AC Thermostat Alternative
03-28-2016, 12:51 PM
#1
AC Thermostat Alternative
I have a 1990 Honda Civic DX, and out of nowhere, my AC quit working. To diagnose the problem, I first verified that all of the fuses were good. After that I followed the HELM flow chart for “Compressor Does Not Come On.” I didn’t bother with the last steps in the flow chart that involved jumping pins on the ECU, or swapping in a known good ECU. Basically I wanted to make sure the compressor would come on, and that the compressor relay was good. Both checked out. Next, I verified that the dual pressure switch was good. This switch is located under the hood on the passenger side along the frame rail. I just had all new O-rings installed and the system recharged a few weeks ago, so I was not worried about the system leaking down. This switch opens when it detects low pressure (below 33 psi, indicating a leaking system) or high pressure (above 340 psi, indicating a blocked system). With the engine off, I just pulled the plug off the sensor and checked for continuity across the switch…it was good. Next, I verified that the dash AC switch was good. I pulled the switch out of the dash and checked for continuity between the 3rd and 4th pins (where the BLU/RED and GRN wires plug into) on the switch when it is in the "ON" position (see PICTURE 1). This checked out good.
The last switch to verify was the AC thermostat. This sensor is located on the side of the evaporator core under the dash. Pull the glove box and you can see it. This switch has a sensor that is buried in the fins of the evaporator core and the switch closes when the temperature of the evaporator is above 36-41F (depending on the particular sensor), and opens when the temperature of the evaporator is below 31-35F (depending on the particular sensor). When your AC is working properly, this is the little guy you can hear clicking away under your dash, cycling your AC compressor on and off. With the AC off and the ambient temperature above 36F, there should be continuity across the switch. Mine came out bad, and is open regardless of temperature.
The bad news is that Honda has discontinued this sensor (part number 80430-SF1-003), and all of the dealerships I checked with don’t have any in stock. There are a few Honda parts websites out there that say they have them, but the one I actually contacted said they do not have it, despite what their website says. Even if you do find one, it's going to be NOS and you’re likely to pay over $100 for a 25 year old sensor. Also, this is not a part I could find at any of my local auto parts stores. My last hope would have been finding one in a junk yard, but have you gone to look for EF’s in a junk yard lately? They’re about like hen’s teeth, and when you do find one, it’s already picked bare. Even if you do find the sensor in a junk yard, what are the chances it will work, and for how long?
The good news is that there is a cheap, robust solution out there. Essentially what I needed was a switch that closes when the evaporator warms up to about 37F, and stays closed until the evaporator cools down to about 33F, when the switch will open again. Thanks to cheap manufacturing in China and Amazon Prime, you can get exactly that. I ended up ordering . This little board has a single pole, single throw (SPST) relay which is normally open (NO), and a header to connect 4 wires, labeled K0, K1, +12V, and GND. . K0 and K1 terminals are the switch side of the relay – these two terminals are electrically connected when the relay coil is energized. +12V and GND are where the power source for the board is connected. I only wanted the circuit powered on when I am running the car’s AC, so the relay’s coil won't just be energized all the time. To achieve this, I needed to find a power source that is only energized when the AC is on.
The dash AC switch has +12V switched ran to it, and this was the closest 12V source to the evaporator, so I used a quick-splice connector to tap into the BLK/YEL wire that runs to the dash AC switch. I tapped into the wire close to the heater control unit (see PICTURE 2) and ran a wire (BRN) to the +12V terminal on the board’s header. Now, the board will have +12V anytime the key is in the ON position. Next I needed to find a ground that was only grounded when the AC is on. If you look at the HELM circuit diagram (see PICTURE 3), you can see that the when turned on, the dash AC switch completes the circuit for the ground going to the AC thermostat (called thermostat switch in the HELM). So, when I turn on the AC switch on the dash, I am completing the circuit (on the ground side) by connecting the GRN and BLU/RED wires at the switch. This BLU/RED wire at the AC dash switch is one of the two BLU/RED wires at the AC thermostat. I tested continuity to determine which one goes to the dash AC switch, and on my particular car, the BLU/RED wire I needed also had silver dots down the length of the wire, which made differentiating the wires easy. Since this wire is the switched ground AND one of the wires that needs to be switched by the relay, I split it between the GND terminal on the board and the K1 terminal. I chose K1 just because it was closer to the GND terminal, but K0 would have been fine too. So, I ran the BLU/RED with silver dots to the K1 terminal, and a jumper (YEL) from K1 to GND. Since K1 is already taken care of, all I needed to do was connect the remaining BLU/RED (without silver dots) wire at the AC thermostat to the K0 terminal on the board (see PICTURE 4). Now, the board will power on when I turn the AC on, and will turn off when I turn the AC off.
