A/C Snowflake light on but compressor does not run

[2008yukonxlslt]

A/C was charged last year and worked fine but slowly got less cold and then the snowflake gave the 3 flash mode. I presumed a slow leak and bought some stop leak and was gonna give a try. I hooked up and system pressure shows 20 psi and is enough I presume to command system on because snowflake light stays on now but compressor doesn’t run. I checked pin 30 and 85 of the a/c relay and strangely they do not have pwr BUT 86 and 87 do BUT the a/c compressor mini fuse does NOT?????
Hmm. I am stumped by these results. Is my relay upside down? It is possible but I never touched it before and to my knowledge neither has anyone else. 2 of the pins should have pwr but it seems like it’s the wrong 2? I am referring to the bottom of the relay to determine the corresponding pin locations on the fuse panel. The way my relay is installed is with pin 30 and 85 on the inboard side nearest the a/c compressor fuse but it’s the outboard pins 86 and 87 that have pwr?? Anyone have any ideas, maybe a different approach.
Also, not sure what those EBEC boxes refer to on the pwr Dist diagram, they are not fusses so I presume they are just connectors.
Thx for your time….long post!!
Looks like I found my answer from the EBEC diagram, 30 and 85 are the outboard pins farthest from the fuse so….. guess my relay was backwards for some reason but, I reversed it but still same issue.

 

[972sparky]

85, 86 and 30 should all show “hot” or 12V to ground if the relay is not on. Relay will only be on when the ecu connects 86 to ground. 87, the fuse and compressor clutch will only have 12V when the relay is on. There is more than just the Freon charge pressure that the ecu looks at before it will turn on the relay.

 

[2008yukonxlslt]

Thx 972 for your time. I analyzed that relay from the fuse panel diagram and sure enough, it’s the outboard posts 85 and 30 that have the pwr which is the way it should be but I realized that the relay was in upside down but I checked it to the fog lamps relay and the fuel pump relay and they were also 180 degrees in the wrong direction and to my knowledge, these are all factory installed, I bought her 1 yr old so I’m sure no one messed with those so I analyzed the schematic of these relays and figured out that it doesn’t matter which way they are in, they still work same, as you say, ecm put grnd on 86 and this commands relay closed and put the beans to the compressor clutch. So, I tried shorting 30 and 87 and sure enough, compressor engagaed!!!! Yeah!!!! I was worried that compressor was f’d. next I was worried ecu was not putting grnd. I realized I was not putting enough pressure on my low side refill cuz the shrader on my refill was acting up. I finally got some pressure into the line and away she went, I put 1 can of leak stop into er and 1 can (6oz) of refrigerant and she is working fine now, gauge in the green and vents are super chill. So, only problem was low pressure. What really stumped me aside from the upside down backwards relays was that the snowflake light would stay on and yet compressor would not run. It showed 20psi. I put just a bit more in the system, 4 oz can of leak stop and then i was in the blue (sorry, think I said green earlier) which is 25 - 55 psi and ecu and compressor were happy. So, it would seem that at 20 psi, snowflake stays on but compressor does not run. I put in 6oz of refrigerant and brought system up to 50 psi and she blows good and cold. Time will tell if the leak stop works but she was a year to loose charge and judging by the fact that I only put in a total of 10oz, I hope she hangs on. I will reply on my post if I have any further issues. Thx for your time and feedback. Have a great evening!!
Thx again.
Ed

 

[Geotrash]

Thx 972 for your time. I analyzed that relay from the fuse panel diagram and sure enough, it’s the outboard posts 85 and 30 that have the pwr which is the way it should be but I realized that the relay was in upside down but I checked it to the fog lamps relay and the fuel pump relay and they were also 180 degrees in the wrong direction and to my knowledge, these are all factory installed, I bought her 1 yr old so I’m sure no one messed with those so I analyzed the schematic of these relays and figured out that it doesn’t matter which way they are in, they still work same, as you say, ecm put grnd on 86 and this commands relay closed and put the beans to the compressor clutch. So, I tried shorting 30 and 87 and sure enough, compressor engagaed!!!! Yeah!!!! I was worried that compressor was f’d. next I was worried ecu was not putting grnd. I realized I was not putting enough pressure on my low side refill cuz the shrader on my refill was acting up. I finally got some pressure into the line and away she went, I put 1 can of leak stop into er and 1 can (6oz) of refrigerant and she is working fine now, gauge in the green and vents are super chill. So, only problem was low pressure. What really stumped me aside from the upside down backwards relays was that the snowflake light would stay on and yet compressor would not run. It showed 20psi. I put just a bit more in the system, 4 oz can of leak stop and then i was in the blue (sorry, think I said green earlier) which is 25 - 55 psi and ecu and compressor were happy. So, it would seem that at 20 psi, snowflake stays on but compressor does not run. I put in 6oz of refrigerant and brought system up to 50 psi and she blows good and cold. Time will tell if the leak stop works but she was a year to loose charge and judging by the fact that I only put in a total of 10oz, I hope she hangs on. I will reply on my post if I have any further issues. Thx for your time and feedback. Have a great evening!!
Thx again.
Ed

The Schrader valves are probably the most common culprit for leaks in the AC system in these trucks. If you replaced those, then you probably won’t have any more leaks.

 

[2008yukonxlslt]

Thx, I’ll try that next if she runs down again. I did see a bit of green leak tek near the high pressure valve so it’s possible that is the problem. Is it the high or the low that is most often to blame? Also, what pressure should I have on the low side, I read another post where guy said he had 115 on low and 125 on high when not running. I’m pretty sure mine was 50 on low when running which is quite a bit lower.

 

[Geotrash]

Thx, I’ll try that next if she runs down again. I did see a bit of green leak tek near the high pressure valve so it’s possible that is the problem. Is it the high or the low that is most often to blame? Also, what pressure should I have on the low side, I read another post where guy said he had 115 on low and 125 on high when not running. I’m pretty sure mine was 50 on low when running which is quite a bit lower.

It's usually the low pressure port that's the problem because that's the one that gets used the most, but I had the high pressure one start leaking on my 2007 this spring. Amazon sells an assortment kit that worked great for me.

When starting with an empty system, the r134a on these needs to be recharged by weight because the pressure will vary significantly according to the outdoor ambient temp. There is a placard in the engine bay that shows what the correct amount is. Then you just use a postal scale with the gauges when you add the r134a. Mine was almost exactly 2 or 3 of those little cans, I forget now.

As for pressures to expect, all r134a systems are the same so this table is a good guide:

 

[2008yukonxlslt]

Ok, thx Dave, that makes sense, mine sitting at 50 so looks like it’s were it should be, was at 20 before I added I can and the table also explains why the top of the blue on my gauge is 55.
I presume that the table is when system is running so, here is a question for you, you said you topped up your system from empty with 2 or 3 cans so, the little cans I have are 6 oz so 3 would be 18 ozs so, when you fill it in this manner, what happens to the trapped air? As I understand, the service machine pulls vacuum prior to the fill up so I presume that would be a better way as just putting in without a vacuum would trap air and I guess would have the effect of diluting the refrigerant. I have never filled a system from empty with the cans so can you feel the diff when not using vacuum or is it negligible.
Thx for your time.