The PBCU provides a ground to the voltage signal, so I suspect you
don't have voltage at the switch due to a failed wire somewhere
between the battery and the pressure switch.
I almost forgot -- some engines are equipped with a wide open throttle
switch to disconnect the AC compressor at full throttle. If that
switch as failed, you won't get AC engagement and no voltage at the
PBCU, just as if the low pressure switch was open. I don't know where
this switch is in the circuit, but the switch itself is on the
throttle linkage.
Check for voltage at the pin socket for the KLIMA, this is supplied
via the keyswitch. If you don't have voltage at the KLIMA socket for
that wire, the AC won't ever come on as the KLIMA relay cannot be
energized. You will have to check the KLIMA wiring, I'm not sure if
that line is separate or internally connected to the switched positive
supply.
If you have voltage at the switch but not the connector at the PBCU,
you have a wiring failure between the pressure switch and the PBCU or
a failed open pressure switch. You can jumper the switch, but you've
tried that, right?
The system works by the PBCU providing a ground for the relay in the
KLIMA, which can be disconnected by an open pressure switch, a failed
speed sensor signal (engine speed vs compressor speed) or in some
cases a temperature or wide open throttle switch.
Peter
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