Pardon me for asking, but how did you verify the pressure is adequate?
Now, regarding how to energize the AC compressor clutch, if you:
- jumper the connection (as you say) at the low-pressure switch, and
- the controls in the cab are set w/ the AC switch depressed (or any other position that would call for AC, regardless of that switch, such as defrost), and
- the high-pressure switch on the AC manifold is closed (measures ~0 ohms), and
- the high-pressure switch on the back of the compressor is closed (check that your connection is present here, it's an easy one to overlook during an engine R&R) then
- the ECU should provide power to the relay that in turn energizes the clutch.
If you have a scan tool, you can verify the ECU's inputs and responses.
The factory service manuals are on-line; if you don't have them consider downloading them for reference. Find them here:
88-98 Service Manuals
Over the years I have collected a bunch of GM service literature. Links to PDF Scans of service and STG manuals and a links to the Parts manuals. You'll need the FREE Adobe Acrobat reader to open these files. Get it directly from Adobe here {http://get.adobe.com/reader/} If you're not...
www.gmt400.com
I've attached the relevant sections from the 1998 FSM, I suspect 1997 is similar. LOOK AT both attachments.
Perhaps you've got a compromised wire or connector in the circuit to the low-pressure switch, or elsewhere. OR, the compressor high-pressure cutout switch, mounted on the compressor, wasn't reconnected after the engine swap. A wiring diagram would guide you, e.g., for where to measure the resistance of the wire(s) and/or voltages to expect on them.
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