I just bought a 2000 blazer. I have had other blazers prior and done work on every part of these trucks. I have come into a weird problem. I have no heat the plenum switch works fine to change vents the ac light can be pushed on or off but for some reason the clutch on the ac compressor is cycling without the ac button pressed. I am assuming this is why I do not have any heat. Is this a common problem with the control unit or relay? I do hear something behind the glove box like a relay not really sure what is going on there but just wanted to report everything that is going on. I also noticed that the hoses going to the heater core are not really hot? I would think I need to get the ac compressor to stop cycling before I start messing with the heater core or the possibilty that the prior owner took out the thermostat. I would assume if the thermostat was missing i would have weak heat instead of ice cold air. I guess I could pull the plug from the compressor but that still does not resolve the issue that it constantly is getting a signal to cycle or work?

 

I'm betting the no heat issue is a clogged heater core. As far as the A/C issue is concerned I'm lost. I'm gonna hazard a guess that the relay is stuck on maybe?

 

Pull the connector on the accumulator {shiny bottle passenger side} to stop the compressor.

If your heater hoses at the firewall are both colder than your rad return hose, probably is fully clogged. Otherwise would guess blend door.
If it was the blend door then both would be about as warm as the rad return.
When it works right with airflow through heater then one is warm other is cooler and warm air.
Partial /mostly clogged, then one will be warm other cooler with airflow but little heat

 

Update...I ended up figuring out the heat issue. The radiator was low because of a crack. Replaced radiator flushed system completely of dexcool. Refilled system with universal prestone. Heater core seems to be ok no sludge in cooling system.

The ac still will not stop cycling and to get hot air you need to put the control on 82 any setting less then that will blow cold air. My next conclusion is the blend door control? My issue is a little different then ones I have read with blend door problems. My major problem is my heat works on one setting and my ac compressor keeps cycling. I was thinking my low pressure switch on my ac has malfunctioned causing the ac to constantly run?

 

have you tried disconnecting the wire above your ac pump (very top 3 pin connector on unit) if it stops blowing cold then your relay is buggered, if it is cold still then the clutch is messed up and wont disengage)

 

Also the A/C will cycle when you have the selector on the dash set to any windsheild defrost setting. This is to "dry" the air blowing against the windsheild. It may be the selector switch or wiring to/from it & yeah, pull the plug ontop of the A/C unit, it'll stop cycling then. But remember to plug it in every now n then so seals don't dry out until you find the issue.