a/c problem on 67

[easyrider]

I am having problems with my a/c. The clutch isn't engaging when I turn on a/c fan. I know on the 67 you have to put the lever on a/c and on cold and then turn on the fan to make it come on. The fan comes on. With all that done, clutch still doesn't engage. I made sure the fuse was good and that the plug on the compressor was plugged in good. I have a relay in the circuit and there is the fan switch itself. Any suggestions as to where to start the tracing process? What is the most likely thing to go wrong. I had the a/c serviced last summer and the truck has set in the shop all winter. Yesterday was the first time that I tried to use the a/c and found that it didn't work. Any suggestions would be appreciated. Thanks!

[AzDon]

I'm far from being an expert about A/C, but I can help you thru the troubleshooting path. I'm going to post and then edit several times so I don't get "timed out" and lose any more posts.
Here Goes:
Your compressor clutch most likely doesn't work because there is a low and or high pressure switch(es) that open when the pressure in the system is not within specification. On stock GM applications of that era, it is a low pressure switch and is located on the side of the dryer/accumulator tank which is the natural aluminum appendage that is upright on the front of the A/C "suitcase" under the hood and has the large (low side) compressor hose screwed onto it.
With the engine running, turn all A/C settings to max. Pull the two-wire plug off that pressure switch and hotwire it and the clutch should engage. You can accomplish the same test by using a test light from each pin on the plug to ground and you'll likely find electricity in, but not out.
When hotwiring the plug succeeds in energizing the clutch, this is more likely an indication of low freon pressure than of a bad switch. BTW if you have 12 volts at the clutch and no engagement, then the clutch is bad.
Anyway, if you've gotten to here and the clutch is going, shut the engine down and get set up to add freon to the low side of the system, which you will do with the engine again running. You will leave the pressure switch plug hotwired while you put in the first can or maybe two. Try to introduce as little air into the system as possible when hooking equipment and changing cans. Once you put the first can (or 2) in, the compressor should run with the plug back on the pressure switch, proving that the system was simply low.
I've simplified this for someone without proper equipment to get their system running. To keep from overcharging, you should not add any more than one can after the pressure switch starts working again. You should also be careful not to breathe freon vapors and wear eye protection.
The more proper way to start the troubleshooting process is to hook up a pair of high/low A/C gauges before hotwiring the clutch. Low pressure should be 30-60 and the high pressure should be 150 to not over 300. When the system has the right freon pressures, the electricals should engage the clutch.
Before doing any of this look the entire system over for signs of leaking. There is oil in the freon charge that will attract dirt. If you have to open up the system, remember that it will be under pressure and need to be bled out and vacuumed down when put back together.
Good Luck

[easyrider]

Thanks for the walk through. I'll print this off and give it a try. I am not an a/c man and I know my limitations, so it is good to have people that know about this to help. Thanks again, easyrider!