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AC cold at idle, warm at highway speed

fastcw on Fri August 14, 2009 3:01 AM User is offline

Year: 1971
Make: Buick
Model: Riviera
Engine Size: 455
Refrigerant Type: R-134
Ambient Temp: 75
Pressure Low: 32
Pressure High: 250
Country of Origin: United States

I have converted the system to R134a, including adjustment of the POA valve.

It blows ice cold at idle. When I drive the car, and get up to 30 mph or so, the vent air temp goes up, and after a few minutes of driving, the vent air is barely cold.

When I stop the car, and idle it, the cold air returns.

When the car is standing still, I can run the engine at 2000-3000 rpm, and the air is still cold. The high and low side gauge readings are normal, around 300/35 psi.

When I drive the car, and get it moving 30 mph or so, the high side reading drops to under 100 psi, along with the loss of cool air. Bring the car to a stop, the high side goes up to 300 psi. Rev the engine with the car stopped, the high side stays at 300 psi.

My first thought is that due to reduced engine vacuum while driving, the ac system temp control is being set higher. But, would that cause the high side reading to drop while driving?

How does the ac temp control on the dash regulate the temp? If it does it by mixing outside air with the fully cold ac air, maybe that is my problem. But would that cause the high side reading to drop to 100 psi?

Any ideas?

Dennis Mowers

HECAT on Fri August 14, 2009 8:43 AM User is offline

Never messed with POA's

Poor air flow at idle? Compressor cycling off? 100 psi static pressure?


HECAT: You support the Forum when you consider for your a/c parts.


fastcw on Fri August 14, 2009 11:24 AM User is offline

I didn't think about the compressor turning off. I'll bypass the clutch control wiring and wire 12V directly to the clutch, and see if that cures the problem.

Dennis Mowers

NickD on Fri August 14, 2009 12:21 PM User is offline

Wow this goes back, did have a 70 Riviera, first car ever with cruise control and did have automatic climate control. Do recall the shop manual, four bucks in those day including shipping and handling from a coupon in the rear of the owners manual had a red cover, and the very rear sheet was a colored diagram of the entire vehicle electrical system. But what was on it is not entirely clear over the years.

ACC was electromechanical, temperature control that blue red knob was a potentiometer whose reference resistance was compared to an in-car thermistor that fed a single transistor circuit to control a small motor that drove the blend door. These climate controls as I recall did not have an Econ position, and the compressor would run all the time if the ambient temperature was above 33*F. I didn't like that, so added a nice black rocker switch under the dash to break the clutch coil circuit. Recall the entire unit was not difficult to remove and recall watching it on my workbench where you could see what was going on. Major problem was dirt build up, and a tad of oil in the blend door bushings so it would run smooth, used a resistance substitution box to emulate the in-car sensor. The blower motor was not so easy, had to remove both the front bumper and the right front fender to replace that. Radiator in that car was crap compared to my 65 Buick that lasted close to 300 K miles, think I had to replace that at around 40K, I also swiped an HEI and a 10SI from Delco, to get rid of those troublesome points in about 1972. Fan clutch was also crap.

I just can't recall the ambient temperature sensor in that car, I think it was something like an underhood thermistor in series with the clutch coil relay solenoid, in those days, this stuff was simple. But I cannot explain why yours is working at idle, but not at highway speeds. Recall a rotary switch with real resistors on it for blower motor speed control that was also motor driven.

Wonder if Tim has a circuit on your vehicle, but from your pressure readings, sure sound like the compressor is not running, has to be electrical. Packard Electrical terminals were used back then, unplated brass that would turn green, always a problem, but still have my PE terminal crimper, silicone grease sure helped.

That was sure a flashy car in those days, heavy, around 5,000 pounds, was stopped at a traffic light when a young woman pulled out of an IGA parking lot and hit me on the right side. Couldn't do a thing about that, wasn't worth repairing, used the insurance money to get a new car. Cars were cheap back then. Think I had around 98K on it. 65 Buicks were made far superior.

fastcw on Fri August 14, 2009 12:53 PM User is offline

I think I found the problem. The AC clutch wiring goes to a vacuum switch mounted on the carb.

I bypassed the switch, and it seems to work with a short test drive. I'll be going 10-15 miles later, and will see if that was the problem.

By the way, the POA adjustment if pretty easy, and makes a big difference in the AC temp. The quick way to check the POA setting is to run the AC at idle, unplug the blower fan, and look at the low side pressure. After a minute or so, the low side will stabilize at the POA setting. Mine was 32 psi, stock setup for R12.

It is easy to adjust, but you have to waste your refrig charge when you open the system. I backed the POA hex set screw CCW .75 turn. You can check the setting without a full charge. I put in about a half a can of R134, and looked at it. The reading was about 27 psi.

Dennis Mowers

NickD on Fri August 14, 2009 11:15 PM User is offline

Thought 28.5 was the normal setting for the POA with R-12, yours was set high for poorer vent temperatures. I don't recall that vacuum compressor cut-out switch on my 70, could be something they added in 71 to gain a couple of extra available HP when hitting the pedal.

mk378 on Fri August 14, 2009 11:40 PM User is offline

That switch is supposed to drop out the compressor at wide open throttle to get more power to the wheels. You can do without it. A POA system's compressor does not cycle in normal operation. It should run all the time that the selector is on "A/C". If less than maximum cooling is desired, the fan or the blend door would be adjusted.

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