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Discussion Starter · #1 ·
This may have already been covered, apologies...

A/C stopped cooling air, fan still blows. removal of fuse in cabin kills radiator fan (fuse is marked AC). further investigation, removal of ac relay shows no signal voltage (no "-" continuity) i do get constant "+" voltage.

Question:
When there is a call for AC, is it the "-" side that is switched and a constant "+" to the relay signal connection? Jumping the relays' "line" connections will engage the compressor clutch, so i'm confident that side (of the relay) functions as it should. Only problem that i see leaving it that way is that the evap will become a block of ice on 95+ days (depending on humidity i'm guessing), and will eventually get no air through the evap to cool the cabin?


Is this another case of the wiring becoming broken as in the case here? (post #2, Thanks again Plain OM for the insight.)

http://www.subaruoutback.org/forums/109-gen-3-2005-2009/425417-back-up-light-switch-location.html

Thanks All.
 

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The right-hand radiator fan is sometimes referred to as the sub-fan or air conditioning fan in Subaru documentation.

The ECU is allowed to cut the A/C clutch by isolating relay coil ground in response to a few different circumstances:

•lack of demand (A/C turned off or cabin cold enough)
•high coolant temperatures (overheat)
•high throttle (momentary A/C cutout for max acceleration)
•stalled compressor detection

That last one is a bit tricky. If the ECU detects that the compressor has stalled, it will prevent the clutch from working again until the vehicle is re-started. This is done to prevent the engine from shredding the serpentine belt, which would cost you alternator and power steering capability.

There is a revolution sensor on the back of the compressor. If the ECU sees that the compressor isn't keeping pace with the crankshaft, it assumes the compressor has stalled and locks it out to save the belt drive.

A faulty revolution sensor or a slipping clutch can fake out this stall detection, and slipping clutches are popping up more now that these models have aged- this feature was first introduced in 2001.

Relevant video:

 

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I doubt it's related to the broken wire syndrome.

A/C stopped cooling air, fan still blows.
Which fan? Cabin fan, or radiator cooling fan(s)?

Jumping the relays' "line" connections will engage the compressor clutch, so i'm confident that side (of the relay) functions as it should.
That does confirm that if the relay switching contacts are closed (relay energized), then battery power is getting to the AC clutch electromagnet, and the AC Clutch will engage.

However, did you find that when wired this way, there was lots of cold air coming from the AC vents? I ask because if the refrigerant pressure in the AC system is low (e.g., some has leaked out), the Pressure Switch disconnects the power for the AC relay energizing coil, so that even if the ECU is calling for AC by grounding the one side of the coil, the absence of voltage at the other side prevents the relay from switching. By jumping the relay switching connections, the compressor clutch can still be engaged, but the lack of or low level of refrigerant might mean poor cooling in the cabin or damage to the compressor.

When there is a call for AC, is it the "-" side that is switched and a constant "+" to the relay signal connection?
When the ignition switch is On, battery voltage is sent to one of the relay switching contacts, and to one side of the relay energizing coil. The other side of the coil goes to the ECU. When the engine is running and there is a call for AC, the ECU grounds that line, thereby providing a path for coil current. This generates a magnetic field that causes the relay switch contacts to close, thereby engaging the clutch.

There's various reasons for the ECU to not ground the relay coil line. rasterman has mentioned a few.

Another possibility is a bad relay. It's a very common cause. The ECU might be signalling the relay to switch, but if the relay coil or switching contacts are bad, the compressor clutch won't engage.
 

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Discussion Starter · #4 ·
The right-hand radiator fan is sometimes referred to as the sub-fan or air conditioning fan in Subaru documentation.

The ECU is allowed to cut the A/C clutch by isolating relay coil ground in response to a few different circumstances:

•lack of demand (A/C turned off or cabin cold enough)
•high coolant temperatures (overheat)
•high throttle (momentary A/C cutout for max acceleration)
•stalled compressor detection

That last one is a bit tricky. If the ECU detects that the compressor has stalled, it will prevent the clutch from working again until the vehicle is re-started.

