Tag Archives: VDC

Variable Displacement Compressor

Worked on a German car this weekend (not my cup of tea, but paid work is paid work).  My buddy joked about the compressor short cycling.  I find it comical when my friends who know nothing of AC try.  I appreciate the efforts though.

His German car has a Variable Displacement Compressor. Basically there is no clutch to engage and disengage. The compressor is spinning anytime the engine is spinning. MACS has a technical breakdown here with some good explanations. https://macsworldwide.wordpress.com/2013/05/29/variable-displacement-ac-compressor/

My buddies Jetta has an electronic solenoid on the back of the compressor that changes the angle of the plate inside. This increases the stroke of the pistons inside thus pumping more refrigerant. The benefit is the control module can push a variable amount as cooling demands. This helps save that 1/2mpg more the EPA demands. Another benefit is the lubrication is always getting moved around preventing slugging and seals from drying out.

That nice how it works, now the tough question.  How do you test it?  Most techs (on a normal system) will dump 12v+ to the clutch coil then watch the pressures. This system requires a special tool to engage the solenoid.  Because the solenoid is on the back side of a low mounted compressor it has a rough service life. So if the solenoid were to crap out, the compressor and cooling system are fine, they just can’t be controlled.  Your newbie in the shop will likely see the equal pressures on the gauges and throw a compressor at the problem praying it fixes it.  When it doesn’t he’ll probably blame the compressor manufacturer as poor quality in the box.

So 4Seasons has this new tester tool.  Turns out they were ahead of the curve by offering a way to manually force the compressor to full displacement.  Now you can bypass the module and harness and directly command the compressor.  This helps a tech rule out an electrical issue… Which as any tech knows not chasing an electrical problem is a huge $$$ saver.

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