Here is a good overview by Wally on Jeannie and David's site:
https://www.928gt.com/T-WallyHVAC.aspx
and Greg's site has lots of good stuff
Porsche 928 Maintenance Tips and Procedures
Do some reading first so you become familiar with the components and common failures:
low refrigerant level, (try jumping the low side switch or send power directly to the clutch), not enough amps from the control head relay, a good clutch coil is 3 ohms nominal,
etc
On the Rennlist seek out Dr Bob
Cheers and have a Merry
Griff