A/C autostop or a switch to control your A/C

Last update:  5/21/2015  Return to Insight home page


As if we don't have enough controls for the A/C you might wonder what else can be done to confuse the issue. That's a fair question.  Allow me to explain what this article is about.  I call this system "A/C autostop".  The difference between this and engine autostop is that A/C autostop happens above a preset speed.

A/C autostop does not affect engine autostop in any way.

To save a little gas and give the car a bit more available power while using the A/C when driving in traffic I installed a gas pedal micro switch on my CRX 22 years ago that would disable the A/C clutch relay above ~40 mph in 5th gear (actually above any preset throttle setting).  The circuit allows the compressor to run normally when you let off the gas pedal to go below ~40 mph or are coasting to a stop.  I found that even in South Florida summer heat that the cabin usually doesn't warm up enough between traffic light stops to be annoying.  My CRX has a white body while the Insight is metallic silver so that may cause the Insight cabin to be warmer.  I shall see when summer gets here.

This circuit allows the A/C to use some of the kinetic energy of the car to cool the cabin when you are slowing down.

A/C autostop actions

You set the A/C in the normal manner (AUTO, ECON AIR ON or ECON AIR OFF) using the normal dashboard switches and set the temperature desired.  To use A/C autostop I just set a SPDT switch on a small added panel to the down ON position (because your foot on the gas pedal controls when it works). To use the A/C without A/C autostop the switch is set to the up ON position.  It's easy to remember, down for your foot, up for the dash.

As you are accelerating, the throttle position is typically set higher than what the car would cruise at the preset cutoff mph in 5th gear.  In that case the A/C fans can run but the compressor clutch is disengaged so the A/C compressor would not be cooling the car.  You can definitely feel the difference in acceleration with the A/C compressor ON or OFF (or even cruising).

With a throttle setting below 45 mph the A/C compressor operates as it normally would without A/C autostop.  If you were driving at a throttle setting above 45 mph in 5th gear and then let up the throttle a little or all the way to coast to a stop the A/C compressor will work normally after the preset delay. The small red LED on the control panel illuminates whenever the A/C clutch relay is energized.

Once the car stops, the A/C would act as it does normally would if the A/C autostop wasn't enabled.  i.e. if the engine didn't autostop the A/C would work normally as if A/C autostop wasn't installed.  If the engine went into autostop the A/C compressor couldn't operate but the ventilation fan might run depending upon how you have the climate controls set etc.

If the Insight has a manual transmission there is a ~3.5 second delay built into the circuit so the compressor doesn't kick in and out while shifting. This is enough time to do a slow shift to get the car into lean burn.

A CVT Insight doesn't need the delay and it isn't included in that installation.

For the Insight I chose 46 mph as the trip point to disable the A/C compressor clutch relay because the speed limit is generally 45 mph in our urban areas and to allow a little more speed for my darker car. So far I've been able to test the system on my Insight with temperatures up to 89 and it works fine. The new dashboard control switch allows you to switch to the stock configuration if desired.  

What needs to be done to install the A/C autostop

This paragraph of information is explained here because I use Electrical Engineering terms in my articles and some people may not be familiar with them.  In electrical design we use a color code to abbreviate part values (numbers) and wire colors etc.  

The colors are abbreviated with three capitalized letters.  The color code is BLK, BRN, RED, ORG, YEL, GRN, BLU, VIO, GRY, WHT for the following colors, black, brown, red, orange, yellow, green, blue, violet, gray, white.  For numerical values the abreviations corespond to the following numbers, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9.  For wires with  a colored stripe on the background color the code might look like this,  GRN/YEL for a green wire with a yellow stripe.

Believe it or not but after a short while you visualize numbers in your mind for part values instead of colors.  And you can read the values left to right or right to left or UP/Down or Down/UP.  There's a phonetic saying to help remember the sequence of colors when learning the color code but we might have young readers here so here is part of it, "Bad Boys R... Our Young Girls But Violet Gives Willingly"

Now on to installing A/C autostop.
Unfortunately you have to get at the ECM box (located on the passengers side floor) to clip wire A17, the RED wire that controls the A/C compressor clutch relay.  While you're in there you might as well clip the fuel pump control relay wire A15 the GRN/YEL wire.  Shush, I'll tell  you about that wire later.

I did consider tapping into the throttle TPS sensor wire to sense the throttle setting instead of using the mechanical conglomeration of parts but one of the goals was to make the electronics build simple for non electrically oriented people.  For that reason I didn't do this installation using the TPS signal.

When you clip the two wires, clip each wire several inches away from the connector so you can solder wire extensions to the four wires you end up with. To play it safe, unplug the A connector from the ECM.  You will probably have to unplug the connectors just to get the ECM out in the open anyway.

