2006 Suburban 1500 5.3 Dual Climate Control System.
When turning on the defrost. Shouldn't the compressor engage? Not getting heat when on defrost.
During the summer my A/C was working great. System was evaced/filled with fresh freon. So compressor was working.

It just recently started to get cold here and I'm experiencing heat issues. Heater was intermittent and now has pretty much gone cold unless ambient temp is warm-ish.
Going to check heater hoses to heater core and see if both are hot to touch.
No heat from rear either on heater or defrost/Aux.

The radiator was replaced in Sept.
I'm pretty sure I got the air of of the system. I did the procedure twice.
Not sure about T-stat. May have one. May not. Bought the truck used. It is on the planned list to put a new one in just for peace of mind.
No engine overheat issues.

I plan on trying a heater core flush but the defrost/compressor is what I'm looking at first.

 

Yes the compressor should engage with defrost. Disconnect the clutch coil and use a test light as a load to verify power and ground to the coil with the engine running at operating temperature and defrost turned on.

If it doesn’t light, the green wire is the power feed from the AC clutch relay & fuse and the black wire is a direct ground on the lower left side of the engine (G102). Connect the test light to battery positive/negative respectively to test each circuit. If there’s no power, and the fuse/relay test good, the HVAC control module or the PCM are preventing the AC clutch relay from engaging. The PCM is in direct control of the relay, but the HVAC module can also request the compressor not be engaged.

If the control circuits check out, it may be useful to have a shop pull trouble codes from the PCM and HVAC modules to narrow down the cause.

 

Thanks for the info.

 

If it's below 40F compressor may not come on. That's normal. If your temp gauge has numbers most of my current and past Chevy's would run normally at the 210 mark on the ones with numbers on the temp. gauges.

 

I haven't checked compressor lockup when it is over 40* ambient. Another thing to look at this weekend.
I have a 20 minute drive to work in the morning, 10 mn. city roads/10 min. freeway @ 65. If it's below 40-50's in the morning I usually just put the heater on vents and fan on 1 or 2 and that's been good enough.
As a side note. Heater was working on way home last night. Go figure....
I started having the heat issue (heater core blockage?) so that's when I noticed the defrost issue. I normally don't need to use defrost mode here in Las Vegas.
Temp gauge normally runs just a tad left of the 210 mark, middle, when fully warmed up.

 

Temperature blend door actuator could be sticking, another option to check if heat is erratic. Core blockage is not common unless some sort of sealer was put into the cooling system.