My compressor works but it will not kick in.

If I supply 12 volts to it, the electric clutch kicks in. There is no power on the blue wire at the low pressure switch. Where does this wire originate(it goes into a huge wiring harness.)

Is there a relay in series with this wire. If so, where is it located?

Jim
1983 300Dt

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1983 300D Turbo
1985 190E 2.3