Splice four wires long enough to bring all four cut wires over to the left side of the dash board.  Use heat shrink or electrical tape over the wire splices. Leave enough slack to come out through the left most push out cover above the covered cubby compartment to the left of the steering wheel.  I found the opening closest to the drivers door is the best one to use for the switch panel so you can see the switches and the LED while driving.  Both sets of wires control only the relay coil currents so they can be a rather small insulated AWG sized wires.

You should use a different color wire for each of the red wires you bring over to the left side of the car.  Write down the color of each wire and especially note which added wire is now connected to the red wire NOT now connected to the ECM (after you have cut the original wire).  It is not necessary to mark either of the added wires to the original GRN/YEL wires but you do need to know which of the four new wires are connected to the GRN/YEL wires.

If you plan on installing a TPS smoothing circuit later, now would be a good time to cut and bring those wires out too. I used two long enough loops of wire to go from the ECM to the left side of the car and back to the ECM so I could drive the car after putting the ECM back in place etc. This will allow you to do the cut & splicing one day and do the actual change over another day if you are limited in work time.

I used the switched 12 volt lead from the DRL fuse (under the dash) for the 12 volts to the delay circuit. You can use the always on 12 volts from the brake switch if you don't want to modify the DRL fuse etc.  The always on voltage circuit will always draw 0.00038 amp. (0.38 ma).  I also power my OBDII gage from the DRL circuit which is a 12 volt switched circuit..

You need to bring both wires from the Insight clutch pedal switch to the clutch switch on the added panel (shown next).  The ground wire shown on the MT schematic micro switch is actually one of the gas pedal micro switch leads that is grounded at the micro switch to it's mounting bracket. I grounded the COM lead but either lead can be grounded and the other lead is connected to the junction of R1, R3 and D1.  The schematic is shown near the bottom of this page.

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A/C auto stop switch panel

This is the small panel I made for the three switches used by my modifications, a small red LED and the delay circuit board.    My delay circuit is wired onto a 0.1" hole spaced prototype board.

The left switch is a SPDT for fuel pump control. The center switch is a SPDT for the A/C autostop (with the small red LED indicator above the switch) and the right switch is a SPDT with center OFF, a momentary ON and an always ON.  This switch is used as a remote clutch switch.  I basically use the momentary ON portion of the switch as a dashboard starter button.  "Press down to start" with the key also in the start position.

Here are links to switches that are the same type as I used.  You might find the same switches on eBay (with free shipping).
  • SPDT ON-ON  2 required.  (Only one switch required for the A/C autostop circuit.)
  • SPDT ON-OFF-MOMENTARY ON  1 required.  (This switch is used for the "clutch" or CALPOD switch.)

Obviously if you have a CVT transmission in your car you don't need the clutch switch.  In that case I would spread out the two remaining switches so they look symmetrical.  Or perhaps use a larger LED and use three horizontal holes in the metal bracket.

The small RED LED indicates when the A/C compressor is running.  Due to the way the ECM  A/C control circuitry is designed the LED illuminates dimly when the key is in accessory position.  I'm not sure exactly why the car system does this but the LED works normally when the key is in the ON position. The LED is not necessary for the circuit to operate so it and the voltage dropping resistor can be left out if you don't want it.

A/C switch panel

The 0.040" thick panel has a rather unusual shape in that the oblong hole it goes into has various internal ridges etc that held the original cover in place. The sides of the panel are bent 90 to extend into the opening to hold the panel in place and are not equal length.  The sides are long and wide enough so the ends of the metal contacts the raised plastic at the far end of the opening which stops the panel from going any further into the opening.  By adjusting the length of the sides you can get the surface of the panel to be flush with the opening.

Because of the chamfered corners of the opening the height of the sides are slightly less than the height of the vertical part of the opening.  The metal panel can be removed for maintenance by pulling on the switch handles.  The delay circuit board is attached to the middle A/C control switch. This was done by using three small terminals on the perf board which are spaced the same distance apart as the A/C control switch solder lugs.

The two RED  A/C wires from the ECM attach to J3 & J5 on the circuit board.  Make sure you connect the wires correctly.

The plastic cover could be drilled for the switches but I didn't want to destroy the cover. Rather than  pry one the covers off the first time I was able to reach the back side of the right side cover from under the dash and push it out.  After that it was easy to reach through the hole and push each of the other covers off.  Leave all the covers off until you are through so you can work the wires over to the left side of the dash.

When drilling the switch holes allow room for the mounting nuts (if that style is used) to be tightened.  If you drill the holes too close to each other one or more of the nuts will jamb on an adjacent nut and you'll end up where you can't tighten all of them.

I found a solar powered way to have enough light to see what you are doing under the dash.  Do your work when the sun is shining through the open door onto the floor of the car and either wear a white shirt or place a white paper or cloth on the floor to lay on as you jamb yourself under the steering wheel.  The sun light will reflect up behind the dash.  Try to -not- get your head jambed under the brake pedal.

