Monthly Archives: February 2016

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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Engine Oil Coolers

Engine oil coolers are more important that you would think.  When most people think of an engine oil cooler they think of a big heavy duty truck (F350) towing and hauling.

Truth is modern cars don’t just push oil through their engines… they more or less chew it up and break it down.  Again I’ll leave the specifics about which oil brand/type is the best for discussion on the BITOG forums.  www.bobistheoilguy.com

So what does this have to do with oil coolers?  Well Ford upped the 5.0L coyote motor to 8 quarts to help keep the engine cool instead of a large engine oil cooler up front.  They also mention that an oil cooler is recommended if you plan on track use… who buys a Mustang and doesn’t put the pedal down to the floor… really???  Ford did use an air-to-water cooler (see below – not mine just found a quick pic on Google) but those are pretty much useless.  Yes they do a good job of warming the oil up to temp with the radiator fluid, but at the track your radiator fluid is typically screaming hot.  Not good for your oil.

OEM Mustang oil cooler

Enter an external engine oil cooler.  Warm-up may take a little longer, but you can always use an external bypass until the temps come up.  A bypass works by short-circuiting the fluid path… basically the fluid short cuts around the cooler and goes back into the engine until it gets warm enough.  Then a spring closes off the short cut and the fluid now is forced through the cooler.  If you plan on any sort of track day an engine oil cooler is a must; its cheap insurance to help your engine oil keep your engine alive.  Those of you that don’t drive on a track but have teenagers at home – your car has seen more ‘track days’ than you think.  Even if you drive slow but live in a southern climate where the summers are a beating – go with a cooler.  110°F outside temp only compounds this problem further.

Many people see an engine oil cooler and think they can short cut with a transmission cooler.  WRONG!  Transmission coolers are designed for lower flow rates and should NEVER be used for engine oil cooling unless the manufacturer says its OK.  Most transmissions run in the 1 to 2 GPM range and use 3/8″ lines and fittings.  Engine oil coolers should have 1/2″ lines and fittings and a cooler to match the flow as well.  An engine oil cooler should support up to 4 or 5 GPM – much less will begin to restrict the oil flow.

P. Heffcac

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