There should be pressure in the system without the compressor running. At least on the low side.

The ambient temp has a lot to do with the internal pressure. There is a chart on most "freon" kits.

 

Just hooked up gauge to our 06, and all works fine all though it is little low on refrigerant. low side reads 80 psi, engine not running. (We have 2 HHR's.) When I hook up to problem vehicle 07 low side reads 300 psi engine not running, when running compressor does not engage. Any thoughts? Thanx.

 

Whaaaat ?
Worked on many automotive AC's years ago. Only times I've seen something like that was when someone did a home repair and overfilled one.
But. Never 300 psi on a low side with the compressor off.

Did you take a high side reading ? It should be the same since the system pressure equalizes when the compressor is disengaged.

Honestly. I would want to have that charge bleed off if it is indeed 300 psi. Your asking for a line rupture or worse.

 

Did you ever check the AC compressor diode, power to the compressor clutch?

I know nothing about AC charging, but that does sound awfully high. Maybe you were looking at Kpa not PSI

 

Thought that was weird also, I'll try checking again. The diode in the #9 slot checks out OK, same as my 06.
Also the system has never been recharged.

 

That sounds high on the low side, and it would cause the pressure switch to keep it from engaging because that will push pressures higher. Your compressor will not kick in if the pressure falls outside a range both low and high.

 

Update. Rechecked pressure not running . After zeroing the gauge then attaching it to the low side I get a reading of 130 psi. Then with vehicle running same reading no engagement of compressor. Tried bleeding off for a few seconds, initial cool air then warm. Indication from tach when pushing A/C knob engine speed increases slightly indicating it is getting the signal. What do you think? Should I bleed off more pressure and try and recharge.

 

Are you 100% positive that's taken on the low side? The reason I ask is this chart. See if you can get your hands on a good set of manifold gauges for this. You might be able to rent some or go to a place and ahve them check.

 

I would not rely on the tach, get out and look. Either the outside face of the pulley is turning or it is not turning.

 

Compressor is not turning. Positive that I am on the low side as gauge only fits on the low port. (larger tubing) I have been comparing all my moves with my other HHR.