Gas pedal micro switch mount

To see an enlarged view of the pictures, click on a picture or right click and select "View Image".
gas pedal mount

Pardon the blurry picture but it's the best of the litter.  This picture shows how the gas pedal is mounted to the firewall. The A/C control micro switch will be mounted using the top 12mm bolt.  I mounted the switch above the pivot point but it might be easier to mount the bracket on the lower bolt. The switch would be below the pivot on the side of the stalk away from the firewall.

The top portion of the stalk has some bends in it above the pivot point that really makes for a tight fit for the switch mount and the micro switch.  My switch ended up mounted between the upper part of the stalk and the firewall.  There's not much room to do this.

The spacer tube shown in the picture in the right picture is 1.375" long and was made out of a piece of thick walled aluminum pipe. Use thick walled tubing for strength. The OD is not critical.

gas pedal mount parts

Here are all the parts I used to mount the micro switch.  The short bolt on the right is the original gas pedal bracket mounting bolt.

I have an extensive electrical/mechanical junk box collection and I chose the micro switch shown because it had the longest lever arm (1.25") of the ones I had. The switch was made by Omron, p/n V-10FLK-IC2-K.

The longer the arm is the easier it will be to find a place to mount the switch so the lever contacts the pedal stalk and has the proper range of movement.

Here is a link to a micro switch that has a longer lever than the one I used.  With care you might be able to straighten the lever out for even more length.

dwg of A/C sw mount

These are the dimensions for my micro switch mounting bracket. Click on the picture to see all the details.  The drawing is not drawn in the same viewing direction as it is mounted in the picture to the right.

The slotted hole is used to allow adjustment of the micro switch to turn ON and OFF with gas pedal depression.

Please realize that if you can't find a micro switch with the same measurements etc as the one I used that your bracket might have quite different dimensions.  Sorry 'bout that.

A/C sw mounted

The lever arm of the switch is under the black stalk of the gas pedal and the end of the lever is positioned so it is ~1/8" to the right of center of the stalk.

I found that 45 mph in 5th gear in lean burn on my car is about 1/4" depression at the bottom end of the gas pedal.  I started the adjustment of the switch position by leaving the two screws & nuts that hold it to the bracket only tightened enough so I could still move the switch through the arc of the far mounting hole. The 12mm mounting bolt was also not fully tight either so I could rotate the switch bracket up and down at first.

I also twisted the switch lever arm to have the end of the lever parallel to the gas pedal stalk where they contact each other at idle. This gave me a little more adjustment range.

Setting the position of the micro switch

This whole project of mounting the micro switch in the correct position is a trial and error affair.  This is caused by the tight area the switch has to be mounted in, the varying dimensions of different micro switches (including the length of the lever) and the fact that different Insights with manual or CVT transmissions are going to have the gas pedal at a different amount of depression depending upon what speed you want the switching action to occur.  Other than all that .............

So take your time and be patient. You may not have the right dimensions on your first bracket or spacer length. I lucked out and managed to hit it the first time by very carefully using small wood blocks etc to hand hold the switch in place at first while taking measurements.  Not easy when your head is sometimes half jambed under the brake pedal.

At idle, the switch lever is pressed by the gas pedal stalk.  For a manual shift car the two wires at the micro switch are connected to the Common and N.C. lugs. One of the wires is grounded to the mounting bracket.  I grounded the COM lead.

For the CVT transmission, use the Common and N.O. lugs since the CVT doesn't use the delay circuit which inverts the signal.  Also bring both wires to the new switch panel.  DO NOT GROUND ONE OF THE MICRO SWITCH LEADS FOR THE CVT CAR.

The rest of the discussion is for a manual shift car.  Just reverse the on/off or open/closed terminology for the CVT car.

When the gas pedal is not pressed the micro switch is an open circuit.  This allows the delay circuit to operate to enable the compressor control relay after the delay time.  As the gas pedal is pressed to an equivalent just above 45 mph the switch contacts close which instantly stops the A/C compressor from working.  You will feel the added engine power if you are accelerating at the time.

When the switch changes to the closed condition it makes a faint click sound.  So what you want to do is set the switch to a trial position and then push the gas pedal 1/4" (in my case) while listening for that click. Always test the position as you press the gas pedal because the closed switch point of the micro switch is not the same as the open switch point.  Also make sure that when the gas pedal is released that the end of the switch lever has a little clearance to the gas pedal stalk. (Mine has about 1/16".)  You do NOT want the micro switch to stop the gas pedal from reaching the normal resting position.  If the micro switch is allowed to stop the gas pedal from reaching it's normal resting position you risk breaking the switch and the engine idle rpms will be higher than normal.

If you don't hear the click or the click occurs too soon or late, you  have to change the position of the micro switch a bit until it operates at the 1/4" pressed  movement.

