Compressor cycles on and off

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340doc

340doc

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I just got the AC in my Duster up and running, all stock components, R12 Freon, fresh oil etc. It blows cold but the compressor clicks off about every 30 seconds. It stays off for about 10 seconds then comes back on. Freon is full, pressure is good. I checked the low pressure switch on the dryer and the power coming to the switch is being interrupted. Is there something in the car under the dash that is cutting power? It hasn't really been an issue because it stays cold but I don't remember them working this way.. Thanks!!
 
OK, as usual, what are we working on, AKA model year, and is this a factory setup, vintage "knee knocker" or modern "Vintage air?"

How do you know the freon is "full" you charged it by weight?

Some of these systems had a stat with a probe in the evap. If that stat starts to go bad this can happen.

The low pressure switch can be bad, or............

you could be low on refrigerant, or............

you could have a problem causing a restriction in the metering. This includes mechanical problems with the metering valve itself, as well as WATER in the system which will FREEZE

If "we" are going to look at a diagram which shows your exact system, we need to know "what" exactly you have.

Have you looked at it with a gauge manifold? What does the suction pressure do?
 
Could be shutting off on low pressure or high pressure. You say its full. Have you put a/c gauges on to check pressure? Did you vacuum the system and put new drier on it before charging? Does it have a sight glass on it that you can look for bubbles in it?
 
67cudafb said:
Could be shutting off on low pressure or high pressure.
Click to expand...

So far as I know the older factory cars don't even have a hi pressure switch. They might have a relief, or blow off valve, no switch
 
OK, car is a 1971 Duster with factory air, all stock components. I replaced the dryer and expansion valve, evacuated and charged system until sight glass ran clear. The pressure on high and low side is fine. The system is working fine, nice and cold even on a hot FL day. The low pressure switch is not cutting power to the compressor. The power coming from inside the car going to the low pressure switch is going on and off. Tell me more about this thermostat..
 
67Dart273 said:
OK, as usual, what are we working on, AKA model year, and is this a factory setup, vintage "knee knocker" or modern "Vintage air?"

How do you know the freon is "full" you charged it by weight?

Some of these systems had a stat with a probe in the evap. If that stat starts to go bad this can happen.

The low pressure switch can be bad, or............

you could be low on refrigerant, or............

you could have a problem causing a restriction in the metering. This includes mechanical problems with the metering valve itself, as well as WATER in the system which will FREEZE

If "we" are going to look at a diagram which shows your exact system, we need to know "what" exactly you have.

Have you looked at it with a gauge manifold? What does the suction pressure do?
Click to expand...
Sorry, suction pressure is steady at 30-35 until comp. cuts off.
 
I would be tempted to put another 1/2 pound in and see if it clears up. No bubbles in the sight glass and cool air out of the ducts doesn't necessarily mean it's fully charged. Duct temps can get down to the mid to upper 30's without freeze up. R12 systems can stand more freon before being overcharged. 134 systems will not tolerate overcharging.
 
if pressures are good and system is cooling stat probe in the evaporator is cycling. stat switch prevents the evaporator from freezing into a solid block of ice reducing air flow. Switch is located on the lower left side of the air box and has a temp probe going into the evaporator. What is the duct temp while cycling?
 
mderoy340 said:
if pressures are good and system is cooling stat probe in the evaporator is cycling. stat switch prevents the evaporator from freezing into a solid block of ice reducing air flow. Switch is located on the lower left side of the air box and has a temp probe going into the evaporator. What is the duct temp while cycling?
Click to expand...
That makes sense, I'll have to get back to you on the duct temp, don't recall what it was. Is the stat accessible from outside the box or do I have to split it? My thanks to all who are trying to help!
 
mderoy340 said:
if pressures are good and system is cooling stat probe in the evaporator is cycling. stat switch prevents the evaporator from freezing into a solid block of ice reducing air flow. Switch is located on the lower left side of the air box and has a temp probe going into the evaporator. What is the duct temp while cycling?
Click to expand...

Exactly.
They HAVE to cycle, and it's the entire reason they have all that clutch hardware on them.:D
And, it sounds like it's cycling at about the right intervals.
I don't think you have a problem.
 
TrailBeast said:
Exactly.
They HAVE to cycle, and it's the entire reason they have all that clutch hardware on them.:D
And, it sounds like it's cycling at about the right intervals.
I don't think you have a problem.
Click to expand...
I understand there supposed to so they don't freeze up but I don't recall these systems cycling so much, maybe someone with a stock running system can offer input on how fast their comp. cycles. I'm not looking to fix something that's not broke, just wondering.
 
toolmanmike said:
I would be tempted to put another 1/2 pound in and see if it clears up. No bubbles in the sight glass and cool air out of the ducts doesn't necessarily mean it's fully charged. Duct temps can get down to the mid to upper 30's without freeze up. R12 systems can stand more freon before being overcharged. 134 systems will not tolerate overcharging.
Click to expand...

I say no. He's determined that it's cycling on the evap temp switch and not low pressure

Undercharged, the evap is not going to get the stat cold enough to cause this

Either what he's seeing is normal.........how often IS it cycling.......?

Or likely the stat itself is going bad

More on the stat? Not much too 'em. Read your shop manual.......it's a cap tube stat that has the tube in intimate contact with the evap coil

The only other thing I could think of is that if a door in the box is effed up and causing recirculation IE blocking airflow through the coil. This seems UNlikely given the description

These diagrams are actually from the '70 manual
 

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67Dart273 said:
I say no. He's determined that it's cycling on the evap temp switch and not low pressure

Undercharged, the evap is not going to get the stat cold enough to cause this

Either what he's seeing is normal.........how often IS it cycling.......?

Or likely the stat itself is going bad

More on the stat? Not much too 'em. Read your shop manual.......it's a cap tube stat that has the tube in intimate contact with the evap coil
Click to expand...

We need to see the pressures, ambient temp, and duct temp to make educated guesses..I agree that the capillary switch is probably the culprit if the charge is correct. tmm
 
toolmanmike said:
We need to see the pressures, ambient temp, and duct temp to make educated guesses..I agree that the capillary switch is probably the culprit if the charge is correct. tmm
Click to expand...

I'm going by he claims he has determined that it's NOT cycling on low pressure. Seems to me this negates worrying about the charge for the moment, and he sounds as if he's electrically astute.

And by the way, I'm a guy who BELIEVES in guage manifolds and temp readings
 
67Dart273 said:
I'm going by he claims he has determined that it's NOT cycling on low pressure. Seems to me this negates worrying about the charge for the moment, and he sounds as if he's electrically astute.

And by the way, I'm a guy who BELIEVES in guage manifolds and temp readings
Click to expand...
Ok, thanks to you all, I'm going to look to see if I have another thermostat and swap it out. Thanks for the direction!
 
They usually cycle, if the air temp coming out of the register is in the upper 30's to about mid 40's on a 85ish degree day it's fine.
 
Stick a temp probe in the center duct. Start the AC and set to MAX AC with fan on HIGH, pay attention to how long (time wise) it takes to start cycling and watch the temp reading as the comp cycles on and off. Set AC to NORMAL and see if the cycle time extends. Make sure the drain tube is dumping water, if its plugged water will accumulate and freeze on the evaporator. As it freezes the airflow will decrease and freeze up even more. You also may have a dirty evaporator, full of old leaves, dirt, etc. restricting airflow.
 
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