Incessant clicking - on, off, on - of ac compressor clutch!

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Old 01-19-10, 11:12 AM
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Incessant clicking - on, off, on - of ac compressor clutch!

Have been told by two people that this is normal when vehicle's ac is even in off position, but control set to defrost, in the cold or winter. (It will not do it, we were told, if you set control to "floor", instead of "defrost".) In fact, this subject has been discussed here, how the mfg wants to have the ac compressor run, to take moisture out of the car in winter?

But would they really have it so the compressor clicks in and out, every couple seconds? That just does not seem normal. The sound and engine draw-down, is obnoxious. And you'd think it might even be hard on the alternator, due to the load coming and going. (In fact his alternator is shot, and he is getting it towed to the shop today) You'd think this action would wear various parts out.

Posting this for neighbor who wants to know - not the fact the clutch kicks in, in the winter - but how it clicks on and off repeatedly.

2001 Ford Ranger.
 
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Old 01-19-10, 02:17 PM
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the reason the compressor cycles is almost always due to a low refrigerant charge. Within the system, there are two pressure sensors that control the compressor and the one that is turning it off is on the low pressure side that when the pressure drops below a predetermined point, the compressor stops. If there is low refrigerant, the pressure in the low side is never as high as it should be so it takes much less time, sometimes merely a second or so, to pump enough additional refrigerant from the low side so as to cause the compressor to kick out.

have the regrigerant checked in your system. I would bet you will find it is low.

you also need to realize that during cold weather, the refrigerant gets cold too and as such, will reduce the static pressure as compared to warmer ambient temps so it is more noticeable and common for the compressor to cycle frequently when the ambient temps are low.

Oh, and it is hard on parts. Not so much the electrical system but the compressor clutch, both the actuating coils and the clutch itself (friction discs). It also tends to cause undue strain on the compressor bearings as every time it engages, there is a sideward thrust on the compressor shaft which can shorten life span of the compressor as well but mainly the clutch is affected.
 
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Old 01-19-10, 04:44 PM
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I knew about the low and high pressure thing, and already thought of that myself. In fact I told that to my neighbor.

But due to......did I say a "couple" of people in prior post?....it was maybe actually 4 people - 2 for real mechanics, and 2 other lay shadetree guys, that said that is normal for that vehicle to do that.

Yet, me nor my neighbor, whose truck it is, can hardly believe that could possibly be true. It only runs literally for a few seconds, quits, then starts it all over again.

I'll tell my neighbor that so far one response is in our favor.
 
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Old 01-19-10, 05:02 PM
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well, I used to be a real mechanic, certified and all by Michigan and ASE in air conditioning as well as a bunch of other stuff.

it is normal for a compressor to run in the winter time due to the dehumidification but it is not normal for it to cycle quickly any more than it is in the summer. Due to the lower pressures due to the ambient temp, it might cycle a little more often but a very quick cycling is not normal.
 
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Old 01-19-10, 05:10 PM
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And then some vehicles (my wrecker for example) have a temperature probe on the evaporator core that runs to a thermostatic device; if the core is in danger of icing up it cuts the compressor out. The thermostat on mine has an adjustable set screw for its sensativity.

Nobody ever mistook me for a real mechanic.
 
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Old 01-19-10, 08:52 PM
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So far as cycling on/off, the A/C is designed to turn off the compressor just before the evaporator coils inside the car reach freezing temperature.

And as noted, air conditioning will remove the humidity from the air. You can notice this by the water dripping from a window air conditioner in the summer.

A car does this too. There is a drain hose at the bottom of the inside evaporator compartment which goes outside. You might notice a water spot on the pavement if you park for a short period in the summer, then move the car.

SO.... There is this water on the evaporator coils inside the car... And air blows through these coils....

If the coils were to reach a temperature BELOW freezing, the coils would freeze up into a block of ice, and air would no longer be able to flow through the coils. (little or no air would come out of the vents.)

So they design the system so the compressor shuts off before the coils reach freezing....

AND... The coils will take a long time to reach the almost freezing temperature on a hot summer day with a lot of heat from the car going into the inside air conditioning intake. AND will also not reach freezing temperatures as soon if the air blowing over the condenser coils in the front of the radiator is hot!

Thus on a hot day, the A/C compressor will not click on/off very frequently.

But on a cooler day, it will click on/off more frequently.

And in the winter, especially if the car is cold on the inside, the coils will reach freezing quickly. So the compressor will click on/off quite frequently.

BTW - I have a manual switch to turn on the A/C in my car and I turn this on when I want to defog the inside windows in the winter. And lots of clicking on/off of the compressor going on in the winter!
 
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Old 01-19-10, 10:33 PM
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P.S. You mentioned engine draw-down...

On modern fuel injected vehicles I have worked on, the A/C compressor signal is routed in various ways to the engine computer. Then the engine computer adjusts idle to compensate for the additional load of the A/C compressor when it is on.

So ideally, if everything is working as it should, the engine would remain a constant speed or increase its speed a bit when the A/C compressor is turned on.

And same thing with lights or a heavy electrical load. I've seen this monitored by the engine computer in various ways to also maintain idle speed.

I don't know how that vehicle is designed though. You would need to dig into the electrical diagrams or read the HVAC section description of operation to see how it should work.
 
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Old 01-20-10, 03:32 PM
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Originally Posted by nap View Post
....., it might cycle a little more often but a very quick cycling is not normal.
Every few seconds seems pretty quick. And obnoxious too.
 
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Old 01-20-10, 03:43 PM
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Originally Posted by the_tow_guy View Post
And then some vehicles (my wrecker for example) have a temperature probe on the evaporator core that runs to a thermostatic device; if the core is in danger of icing up it cuts the compressor out. The thermostat on mine has an adjustable set screw for its sensativity.
Makes sense. I wonder if most vehicles now have that? And if it is out of callibration, maybe that could cause a vehicle to do like my neighbors.
 
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Old 01-20-10, 03:49 PM
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Originally Posted by Bill190 View Post
P.S. You mentioned engine draw-down...
I can't remember exactly what it does. But you know it when the clutch engages - let's put it that way. I will be seeing my neighbor when he gets his truck back from the shop and I will pay closer contention, and may post about it.
 
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Old 01-20-10, 04:12 PM
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Like Nap said, if it's cycling very quickly, most likely it's low on freon.
 
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Old 01-22-10, 06:00 PM
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OK guys, we're talking Ford here...........The first thing to check would be low pressure.........BUT.........If the compressor is starting to come apart internally, and debris clogs the orifice tube......When the compressor turns on the high side rises and the low side goes into vacuum........and the low pressure switch cuts off the compressor till high and low start to equalize and the process begins again........Either way, first thing to do is hook up gauges
 
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Old 01-23-10, 11:57 AM
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The neighbor has his truck now sitting back in the driveway. Can't wait to find out what was found out if alternator was the problem and if any more was made of the clicking compressor clutch. It was that mechanic who told him it was normal. But Ford truck owner neighbor does not believe it and maybe talked to him more about it. I'll let you all know what I find out. Not sure if neighbor will tell me what it all costs, though, especially since I told him the other neighbor would redo the alternator for the fun of it + cost of parts, only.
 
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