To get a little more adjustment range I twisted the end of the switch lever arm so it was parallel to the angle of the gas pedal stalk where the parts touch each other.  Use two small pliers to twist the lever if you need to so you don't break the rather brittle switch body.

Once you hit the magic position tighten the 12 mm bolt and the smaller micro switch hold down bolts and check the switch action again. Then test drive the car to see what speed it actually switches at.  I temporarily attached a low wattage 12v bulb to the switch to test the speed that it switched at.  Be happy that after I did all this on my CRX I drove the car for 22 years and never had to adjust a thing about the switch or the circuit.

The delay circuit

If you are going to build this circuit yourself I assume you know enough electronics design practices so I don't have to teach a PCB layout course here.  If not, perhaps you have a friend that can help you build it.

I laid out the delay circuit so it mounts right onto the three pins of the control switch.  It is built on what's called 0.1" spaced "prototype board" or what is sometimes called "perf board" (radio shack sells small boards that can be used).  In affect I design a circuit and once the part values are determined I lay out a printed circuit board as if I am going to have it manufactured and then hand wire the circuit onto the board.  I end up with a hand wired [lumpy] equivalent to a printed circuit board.

The N channel Mosfet can be any low power device that has less than 0.7 ohm Rds.  I used a IFRD110 which is a strange [old] 4 pin mounting device. There is very little power dissipated by the Mosfet so no heat sink is necessary.

You might be able to substitute a NPN transistor that can handle the relay current (should be less than 150 ma) instead of the Mosfet.  In that case you may have to adjust the values of R1 and/or R3 on a 5 speed car so the delay circuit switches ~3-1/2 seconds after applying 12 volts to J1.  You should also include a back EMF protection diode for the transistor.

No matter which device you use, test the circuit on the bench before installing it in the car.

Notes for the schematics below
  1. J1 is the switched 12 volts input from the DRL fuse under the dash.  Or you could wire it to the all time 12 volts at the brake switch.
  2. J2 is connected to the remotely mounted RED LED anode.
  3. J3 is connected to the remotely mounted RED LED cathode.  J3 is also connected to the RED wire which was routed to the switch panel from the ECM area.  It is the RED wire that was not connected to the ECM after the wire was cut.
  4. J4-2 COM is the micro switch wire grounded to the micro switch mounting bracket. There is no actual ground wire on the delay board.
  5. J4-1 N.C. is connected to the single wire from the micro switch N.C. connection.
  6. J5 is the original RED wire that was routed to the switch panel from the cut RED wire at the ECM.
  7. The schematic isn't as clear as it ought to be.  Diode D1 is a 1N3064 and C1 is a 22 mfd, 16 volt or higher voltage. LP1 is a red LED.
Switch connections
  1. Connect one wire of the cut fuel pump wires to added fuel pump switch center terminal and the other wire to either side terminal of the added fuel switch.  I positioned the switch on the added switch panel so up is ON.
  2. The clutch switch (if used) is wired so that the two outside terminals are wired together.  One lead from the actual Insight clutch switch is connected to the added clutch switch center terminal and the other wire to either outside terminal.  I positioned the switch so the momentary movement of the switch is down.
The CVT car connections are similar to the manual transmission car except the clutch switch is not installed.

You will probably have to click on the schematics to see all the details.
MT delay schematic CVT ac schematic

Is it really worth all this work?

Part of my answer is ...... do we really need skirts or strakes on our Insights?  Probably not but it sets the car apart from most other cars on the road and they do help somewhat with gas mileage.

The main advantage of A/C autostop that is immediately noticeable is that when the A/C compressor switches off you can feel the car has more power while accelerating even with assist working.  You can also recover more of the kinetic energy of the car while coasting.  Kinetic energy not used while coasting is lost energy.

I notice about a 10 mpg drop and a few mph drop in speed; depending upon how fast you are going when the A/C switches ON and OFF.  So yes, I'd say it is worth all the work.

The main disadvantage is that you might not be as comfortable if the traffic is such that the A/C autostop doesn't allow the A/C to run long enough.  In that case you might be able to adjust the temperature lower, adjust the micro switch setting to switch at a higher mph or just flip the A/C autostop switch to off.   And as a last resort set the A/C control panel to "Auto" so the gas engine continues to run and the A/C will cool the cabin at stop lights.

Final tweak to micro switch

After using this circuit for awhile I tweaked the switch position to increase the switching to occur at 49 to 50 mph.  The actual speed varies because it is really controlled by the gas pedal position to maintain the speed.  Some days due to outside conditions etc, the speed may still be near 46 mph.  Other times/days it may be 50 mph.

With a passenger in the car you have to use a little more pedal to maintain the same speed compared to when a passenger isn't in the car.  Therefore the switching mph is reduced depending upon the weight of the passenger, the wind direction etc etc.

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