What happens to a direct drive, clutchless, solenoid controlled A/C compressor when the system loses its refrigerant? (e.g. a hose leak develops). Without refrigerant flow, there is no oil flow; the compressor spins but is not lubricated. Therefore, the compressor will eventually lockup, and the belt-driven pulley overload protection will break in order to allow the serpentine belt-driven pulley to continue to turn. All comments appreciated.
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Welcome. First off please know I personally don't have any hands on knowledge proven to me about that system's use of oil and how. Also, here or wherever do state the year, make model will help a lot.
So you and I do know low charge can and will mean oil starved compressors BUT this style IDK if too low if it wears/burns it up or can run on just bearings without disabling this vehicle! I want to know and you need to - never mind having A/C it needs to run and not lose the belt take out the use of it totally.
By all odds and guessing if you unplug wires to actual unit it should default to a non destructive mode and suggest you at least do that possibly get some warning light over it?
What you can and should do now is like anything that's leaking find exactly what and where plus part(s) needed to be ready. How much oil evidence if any is there noted. If it can hold and pressure or has some now take it, just static pressure like tire pressure taking note the temp in the area also. That's info for right now about how low it is or can say if pressure in PSI is lower than real temp in F. is a serious warning system is essentially empty. That's important to know now.
For today just that then let's see what we can do to stop any damage then go on for a fix,
Tom
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Hi Tom,
Thanks for taking a stab at this issue.
The question I pose is for general knowledge. Numerous vehicles are now using direct drive compressors fitted with refrigerant control solenoids including Cadillac, Lexus, Chrysler, VW, Audi, and others. I’m curious to know what happens to the compressor when the A/C system develops a leak? Just like cycling clutch systems, direct-drive compressors require a mist of oil carried by the refrigerant for lubrication and heat dissipation purposes. But unlike cycling clutch systems, direct-drive compressors lack the ability to turn off – they are always running, even when A/C is not requested. The only compressor wires are those attached to the refrigerant solenoid. Disconnecting those wires will only disable the solenoid and have nothing to do with stopping the compressor from turning. Direct drive pulleys are a ‘break-away’ style. If the compressor shaft seizes, the pulley-to-shaft coupling will shear in order to allow the serpentine belt compressor pulley to continue turning. When the engine turns, the compressor turns. The compressor only stops turning when the pulley clutch pin shears off.
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Let me 1st say I have no 1st hand experience with them. My understanding is they are a swash plate type piston compressors. when disabled, the plate goes vertical, and no piston movement. that way you just have a shaft, rotating on set of bearing, hopefully with a little oil for the bearings.
Theoretically, you could use the static pressure of the refrigerant to move the swash plate, so as the pressure is lost, so is the angle of the swash plate. Having said that, I don't see it as a good system.
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Jimbob,the direct drive compressor at low refrigerant will still be lubricated because the electric solenoid will be open more reducing the compression rate so the oil won't be leaving the compressor with low refrigerant.The electric solenoid is control by the ECU which also has the pressure reading from the P/Switch which these system use a transducer PS ( 3 wires plug),so at low pressure reading ECU apply lower voltage to the solenoid control valve and the needle inside open more causing the head pressure to drop. Think of this control valve as the admission/inlet and exhaust valve in an engine,when you have both inlet and exhaust valves open your engine spin freely with no resistance,that is exactly what happens with the compressor when AC is OFF no power to the control solenoid which is in open position creating a bypass between the suction and discharge chambers.Comment
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YT video on solenoid clutchless compressor.
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Nickyanc, thank you for your insightful answer. If I understand correctly, the pressure transducer tells the ECU that usually low pressure is present in the A/C circuit, and the ECU responds by decreasing the refrigerant solenoid duty cycle to zero, even if A/C is being requested. This effectively closes the compressor suction and discharge ports and keeps the lubricating oil inside the compressor. The oil remaining in the compressor will suffice for the moment, but it will be dissipated after a limited amount of time – when the refrigerant pressure is low, the catastrophic destruction clock is ticking. I am guessing that on direct drive compressor equipped vehicles, low A/C system pressure illuminates a warning telltale or some light flashes continuously. (can anyone verify this?)Originally posted by nickyanc View PostComment
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No it doesn’t close the suction or discharge ports,it actually open up and link them together so pressure is equal on both sides .no suction and no compression.The oil in the compressor stays in there cause there is no refrigerant movement to carry it.There is no flashing/warning light on these new AC systems with transducer PS,the only way you can tell is by hooking up the gauges on the AC or computer scanner which will read the pressure to you.Comment
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Nickyanc, Can you provide a "ghost drawing" or a YT video of a compressor that works like that? Everything clutchless on YT that I found were swash plate type compressors. I just can't see it working that way with the pistons still moving and little to no oil in the system? I also don't see the solenoid to control headpressure anywhere. The solenoid on a swashplate compressor only controls the swashplate angle and therefore the effective displacement.Comment
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The attached link provides an excellent explanation of variable displacement compressors.