So if that is the case, wouldn't i be able to get at least a momentary engagement of the clutch when i restart the engine? Because i don't get even a click from the clutch upon restart. Can a bad sensor defeat a reset?

This is done to prevent the engine from shredding the serpentine belt, which would cost you alternator and power steering capability.

There is a revolution sensor on the back of the compressor. If the ECU sees that the compressor isn't keeping pace with the crankshaft, it assumes the compressor has stalled and locks it out to save the belt drive.

A faulty revolution sensor or a slipping clutch can fake out this stall detection, and slipping clutches are popping up more now that these models have aged- this feature was first introduced in 2001.

Relevant video: Excellent resource, Thank You

https://www.youtube.com/watch?v=7ptUvOv_AfY
I doubt it's related to the broken wire syndrome.

Good to hear, i was dreading that.

Which fan? Cabin fan, or radiator cooling fan(s)?

Radiator fan

That does confirm that if the relay switching contacts are closed (relay energized), then battery power is getting to the AC clutch electromagnet, and the AC Clutch will engage.

However, did you find that when wired this way, there was lots of cold air coming from the AC vents? I ask because if the refrigerant pressure in the AC system is low (e.g., some has leaked out), the Pressure Switch disconnects the power for the AC relay energizing coil, so that even if the ECU is calling for AC by grounding the one side of the coil, the absence of voltage at the other side prevents the relay from switching. By jumping the relay switching connections, the compressor clutch can still be engaged, but the lack of or low level of refrigerant might mean poor cooling in the cabin or damage to the compressor.

Air was ice cold, until the evap froze up and stopped passing air. So i think (that side of) the system is ok

When the ignition switch is On, battery voltage is sent to one of the relay switching contacts, and to one side of the relay energizing coil. The other side of the coil goes to the ECU. When the engine is running and there is a call for AC, the ECU grounds that line, thereby providing a path for coil current. This generates a magnetic field that causes the relay switch contacts to close, thereby engaging the clutch.

There's various reasons for the ECU to not ground the relay coil line. rasterman has mentioned a few.

Do you know of any others?

Another possibility is a bad relay. It's a very common cause. The ECU might be signalling the relay to switch, but if the relay coil or switching contacts are bad, the compressor clutch won't engage.
I think the relay is ok. I'm not getting a "-" connection from the relay socket. Rastermans explanation of the ECM's opening of the "-" side due to a sensor problem mimics my problem.
 

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So if that is the case, wouldn't i be able to get at least a momentary engagement of the clutch when i restart the engine? Because i don't get even a click from the clutch upon restart. Can a bad sensor defeat a reset?
As I understand it, the system should try to engage at least once upon fresh start. If the revs don't match, then the computer disengages the clutch and "marks it out" so it won't re-engage until the next engine start.

If yours isn't engaging at all, I would look at the combination pressure switch. That switch can interrupt the ECU's signal to the relay if the refrigerant pressure is either too high or too low.

Alternately, just hang a set of gauges and see where your pressures are at- I think you're headed there anyway. I'm inclined to think it's far enough off that the switch doesn't want to allow the system to start. If the pressure is off, get the remaining refrigerant recovered, replace the o-rings and schrader valve seals, and then evacuate & recharge.

There is a good thread on this board covering the o-rings and valve seals. Very cheap and easy to do.
 

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Just wondering . . .

If I understand the reports so far, when the AC is turned on, the radiator fans come on. I presume (subject to verification) that when the AC is then turned off, the fans go off (unless, of course, they stay on because the coolant temperature is high).

If the ECU is locking out the compressor, why would it still turn on the fans? There's no need for cooling the condenser, and turning them on is adding unnecessary electrical load on the engine.

According to the auto AC wiring diagram, when the pressure is low the pressure switch cuts the 12 V power for the AC relay energizing coil, thereby preventing the relay from switching. It also cuts the same line going to the auto AC control panel, which I believe is to signal that the pressure is low. In this situation, i.e., when the pressure is low and the AC won't work, do the radiator fans turn on when the AC control is turned on?
 

