P38 air con

I am going round in circles on this one. If i switch on the air con the clutch pulls in momentarily (1 second) then drops out. if I jump the output from the HVAC unit then all works fine. This means that the electrical circuit is all intact including the pressure switches and relays as they are all still in use.
It has recently been regassed (twice) so that is also OK.
The grant signal to HVAC from the engine ECM (4.6 petrol) remains good.
I do not see that any other inputs to the HVAC can demand the clutch to disengage except the serial line from wherever BCAM perhaps. Is this unit able to measure the current to the A/C clutch and signal an out os tolerance. If this is the case can anyone let me know where to look. Thanks

Re-gassed twice? suggests you have a leak, no gas = no go.

Thanks but no leaks just had it done a second time to ensure that it was FULL. When the clutch is energised with the jumper it is lovely and cold, so again plenty of gas. I have learned that if there is less than say 80% gas the pressure switches will inhibit clutch operation, it is not that fault.

Check the under bonnet fuse box for signs or smells of burning.

There was a recall to earlier GEMS engined P38's which involved fitting a small wiring harness and an additional relay to operate the compressor clutch. There was a problem with the clutch coil supply not being able to maintain enough power to hold the coil energised and it kept dropping out. From memory, the original signal/feed wire to energise the coil was used to energise the relay coil instead as less current was needed. The relay then switched a fused supply directly to the compressor clutch coil.

Thanks Irishrover I think that you may be close. Mine is a 99 thor car but I wonder if the HVAC unit has been changed for an early unit, (I know it was changed before I bought it). Thus if the unit has a current sensing function it would be looking for over 3amps for the clutch but only seing less than half an amp for the relay.

Does anyone know the part numbers for the HVAC unit early and late.

Thanks again fellow Brummie