The board came with a temperature probe that is a K-type thermocouple. Ideally, the probe would be placed in the fins of the evaporator core.However, the probe provided is larger in diameter than the OEM sensor, so I would have had to change to a smaller diameter K-type thermocouple probe and wire it in to the board. If you do wire in a new probe, make sure you wire it in correctly (not backwards), or you will get an incorrect temperature reading. Another problem with placing the probe in the evaporator is that to open the cover on the evaporator, the evaporator must be removed from the car, requiring the system to be evacuated of refrigerant. Seeing as I just had my system recharged, I did not want to do this, so I decided to just go with the probe provided with the board and not bury it in the evaporator, but just put it in the cold airstream. Since I am not going to put the sensor in the evaporator, I want to put it as close to the evaporator as possible, on the downstream side. To do this, I removed the band between the evaporator and the heater control unit, fed in the probe, and then reinstalled the band (see PICTURE 5). Also, I mounted the board to the evaporator using
.
Now, cranking the car, turning on the fan, and turning on the AC powers on the board. By short pressing (less than 5 seconds) the SET button, the temperature set point will flash. Use the “+” or “-“ buttons to set the temperature (in degrees Celsius) that the controller will cool down to. Since my sensor is not in the evaporator, it will actually see a warmer temperature than the actual temperature of the evaporator. You do not want the evaporator to stay below freezing (0C) for long, or it will begin to form ice. If the evaporator is allowed to build up ice, it will reduce the cooling capacity (you’ll start to get warm air) and overwork your compressor (not good in the long term). I set my temperature set point at 2.0C (35.6F) for now, and may adjust as I get it figured out more. 10 seconds of inactivity will take the display back to showing the actual temperature. Long press (5 seconds or more) the SET button to enter the sub-menus. The display will show “P0”. Pressing “+” or “-“ will change from P0 to P6, to access the seven sub-menus. The sub-menus are as follow:
P0 – Heat or Cool
P1 – Hysteresis
P2 – Upper set point limit
P3 – Lower set point limit
P4 – Calibration
P5 – Delay
P6 – Over Temp Alarm
Hysteresis is default set to 2.0C. I adjusted mine to 2.5C. This way my car will cool to 2.0C (set point) and then the compressor shuts off and stays off until the probe reaches 4.5C (set point + hysteresis, or 2.0 + 2.5). I did not adjust any other settings on the board. Also, the board stores the settings, so powering the board off and on will not require you to reprogram it each time.
***UPDATE 12-Oct-2017***
I ended up replacing my evaporator core recently, and went ahead and installed the sensor in the fins of the evaporator, in the original location while I had everything apart. The probe is larger in diameter than the OEM sensor, however you can easily cram it in between some fins. Now the board is getting a more accurate reading of the evaporator's temperature. I did have to adjust my temperature setpoint and hysteresis up a few degrees, as I was running into the issue of the evaporator freezing up on trips over about an hour in length, but after the adjustment everything is running fine.
The last switch to verify was the AC thermostat. This sensor is located on the side of the evaporator core under the dash. Pull the glove box and you can see it. This switch has a sensor that is buried in the fins of the evaporator core and the switch closes when the temperature of the evaporator is above 36-41F (depending on the particular sensor), and opens when the temperature of the evaporator is below 31-35F (depending on the particular sensor). When your AC is working properly, this is the little guy you can hear clicking away under your dash, cycling your AC compressor on and off. With the AC off and the ambient temperature above 36F, there should be continuity across the switch. Mine came out bad, and is open regardless of temperature.
The bad news is that Honda has discontinued this sensor (part number 80430-SF1-003), and all of the dealerships I checked with don’t have any in stock. There are a few Honda parts websites out there that say they have them, but the one I actually contacted said they do not have it, despite what their website says. Even if you do find one, it's going to be NOS and you’re likely to pay over $100 for a 25 year old sensor. Also, this is not a part I could find at any of my local auto parts stores. My last hope would have been finding one in a junk yard, but have you gone to look for EF’s in a junk yard lately? They’re about like hen’s teeth, and when you do find one, it’s already picked bare. Even if you do find the sensor in a junk yard, what are the chances it will work, and for how long?
The good news is that there is a cheap, robust solution out there. Essentially what I needed was a switch that closes when the evaporator warms up to about 37F, and stays closed until the evaporator cools down to about 33F, when the switch will open again. Thanks to cheap manufacturing in China and Amazon Prime, you can get exactly that. I ended up ordering . This little board has a single pole, single throw (SPST) relay which is normally open (NO), and a header to connect 4 wires, labeled K0, K1, +12V, and GND. . K0 and K1 terminals are the switch side of the relay – these two terminals are electrically connected when the relay coil is energized. +12V and GND are where the power source for the board is connected. I only wanted the circuit powered on when I am running the car’s AC, so the relay’s coil won't just be energized all the time. To achieve this, I needed to find a power source that is only energized when the AC is on.
