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This is a follow up to two recent threads relating to the air conditioner compressor. The first thread is http://www.subaruoutback.org/forums...-red-dust-around-c-compressor-whats-deal.html where the discussion was about red dust apparently coming from the compressor. The second is http://www.subaruoutback.org/forums...26071-c-intermittent-failure-flaky-relay.html, where a bad AC relay caused poor AC performance.
My car is going through it's fourth summer since new and, for the first time, I noticed some red dust in the area behind the AC compressor. (Photo attached).
Having been involved with the other two discussions, I decided to do some pre-emptive diagnosis. It was pretty short. As I noted in a post in the first link,
According to a Subaru HVAC training manual, the signal to the magnetic clutch solenoid coil "must not drop below 10.5 V for the coil to be effective". [Lower voltage/current to the coil] would lead to slippage, and perhaps a source of the highly noticeable red dust. The problem of low voltage/current could also be caused by something as simple as a bad electrical connection at the compressor or along the way.
I checked continuity in the supply and to ground and it was fine. So I pulled the AC relay, and tested it by applying 12 V to the coil terminals and measuring the resistance across the main switch terminals with a low-Ohms meter. Each time I applied 12 V, the readings differed, just as halfapie reported in the second linked thread.
I also pulled the sub-fan relay, and found the connection to be no more than 0.02 Ohms consistently.
Given the requirement for sufficient current to the clutch, the obvious poor connection being made at the relay, and the absence of red dust until this year, I'm pretty well convinced the red dust is from clutch slippage.
My reasoning is based in part on the fact that there was no dust on the belt or pulley. But more important is the fact that the clutch needs sufficient current to stay solidly engaged. The clutch coil measures 4 Ohms. At 12 Volts, this means it draws about 3 Amps. (This is consistent with the 35 Watt power consumption rating in the FSM.) For the voltage to be only 10.5 Volts (assuming the system voltage is 12 V), the current would be about 2.6 Amps, which isn't much less. It would take less than an Ohm of resistance in series with the coil to reduce the current to this critical point. Even when the system voltage is around 14 V (engine rpm above idle), it would take less than 1.5 Ohms to reduce the current to the 2.6 Amp limit. The measurements on my relay were above this.
I will get a replacement relay and clean up the area in behind as best I can. I suspect that there won't be any deposits afterward, at least until the relay begins to fail again.
Also, as soon as I have the new relay in, I'll crack open the old one to see if I can find exactly what is happening, and will report here.
In the meantime, if you notice red dust behind the compressor, start by checking the AC relay. And, in my case, I will add a periodic relay test to my routines.
Edit: opened the old relay today -- see post #3 below
My car is going through it's fourth summer since new and, for the first time, I noticed some red dust in the area behind the AC compressor. (Photo attached).
Having been involved with the other two discussions, I decided to do some pre-emptive diagnosis. It was pretty short. As I noted in a post in the first link,
According to a Subaru HVAC training manual, the signal to the magnetic clutch solenoid coil "must not drop below 10.5 V for the coil to be effective". [Lower voltage/current to the coil] would lead to slippage, and perhaps a source of the highly noticeable red dust. The problem of low voltage/current could also be caused by something as simple as a bad electrical connection at the compressor or along the way.
I checked continuity in the supply and to ground and it was fine. So I pulled the AC relay, and tested it by applying 12 V to the coil terminals and measuring the resistance across the main switch terminals with a low-Ohms meter. Each time I applied 12 V, the readings differed, just as halfapie reported in the second linked thread.
I also pulled the sub-fan relay, and found the connection to be no more than 0.02 Ohms consistently.
Given the requirement for sufficient current to the clutch, the obvious poor connection being made at the relay, and the absence of red dust until this year, I'm pretty well convinced the red dust is from clutch slippage.
My reasoning is based in part on the fact that there was no dust on the belt or pulley. But more important is the fact that the clutch needs sufficient current to stay solidly engaged. The clutch coil measures 4 Ohms. At 12 Volts, this means it draws about 3 Amps. (This is consistent with the 35 Watt power consumption rating in the FSM.) For the voltage to be only 10.5 Volts (assuming the system voltage is 12 V), the current would be about 2.6 Amps, which isn't much less. It would take less than an Ohm of resistance in series with the coil to reduce the current to this critical point. Even when the system voltage is around 14 V (engine rpm above idle), it would take less than 1.5 Ohms to reduce the current to the 2.6 Amp limit. The measurements on my relay were above this.
I will get a replacement relay and clean up the area in behind as best I can. I suspect that there won't be any deposits afterward, at least until the relay begins to fail again.
Also, as soon as I have the new relay in, I'll crack open the old one to see if I can find exactly what is happening, and will report here.
In the meantime, if you notice red dust behind the compressor, start by checking the AC relay. And, in my case, I will add a periodic relay test to my routines.
Edit: opened the old relay today -- see post #3 below
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