Yes, as drawn, the opto circuit is not isolated.
It would be isolated if the opto output was used to short across the PB without a connection to +12V, with an added Darlington or Sziklai stage if needed (the Sziklai having a one diode lower drop).

The question is still, of course, how much current is the switch conducting when on, and how low a voltage indicates a PB press..

 

It would be isolated if the opto output was used to short across the PB without a connection to +12V,

I don't think it could be fully isolated (or need to be) if the Thermostat is powered by the same 12 volts as the air conditioner controller.

 

That is correct which was my request in post #37.
The use of the opto was never meant for isolation.

The output across that PB MUST be isolated because we have no hint as to what the interface circuit is. It could be 3.3 volt logic or even a scanned array of inputs. Or something else.

 

The output across that PB MUST be isolated because we have no hint as to what the interface circuit is. It could be 3.3 volt logic or even a scanned array of inputs. Or something else.

That will be determined if the TS can figure out the switch and any voltages present. My original circuit uses a relay to avoid the uncertainties but as I stated it could be simply switching 12 volts momentarily. The thought was to eliminate unnecessary components.

 

That will be determined if the TS can figure out the switch and any voltages present. My original circuit uses a relay to avoid the uncertainties but as I stated it could be simply switching 12 volts momentarily. The thought was to eliminate unnecessary components.

Making progress - half the dashboard is in bits and finally got to the connector which connects to the AC button. The plan is to extend its cable and include a male and female connector in the extension and loop that into a more accessible location where I can put everything back together and then do some testing. If Fiat are consistent with the headlight circuit operation which I have interfaced with previously, then I'm hoping the switch will just pull a connection to ground when operated, as the headlight switch does.

I will also be looking for the connection to the LED in the AC button which turns it on to indicate the AC is running, as I may be able to use that to block the output from the thermostat if it tries to turn on the AC when it is already on. I have noticed that there is some logic processing applied to the AC state as it will not operate if the heater fan speed is set to 0 so maybe explains why it is not a simpler arrangement.

 

An examination of the service manual for the AC portion of the system would reveal if the system monitored the compressor operation or not, which does not seem like it would be monitored. And still, interrupting the power feed to the compressor clutch at the compressor seems like it would be much simpler than any other scheme.
AND I am wondering where the added temperature sensor/controller will actually sense the temperature. Will it be the delivered air temperature or some place in the cabin?? And where will that additional control be located? And where will that added electronics assembly be mounted??

 

Unfortunately it doesn't seem possible for an individual end user to get a workshop manual for this vehicle which is a shame because I would very much like to have a full wiring diagram. I have looked into and under the very tightly packed engine bay but not been able to identify the AC components, so for now tapping into the cabin AC switch is my best hope.

To answer your questions: I will place the temp sensor somewhere near the dashboard but out of any direct sunlight and away from air vents so that it reflects the average air temp inside. As this is a van the cabin is larger than a car and there is loads of space under the dashboard panels for any additional electronics, and I will mount the thermostat control box/display somewhere on the large central console, possibly recessed into it.

 

----> https://www.ebay.com/itm/3552849460...ceY19NangeojLtFcZ/tYhxmw==|tkp:Bk9SR-jAvc2XZA

Or : ----> https://www.ebay.com/itm/3647487157...G7wTb7AWgieF/FImSXpN4xTHDV|tkp:Bk9SR-rAvc2XZA

Or canibalize the one in a discarded refrigerator or air conditioner.

 

----> https://www.ebay.com/itm/355284946045?itmmeta=01J2YDFC18EB75PJ0PRRGR540V&hash=item52b8a20c7d:g:wXcAAOSwKeBleU~D&itmprp=enc:AQAJAAAAwIDSkjyX32Vx9uoo9HaX8zkEcbUU6ugH7mkc76fW57HU3yp7pJq2USmV+BfgldiKGeslDY7sD4I2abfNwrJjIjRDopJ4zBYXTEGSW099Q314HQufObxpfTXIJH3n23XVEzDilsK92IA0ggPI1h40sNAB26Lb238imrN/TCmtl1dff4FoCchyCYId5+0fL1hqbcC6ZU7bG4/kl09zEf/mtibNpqXaoeyL1f+Dirtm6mceY19NangeojLtFcZ/tYhxmw==|tkp:Bk9SR-jAvc2XZA