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Discussion Starter · #7 ·
Just wondering . . .

If I understand the reports so far, when the AC is turned on, the radiator fans come on. I presume (subject to verification) that when the AC is then turned off, the fans go off (unless, of course, they stay on because the coolant temperature is high).

If the ECU is locking out the compressor, why would it still turn on the fans? There's no need for cooling the condenser, and turning them on is adding unnecessary electrical load on the engine.
Beats me, thats why i'm here...
According to the auto AC wiring diagram, when the pressure is low the pressure switch cuts the 12 V power for the AC relay energizing coil, thereby preventing the relay from switching. It also cuts the same line going to the auto AC control panel,
Does that mean that when i touch the "auto" button on the evo control that it, (the cabin fan) wouldn't come on? Because it still does.
which I believe is to signal that the pressure is low. In this situation, i.e., when the pressure is low and the AC won't work, do the radiator fans turn on when the AC control is turned on?

Again, i don't think this is a low refrigerant problem. System blows cold air till it freezes up. That being because when i have it jumped, it runs without control. Normally, it would (with control) cycle to keep from icing.
Thanks for the continued troubleshooting.
 

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My reference to fans was about the radiator fans, not the cabin fan. The AC is used to cool the car, but it's also used to dehumidify the air. When the HVAC is set to one of the two windshield demist positions, and the fan control (or auto control) is On, the AC is automatically turned on so that air being blown onto the windshield will benefit from the dehumidifying action. But, if the AC is locked out, you would still want fan to blow onto the windshield, for safety. The cabin fan would not be disabled when the AC compressor is locked out.

When the pressure is below the pressure switch low pressure threshold, it cuts the power to the relay. However, that doesn't mean that the system is void of refrigerant. Consequently, when the relay switching contacts are jumpered the compressor might run, and there might be cold output from the cabin vents, but the system isn't running properly. If I recall correctly, icing of the evaporator can be a symptom of low refrigerant.

How and where are you measuring the relay control signals?

There's a copy of the relevant part of the wiring diagram attached here. (There's additional diagrams in post #6, and the whole thread might be of interest.)

Terminal 28 of the relay connector should have system voltage when the ignition is at ON. Terminal 29 goes to the clutch electromagnet and from there to ground, so there's no voltage there when the relay is not switched on.

Terminal 31 will have system voltage on it when the ignition is at ON and the pressure switch is closed, i.e., the pressure is good. So with the relay out, if there's power at terminal 31, the pressure isn't the problem.

The ECM switches the relay by grounding terminal 30. This is probably through a switching semiconductor, but if the ECU is locking out the compressor there won't be any apparent change there.
 

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Evaporator ice (above normal levels) has been linked to clogged airbox drains, low refrigerant level and failed expansion valves.
 

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I'm usually one for making use of the data that is available from the ECM via the OBD port. In our present case, there's several parameters that are available from the K-line SSM data series (not OBD), including the Air conditioning switch signal, the A/C compressor relay signal, and the A/C lock signal. The latter two will verify if and when the AC relay is being commanded on by the ECM (even if for some reason it isn't actually working), and whether the ECU is in AC lock-out mode.

These parameters ("switches") can be monitored using the free Romraider logger program on a laptop with a Vag-Com KKL adapter cable. (There's threads here on setting up and using Romaider)
 

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Discussion Starter · #11 ·
Thanks guys for all the help...

Update: Upon replacing, or trying to replace the struts, i couldn't break loose one of the lower shock bolts and decided to bring it to the local shop and have them brake them loose, then re-torque them by hand and i would continue this weekend to replace. When i dropped it off, i decided to tell the guy about the A/C and asked him to take a quick look at it. Before i dropped it off i took the relay and pushed it back into place and handed him the keys. I get a call later that day and said that "it is ready to pick up, couldn't find a problem with the A/C".

Great, now i have a gremlin...
 
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