Question: Is the following statement correct? "All direct-drive compressors are variable displacement." I think this is true but would appreciate confirmation.Comment
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I've spent the morning reading up on everything I can find on them, and yes as far as I know all are of the variable displacement swashplate type. It is worth noting however that while all clutchless appear to be variable displacement, not all variable displacement are clutchless.
Swashplate compressors have been around since before the A-6 came out in the early 60's The A-5 that proceeded the A-6 was also a swashplate design.
I have not seen anything like what nickyanc describes, a fixed displacement with a "bypass" solenoid. which is why I ask for some drawingsComment
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As Cornbinder89 stated in an earlier post, "when [the compressor is] disabled, the [swash] plate goes vertical, and [there is] no piston movement. [When the swash plate is positioned vertically] only the shaft is rotating on a set of bearings." With no piston movement, lubrication requirements would be minimal. The oil remaining in the compressor when the A/C system became inoperative will obviously not last indefinitely, it needs to be replenished, but should allow compressor rotation for some time. The amount of time the compressor can continue to rotate with only residual oil is a question I am seeking to have answered.Originally posted by Cornbinder89 View Post
It is my understanding that all direct-drive, clutchless compressors, are of variable, swash plate style design. Perhaps someone can comment on this.
The compressor refrigerant solenoid controls the A/C system refrigerant flow and therefore is directly responsible for system head pressure (and low-side suction pressure).Comment
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I don't have an answer to your question about how long you could run it, but everything I have read says that it is very important to keep the system charged.
I may be wrong about the swashplate going vertical, further reading tends to point to "minimum" angle, so there would still be some piston movement, and that need lubrication.
I have the same understanding that you do Jimbob, and I have spent the morning reading what can be found on the .net. Nothing I have read changes substantially what I knew before.
It is kind of interesting (to me anyway) , the progression of designs from the 50's to today. In the beginning, the compressor ran continuously when A/C called for and the evaporator pressure, and therefor temp was controlled by valves or regulators (POA, STV, EPR) then they cycled the clutch to control evaporator temp ( harder on clutches, but cheaper to make) then we got the variable displacement compressors, and next I suspect we will see electric compressor, 3 phase with variable frequency drives, to control compressor speed. I suspect these may already be the case in all electric vehicles, but don't know for sure.Comment
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Hello again Cornbinder89, Your comment about the swashplate going vertical is essentially correct. The video link posted in this thread explains that the swashplate default position is "minimal stroke" which would be accomplished with a near-vertical position. I read that a direct-drive compressor's minimum duty cycle is 2%, just enough to maintain sufficient refrigerant flow with captured lubricant. When additional cabin cooling is requested, the refrigerant solenoid is commanded 'On' (increased duty cycle) which causes a longer stroke and more refrigerant displacement.
I speculate that if the refrigerant pressure sensor (transducer) sees little or no A/C system pressure, it would command a zero percent duty cycle which would cause the swashplate to assume a vertical position and stop all piston movement thus minimizing lubrication requirements and preventing internal compressor damage.
My interest in this matter is from a repair perspective. Direct-drive replacement clutches are available, but if the clutch has broken loose, it seems as though internal compressor damage would have already occurred and compressor replacement is required. Unlike cycling clutch compressors, direct-drive clutch replacement sounds like a bad idea.Comment
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I may be wrong but I believe the 2% duty cycle refers to the PWM (voltage pulse ie. square wave) to the solenoid. I'm not sure it goes true vertical, but even if it did, it wouldn't last long without oil to lube the bearings and "shoes" on the swashplate. I read minimal angle as too mean that the plate can't be placed in a less (vertical) stroke position.
It bears a lot of similarities with hydrostatic hydraulics.
The break a away drive sounds like it wouldn't be field replaced, I agree. If it failed, you'd need a new rotating parts and only the case would be re-used.
I too am interested, but all of my stuff is in heavy trucks, and I don't have to deal with the newer stuff, but I do like to keep up on it.Comment
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