Or : ----> https://www.ebay.com/itm/364748715752?itmmeta=01J2YDFC193NN1STN8JJ9F4KWV&hash=item54ecb7b6e8:g:2cIAAOSwcoBl2AIW&itmprp=enc:AQAJAAAA4EU5LVzE6SaPPuhnFOytowJiJrvs0+yDCWJzwxZOiIU0OZgGPj009hCVVpuKwl7jDN01tgQl3/7GkwBQqbPX67N6zYz8QDY5kYpw8AhusKrC8GnWFSJoY5+JYpAz6rJz4kQrvMXb5Z+Kuz0h+5DGBLU6ZYHo5FlkVDD23KpVJHza4hOa++TaQLWA6fkZq21P2jZo6jx2kZB5S4w3Ou2xEOWp3As5btZ5I9qYGw+3xCkWvKLKnZo4FvrtQiZMTcqSmjT86s5RYtWBmt/D1SG7wTb7AWgieF/FImSXpN4xTHDV|tkp:Bk9SR-rAvc2XZA

Or canibalize the one in a discarded refrigerator or air conditioner.

I am using one similar to your first link. However you have to be careful which variant you buy as they come in three flavours, powered by 12V, 24V DC or 110-220 AC. The spec of the relay they contain includes 20A 12V which can be confused with the power needed by the device. Amazon sent me an AC powered one even though the description said powered by 12V, so have had to send it back and reorder.

I don't think the refrigerator thermostats would be suitable without additional circuitry and difficult to place unobtrusively in the vehicle's cabin which is where I need to monitor the temperature.

 

I have looked into and under the very tightly packed engine bay but not been able to identify the AC components

You should be able to identify the compressor driven by the equipment belt.
It should have one or two wires with a connector going to it, which is the clutch connection.

 

Or one of maaaany flavors :

----> https://www.amazon.com/UAC-SW-3004C...08-84ef-0d61d4df70c5&pd_rd_i=B003R3WVO0&psc=1

Or canibalize one from the wreck yard with all its wiring and connectors... Perhaps from/for the same brand of your vehicle, then you will learn where it fits and connects.

 

Unfortunately it doesn't seem possible for an individual end user to get a workshop manual for this vehicle which is a shame because I would very much like to have a full wiring diagram. I have looked into and under the very tightly packed engine bay but not been able to identify the AC components, so for now tapping into the cabin AC switch is my best hope.

To answer your questions: I will place the temp sensor somewhere near the dashboard but out of any direct sunlight and away from air vents so that it reflects the average air temp inside. As this is a van the cabin is larger than a car and there is loads of space under the dashboard panels for any additional electronics, and I will mount the thermostat control box/display somewhere on the large central console, possibly recessed into it.

Locating the AC compressor should be simple because it is engine driven, meaning that it is on the end of the engine with the drive belts. And while the vehicle is a camper/motorhome/RV of some size, it probably uses an engine/ drive system by a larger automotive or truck manufacturer. So an on-line search should be able to locate pictures of the engine with accessories.
And in a worst-case situation a dealer for that same brand of vehicle as the engine should be able to have a mechanic point out the AC compressor and the clutch power connection for a manageable price.

 

This because if the AC is on when the vehicle ignition is switched off it is also on when the ignition is switched back on, but pressing the button while the ignition is off does not change that. This proves to me that the state is not maintained mechanically by the button but some controller elsewhere in the vehicle's electronics.

of course a momentary switch only works in this case when there is a memory circuit to latch its transition and toggle the on/off states (clap on / clap off) with the switch logic transition.
The controller saved the current state of on/off whenever it's toggled, and this is how it restored the state when the ignition turned on.

 

My vehicle, a camper van, was not able to be purchased with the optional full climate control system, so I am stuck with a manual system whereby the cooling function is turned on or off by a simple button on the heating control panel.

Are you in the US?
It's hard to believe that the DOT don't require auto manufacturers to have thermostat controlled AC system as this would waste gasoline and add more pollution to the environment which everyone is focusing on.
AC compressor on/off controlled by a manual switch -- it's fact or friction in 21st Century?

 

I will also be looking for the connection to the LED in the AC button which turns it on to indicate the AC is running, as I may be able to use that to block the output from the thermostat if it tries to turn on the AC when it is already on. I have noticed that there is some logic processing applied to the AC state as it will not operate if the heater fan speed is set to 0 so maybe explains why it is not a simpler arrangement.

How do you use the LED signal to gate the Tstat signal?
Relationship of blower fan and compressor: with fan off, the controller disable compressor to prevent freezing up the evaporator.

