

By observation, many people look at the control head (in the instrument panel) as their first thing to replace with a/c problems. It might command certain functions, but said functions are also influenced by other inputs to the control system by sensors and switches elsewhere in the vehicle. The price of those control heads (when the cars were newer) usually made people look elsewhere or want to try something else less expensive first.

Just like most blower motor "not acting right" issues (with automatic hvac systems) are the result of a (more common) failed "Blower Motor Module" in the hvac housing. And, on the vacuum-controlled actuators, with the air coming out of the heater and defrost ducts (on a/c or otherwise non-heater-related functions) was the fault of a decreased vacuum source to the actuators (usually from a cracked vacuum t/check valve for the cruise control (usually located near the power brake booster) from a hair-line crack in the vac tee part of that part (which is the vacuum source for the actuators).

And, of course, checking the orifice tube (which would require discharging and recovering the freon gas for recycling and recharging) might indicate that something in the compressor is or has or is getting ready to "come apart". A clogged orifice tube screen is just that, a "clog", so cooling capacity would be lessened somewhat.

So, if you have the a/c shop do anything, you might first hav them to recover the freon in the system, discharge the system, check the orifice tube for "debris", replace if necessary, recharge the system and check for leaks. This should be a pretty basic situation. Hopefully, what freon in the system hasn't been "contaminated" with non-approved gasses so it won't contaminate the recovery system at the a/c shop.

From that point, you should know the system is fully charged and leak-free. Then it's time to do other checks and such, plus possibly cleaning the condensor at a high pressure car wash. Doing these things will then get things more ready for a better research and diagnosis of what's happening and why--typically.

If you get the orifice tube replaced and things work fine for a while, then decrease, then find the orifice tube is again clogged, it might be a sign that the a/c compressor itself might be an issue.

Just some thoughts,


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Thankd NTX5467 for the reply. As I read it I thought of something important that I did not mention: That is, the "Service A/C" light comes on when the cooling stops. The blower keeps blowing, just not blowing cold. As an expermient I just went out and adjusted the temperture higher than the ambient temperature to see if the blower would shut down and it did---the blower shut down as it should have.

So, what condition causes the "Service A/C" indicator to come on? It seems that the compressor is being turned off but the blower is left running.

Again thanks for your suggestions. I will use these when I go back to a qualified A/C service person.


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