low voltage

I will keep this short! 54 plate V6 3.0 Cabriolet. AC not working. Recharged, fuses OK, new high pressure sensor. 8.4 Volts at the compressor. If I power the compressor from the battery directly with a fly lead, it clicks at works and the AC operates. So, where is the voltage being lost...

Hi All A while ago battery light came up so i replaced battery. I have used OBD Eleven to code but charging was around 13.4v. I swapped original battery for Bosch 110ah/920a. No one could sort low charging our and my garage suggested new alternator. Bran new part fitted and problem still the...