I think Mr.Bill2's idea of controlling the compressor clutch on/off with the added Tstat is the best solution (might need to gate with the fan running signal for protection). I also bet that the computerized controller doesn't check the on/off states of the compressor -- who what reason that the designer wanted to do this? To see if someone like you added a DYI circuit to turn off the clutch when it's too cold? System error codes are for diagnosis to aid in the process of repairing, and so what is the reason to detect the compressor off state (disabled by your added Tstat circuit) when the PB is on? Check its own relay?
It's hard to believe that this AC system has no temperature input setting.

My vehicle, a camper van, was not able to be purchased with the optional full climate control system, so I am stuck with a manual system whereby the cooling function is turned on or off by a simple button on the heating control panel. On a recent trip to sunnier climes, this led to us alternately freezing and sweltering due to having to remember to manually control this in a timely manner.

How do you set the heater temperature in winter -- no setting but just on and off switch manually activate? It would melt everything at 96C hot coolant if someone forgot to turn off. So it must have a temperature input setting. Perhaps the same heater input setting is also used to set the cooled temperature with a Tstat already built in.
It's crazy to think the system has no Tstat feedback.

 

Are you in the US?
It's hard to believe that the DOT don't require auto manufacturers to have thermostat controlled AC system as this would waste gasoline and add more pollution to the environment which everyone is focusing on.
AC compressor on/off controlled by a manual switch -- it's fact or friction in 21st Century?

I am in the UK and my camper was built in the EU on a Fiat Ducato base vehicle (badged a Dodge RAM in the US I believe). Although the base vehicle is a commercial van it has many what might be considered luxury options available such as climate control, electronic rear view mirror, automatic parking assist, adaptive cruise control, reversing camera, fully electronic instrument panel, etc, etc. Unfortunately to keep down costs and standardise the base vehicles as much as possible, most camper converters only offer a limited selection from the base vehicle options so in my case I only have manually controlled aircon, basic cruise control and am missing a few other things I would have liked, although I was able to specify the top of the range 180 bhp engine and 9 speed auto gearbox and have added my own electronic rear view mirror and a Pioneer head unit with reversing camera.

However, I would disagree with the notion that an automatic climate control type of AC would be less wasteful, as with these systems people tend to have them operating continuously, whereas I find that with a manual system I only really switch it on when it gets uncomfortably warm, and switch it off once it has cooled the cabin down a bit. If I add thermostatic control to mine it will probably mean that it will be operating more!

 

How do you use the LED signal to gate the Tstat signal?
Relationship of blower fan and compressor: with fan off, the controller disable compressor to prevent freezing up the evaporator.

I think Mr.Bill2's idea of controlling the compressor clutch on/off with the added Tstat is the best solution (might need to gate with the fan running signal for protection).
It's hard to believe that this AC system has no temperature input setting.

How do you set the heater temperature in winter -- no setting but just on and off switch manually activate? It would melt everything at 96C hot coolant if someone forgot to turn off. So it must have a temperature input setting. Perhaps the same heater input setting is also used to set the cooled temperature with a Tstat already built in.
It's crazy to think the system has no Tstat feedback.

Not given much thought to the LED signal yet, but I think that would be easy to achieve using a logic gate arrangement of some sort depending on whether the LED is being activating by switched 12V or switched 0V.

I still prefer using the existing control system, effectively just automating the button presses, as that maintains any overrides and protections built into the system. Although I see the appeal of controlling the compressor directly, to me this is effectively controlling it by repeatedly introducing a fault (loss of power) into a running system. I am not convinced that this would have no adverse effects such as generating system errors in the ECU.

The system has the usual fan speed selector, ventilation air distribution control and temperature dial for heating.

 

The system has the usual fan speed selector, ventilation air distribution control and temperature dial for heating.

What is the min value on the temp dial? How do you know it's only used during heating but not cooling?

 

What is the min value on the temp dial? How do you know it's only used during heating but not cooling?

For temperature control there is an old fashioned dial such as cars have had for decades which I believe simply operates a valve controlling hot water flow into the heater core. The red section of the dial permits varying rates of hot water flow and when you go into the blue section the heating water flow is stopped and you just get ambient temp air flow unless you turn on the AC. I am pretty sure there is no thermostatic control and in fact it appears that the dials operate cables to remotely adjust airflow and control hot water flow, so they would need to be motorised if there was any thermostatic control, which they definitely aren't.

 

On my last few cars which all came with AC/ the compressor was sized for cooling on a really hot day, and so on just warm days the compressor would cycle off and on, about a 2 minute cycle. AND looking at the circuits of some automotive AC systems they do have an "evaporator pressure control" pressure switch to optimize the pressure for when the car is not idling stuck in traffic.