1973 Corolla TE21 – A/C Clutch Cycle Control

By: Bob Goodson
August 30, 2020

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Author: Michael Yount
As mentioned in the last post about the A/C, I did end up using the rear window defroster switch to cycle the A/C clutch.  Just a matter of extending the wire going to the clutch under the dash and running it through the switch.  It was a complete success.  Turn on the A/C switch and pull out the defrost switch – bingo – compressor kicks on.  As things got right chilly inside the car while driving, just push the defrost switch in (opening the switch) and the compressor kicked off.  As things began to warm up – pull it out again.  Worked beautifully.
With that in mind I broke my recent ‘no Chinese stuff’ rule and ordered a $10.99 digital thermal controller through Amazon.
You place the sensor wherever you want to sense temps, and then program the unit to help you do that.  It operates like a relay — will manage 10A from 5V-30V on the input side.  Has Heat and Cool modes — when set in the Cool mode, as the sensor senses a low temp that you set — it opens the switch disrupting the flow of current to your load.  In this case that will be the compressor electro-clutch.  Then you can set a delta-T (hysteresis) that when added to the low temp setting, if the sensor senses that higher temp (set temp + delta T) it closes the switch.  This will allow it to cycle the clutch for me.  Plan is to mount the sensor against the copper tube downstream of the expansion valve — feeds right into the evaporator.  I set it to open the switch (killing the compressor) at 39F; and to re-close the switch at 47F.  We’ll start there and see what happens.
Have to think through where/how to mount it so I have access to play with settings.  You can see it’s not very aesthetically pleasing.  Probably put it in the glove box (which is right above the A/C unit) somehow.
So, decided to mount the unit in the glove box next the stereo head unit.  About the only place I can.  The previous owner fashioned this little “box” out of the glove box plastic and some medium density fiberboard – he had it mounted using two sheet metal screws going upwards through the plastic and into the inside lip of the dash.  So it’s just sort of ‘hanging’ there.  It seemed secure enough – but I added a little metal tab at the bottom to secure it there too.  Also, makes it MUCH easier to install as trying to hold it while getting a screw started was dang near impossible.  I plan on creating a plug for the 5 wires that will attach to this — it will make removing this whole piece much easier.  That way I can pull the plug through that opening you see at the upper left (that makes room for the magnet on one of the front stereo speakers), unplug the device and just pull it/stereo out together.

If you look carefully you can see I’ve tie-wrapped the t’stat temp sensor to the evaporator inlet line, just downstream of the expansion valve.

Added male/female terminals on the digistat so I can undo the wiring and remove the stereo again if need be.  Everything fell into place.  The little tab on the bottom of the plastic surround made reinstallation SO MUCH easier.  And it serves as a great place to ground the digistat.

Test drive was perfection.  Cycled just as I hoped.  In fact, with mid 80’s ambients, the 3.9C low/switch-off temp was a bit too chilly.  So I upped it to 4.5C while maintaining the 4.5C degree delta.  You can see the read out – as the clutch cycles on, at 9.0C, you can watch the temp drop as the freon is cooling the evap.  As soon as the sensor temp hits 4.5C, it cycles off.  And because the system is “working”, it continues to get a bit cooler – dropping down to 3.8, 3.9C, and then starts climbing.  Back up to 9.0C whereupon the unit re-engages the clutch.  Couldn’t be happier.


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