The dash AC switch has +12V switched ran to it, and this was the closest 12V source to the evaporator, so I used a quick-splice connector to tap into the BLK/YEL wire that runs to the dash AC switch. I tapped into the wire close to the heater control unit (see PICTURE 2) and ran a wire (BRN) to the +12V terminal on the board’s header. Now, the board will have +12V anytime the key is in the ON position. Next I needed to find a ground that was only grounded when the AC is on. If you look at the HELM circuit diagram (see PICTURE 3), you can see that the when turned on, the dash AC switch completes the circuit for the ground going to the AC thermostat (called thermostat switch in the HELM). So, when I turn on the AC switch on the dash, I am completing the circuit (on the ground side) by connecting the GRN and BLU/RED wires at the switch. This BLU/RED wire at the AC dash switch is one of the two BLU/RED wires at the AC thermostat. I tested continuity to determine which one goes to the dash AC switch, and on my particular car, the BLU/RED wire I needed also had silver dots down the length of the wire, which made differentiating the wires easy. Since this wire is the switched ground AND one of the wires that needs to be switched by the relay, I split it between the GND terminal on the board and the K1 terminal. I chose K1 just because it was closer to the GND terminal, but K0 would have been fine too. So, I ran the BLU/RED with silver dots to the K1 terminal, and a jumper (YEL) from K1 to GND. Since K1 is already taken care of, all I needed to do was connect the remaining BLU/RED (without silver dots) wire at the AC thermostat to the K0 terminal on the board (see PICTURE 4). Now, the board will power on when I turn the AC on, and will turn off when I turn the AC off.
The board came with a temperature probe that is a K-type thermocouple. Ideally, the probe would be placed in the fins of the evaporator core.
Now, cranking the car, turning on the fan, and turning on the AC powers on the board. By short pressing (less than 5 seconds) the SET button, the temperature set point will flash. Use the “+” or “-“ buttons to set the temperature (in degrees Celsius) that the controller will cool down to. Since my sensor is not in the evaporator, it will actually see a warmer temperature than the actual temperature of the evaporator. You do not want the evaporator to stay below freezing (0C) for long, or it will begin to form ice. If the evaporator is allowed to build up ice, it will reduce the cooling capacity (you’ll start to get warm air) and overwork your compressor (not good in the long term). I set my temperature set point at 2.0C (35.6F) for now, and may adjust as I get it figured out more. 10 seconds of inactivity will take the display back to showing the actual temperature. Long press (5 seconds or more) the SET button to enter the sub-menus. The display will show “P0”. Pressing “+” or “-“ will change from P0 to P6, to access the seven sub-menus. The sub-menus are as follow:
P0 – Heat or Cool
P1 – Hysteresis
P2 – Upper set point limit
P3 – Lower set point limit
P4 – Calibration
P5 – Delay
P6 – Over Temp Alarm
Hysteresis is default set to 2.0C. I adjusted mine to 2.5C. This way my car will cool to 2.0C (set point) and then the compressor shuts off and stays off until the probe reaches 4.5C (set point + hysteresis, or 2.0 + 2.5). I did not adjust any other settings on the board. Also, the board stores the settings, so powering the board off and on will not require you to reprogram it each time.
***UPDATE 12-Oct-2017***
I ended up replacing my evaporator core recently, and went ahead and installed the sensor in the fins of the evaporator, in the original location while I had everything apart. The probe is larger in diameter than the OEM sensor, however you can easily cram it in between some fins. Now the board is getting a more accurate reading of the evaporator's temperature. I did have to adjust my temperature setpoint and hysteresis up a few degrees, as I was running into the issue of the evaporator freezing up on trips over about an hour in length, but after the adjustment everything is running fine.
Last edited by DaX; 10-12-2017 at 05:18 PM.
05-11-2016, 10:08 AM
#2
Honda-Tech Member
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Re: AC Thermostat Alternative
This is awesome. I am dealing with this issue now. I am going to be testing mine and if it reads bad ill be following your plans. Thanks.
07-30-2016, 06:41 AM
#3
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Re: AC Thermostat Alternative
This is freaking badass! Really hoping I can do this same thing on my 95 Integra. It should be the same principal. I'll order it and reply back on here
07-30-2016, 08:17 AM
#4
Old Fart
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Re: AC Thermostat Alternative
Universal A/C thermostats are available at any car parts stores that carry Evercool or Four Seasons A/C parts.
They are mechanical, no wiring required except the in and out control lead. 94
They are mechanical, no wiring required except the in and out control lead. 94
08-01-2016, 09:45 AM
#6
Old Fart
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Re: AC Thermostat Alternative
Nothing really, just in line with making everything electrical as complicated as possible, the A/C thermostat has one function, to prevent evaporator from freezing up by cycling the A/C compressor, a universal A/C thermostat, [mechanical] wiil do that without any wiring other then the in and out leads, no power lead is even needed, and the in and out lead can be either a pos.(+) or in this case a neg.(-) the control is mechanical, simple. 94
08-01-2016, 02:22 PM
#7
Re: AC Thermostat Alternative
so would one of those $8 UAC ac thermostats work well? something like this https://www.amazon.com/Universal-Con...S5XGF0HC843ZGQ
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08-02-2016, 02:33 PM
#8
Old Fart
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Re: AC Thermostat Alternative
so would one of those $8 UAC ac thermostats work well? something like this https://www.amazon.com/Universal-Con...S5XGF0HC843ZGQ
10-02-2020, 06:24 AM
#9
Re: AC Thermostat Alternative
Just wanted to check back in and say this thing is still working - over 4.5 years without issue. I did change the set points for when the switch opens and closes. I have since converted to a full R134 system with parallel flow condenser and TRS090 compressor (see this thread), and this switch still is working fine.
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