for the A/C gurus

[LT2771]

'87 Grand National A/C blows air but it's not cool.
I checked the freon low pressure and it is about 36 psi.
I jumped the cycle switch and the compressor clutch kicks in
but still no cold air.
Also not sure if it's related but the max speed on the blower does
not work.
Thanks for any help you can give me.

 

[mgmshar]

When the AC system has been off (not running) for a while, the freon pressure evens-out throughout the entire system. When the AC system is running (compressor is clutched and turning), the "high pressure side" and "low pressure side" are created by the compressor.

With the compressor running, the low-side pressure should be in the neighborhood of 15 to 40 psi or so - it depends on the outside temperature and humidity. There are tables available on the internet that you can look up such as the one I've linked below. Internet tables are approximate - the tables in the GM service manual are best (assuming you are still running R-12).

http://www.ackits.com/aacf/ptchart.cfm

When the AC systems has not been running for a while, the freon pressure measured at either port (low or high side) will be the same. Based on my experience, a fully charged system will show at least 70 or 80 psi at both ports when the system has been off for a while. If you are getting 36 psi with the AC system off for a while, you are definitely low on freon.

When low on freon, the low-side pressure switch on the receiver/dryer will sense the too low pressure and send a signal to shut the compressor off. This is done to protect the compressor and prevent freezing conditions in the evaporator. You can jump the swtich to override it, but you should only do it for a short time for diagnosis and to help charge the system with a can of freon attached to the low-side fitting.

Bottom Line: You need more freon.

Good Luck,

 

[Nick Micale]

High blower and fan are on 2 different circuits.

On a GN AC operating as normal, we see ~25 psi on the low side.

You may have more than one issue with your system such as an original 25 year old compressor, or low on freon?

Without the proper gauges and some knowledge/experience, it is difficult to diagnose correctly.

Do your self a favor and spend a few bucks to have it professionally diagnosed, as the best you can get here with almost no real data is just a wild guess.

Having diagnosed hundreds of GN AC issues, most of my guess has been good, but not always, even after spending some time with the car.

Have not figured out how to diagnose through my computer yet?

 

[LT2771]

That's some great info/advice guys. I appreciate it.

 

[bose]

That's some great info/advice guys. I appreciate it.

My GN was not blowing cold air even though the system was properly charged and the compressor was engaged. I checked it out and my water valve on the heater hose was closing under vacuum but the valve was letting hot water leak through thus canceling out my cold air. I replaced the valve and now have ice cold air again. Bruce

 

[northerngn]

My GN was not blowing cold air even though the system was properly charged and the compressor was engaged. I checked it out and my water valve on the heater hose was closing under vacuum but the valve was letting hot water leak through thus canceling out my cold air. I replaced the valve and now have ice cold air again. Bruce

Been there done that!