AC compressor clutch cycles on and off

[ScottH]

After fixing the blower motor & vent issues on the 87 V10 I recently bought, I tried the AC to see what would happen. It appears that the clutch on the compressor (I may not have the correct term) engages then stops right away and keeps doing this. I have heard that if it is low on Freon this could cause this. Can anyone verify this or offer other suggestions as to what the issue might be. Have no idea last time the AC was used in this truck.

Also is there any way to tell if the truck was converted to R134? If I need to add Freon it would be nice to know what kind to add.

Appreciate any help.

Thanks
Scott