Clicking Sound In Dash And A/c Compressor Cycling

[87MJJeep]

I had a new compressor put on back in the summer (and converted to R-134a), and after about two weeks of blowing really cold and continuously, the compressor seemed to start cycling - or cycling more.  I can't recall if it cycled any while the air felt cold 100% of the time.  After a while, I realized I hear a flimsy clicking sound that seems to be in sync with the compressor's cycling.  Click on and click off. 

 

If it is the A/C clutch cycling, I would be surprised, because the sound is so distinct, and I would bet money it is coming from behind the dash or over the passenger kick panel.  I wondered if there is any sensor/switch that is mounted on the evaporator that would make such a noise and send a signal to cycle the compressor. 

 

When the compressor is on, the air out of the middle vent with it set to MAX A/C gets down between 40-45F. 

 

It does the same clicking/cycling now that I am using the defroster. 

 

Thanks for any input. 

[87MJJeep]

I just remembered something that may be useful.  When there were still summer temps, it seemed like the compressor cycled less frequently if I ran A/C instead of MAX A/C. 

[gogmorgo]

If the pressure on the low side of the system is too low (i.e. there isn't enough refrigerant) then the system will shut the compressor off. Then the refrigerant bleeds down to the low side, and there's enough pressure there for it to run again... briefly. Probably you either have a very slow leak somewhere, or else it wasn't fully charged. When recharging an a/c unit from completely empty, my experience is that the compressor simply won't turn on, then it starts cycling, staying on for longer and longer until eventually it stops cycling and is just running constantly.

The switch that controls the vent setting is a vacuum switch. If there's a leak in that system, it won't change the vent setting. Are you actually getting air out of the defrost vents?

[87MJJeep]

Thank you for the explanation. I'll ask the shop to check the charge level and maybe add a dye to determine where it's leaking if that is what's going on.

 

The vacuum operated air controls work fine. It blows where it should for each setting. One of the duct "doors" squeaks when it closes. That's the only thing wrong in that regard.

 

It's just odd I hear that click every time the compressor cycles. To my ears, it is a very flimsy click. Like small metal pieces coming together or similar to the sound plastic makes if you break it. Maybe it is the clutch, and just doesn't sound as "solid" on inside of the truck.

[gogmorgo]

More than likely you're hearing the relay clicking that's activating the clutch.

However, I'm a little concerned about something else. I'm reasonably confident the a/c shouldn't be coming on at all in the defrost position, which is why I was asking that.

It's definitely the a/c compressor doing the cycling (you've observed it with the hood open and the engine running), right? The reason I'm asking is it just occurred to me that if your blower motor were to have a loose connection, that could cause the blower fan to cycle on and off, with a "click" sound each time. But that's just a guess.

[Eagle]

owever, I'm a little concerned about something else. I'm reasonably confident the a/c shouldn't be coming on at all in the defrost position, which is why I was asking that.

It's definitely the a/c compressor doing the cycling (you've observed it with the hood open and the engine running), right? The reason I'm asking is it just occurred to me that if your blower motor were to have a loose connection, that could cause the blower fan to cycle on and off, with a "click" sound each time. But that's just a guess.

 

The defrost cycles DOES run the a/c. It's designed that way.

[gogmorgo]

Huh, weird. Why would you want it to do that? I don't think mine does, but then again, I've never tried using the defrost with cold air... and that would seem counterproductive, as in it would be a pretty good way to contribute to condensation on the outside of the windows in my opinion. I was always under the impression that apart from the three a/c settings, it wouldn't be on.

[ComancheKid45]

Its designed to dry the air out that comes out the defrost vents.....After all fog/frosting is a result of moisture in the air ;)

[Kerry]

If you're getting 40-45 deg air then you're charge probably isn't low. The AC does come on with the defrost and it most likely is the relay you're hearing. It's normal for the compressor to cycle some and it will do it more often in cooler weather. How often on the average is it cycling (count an off and on as one cycle not two)?

[87MJJeep]

When I was driving in the hot weather, I'd say it cycled once per minute, maybe twice.  Maybe the last bit, it would cycle up to three times in a minute.  With the defrost the last few days, I'd guess it cycled as often as once every 10-15 seconds. 

 

Where is the relay you're thinking about?  What controls the relay?  In hot weather, it definitely cycled less with it on A/C vs. MAX A/C.  I sorta assumed that was due to MAX A/C running colder, and it was cycling out of necessity by design, so with it running on regular A/C and cooling the incoming outside air, the sensor(?) wouldn't call for it to cycle as much.   

 

If you're getting 40-45 deg air then you're charge probably isn't low. The AC does come on with the defrost and it most likely is the relay you're hearing. It's normal for the compressor to cycle some and it will do it more often in cooler weather. How often on the average is it cycling (count an off and on as one cycle not two)?

[Kerry]

That seems a little excessive. The AC clutch relay is actually under the hood next to the right side shock tower. Underneath the plastic cover (if the cover's still there) is a row of relays and the AC clutch relay is the one nearest the cab. It is energized by the ECU when it sees an "AC Request" signal which is based on the AC controls being on and the thermostat calling for cooling. The thermostat is located under the dash in the passenger side area. The low pressure switch which is located on the drier (can looking thing) under the compressor will also stop the "AC Request" signal by breaking power before the thermostat. The low pressure switch is mostly a safety to prevent your compressor from running without freon but it will also open if the stat is stuck on or bypassed because low temp equals low pressure. If there's any warranty left from your compressor replacement I'd let them take a look but if not and you have a multimeter PM me and I can email you a wiring diagram. Also the info above is based on a 4.0 Renix but all should be similar.

[gogmorgo]

Its designed to dry the air out that comes out the defrost vents.....After all fog/frosting is a result of moisture in the air ;)

Yeah, but cooling down the window causes the fog/frosting to deposit itself on the outside of the window... the only reason I've ever found to use the defroster is to warm up the windshield to prevent/eliminate condensation. Is there any relation between the temperature setting and the a/c coming on with the defroster? Would it come on with other settings, too?

 

 

The AC clutch relay is actually under the hood next to the right side shock tower. Underneath the plastic cover (if the cover's still there) is a row of relays and the AC clutch relay is the one nearest the cab. [...] Also the info above is based on a 4.0 Renix but all should be similar.

My HO is the same, although I'm not positive on exact placement of the relay in the fuse/relay block. But it's written on the cover... if the cover's still there. It's also in the owner's manual... if that's still there.

[87MJJeep]

That seems a little excessive. The AC clutch relay is actually under the hood next to the right side shock tower. Underneath the plastic cover (if the cover's still there) is a row of relays and the AC clutch relay is the one nearest the cab. It is energized by the ECU when it sees an "AC Request" signal which is based on the AC controls being on and the thermostat calling for cooling. The thermostat is located under the dash in the passenger side area. The low pressure switch which is located on the drier (can looking thing) under the compressor will also stop the "AC Request" signal by breaking power before the thermostat. The low pressure switch is mostly a safety to prevent your compressor from running without freon but it will also open if the stat is stuck on or bypassed because low temp equals low pressure. If there's any warranty left from your compressor replacement I'd let them take a look but if not and you have a multimeter PM me and I can email you a wiring diagram. Also the info above is based on a 4.0 Renix but all should be similar.

Thank you for the help. I will send you a PM.  I still have warranty left, but I'd like to see what I can determine myself.  I suspect the sound I hear is the thermostat you referenced.