For any discussion not specifically related to your project
We are dipping our toes in replacing the Subaru 1990ish ECU with a Speeduino. The biggest issue appears to be the tight integration of the HVAC with the OEM ECU. Most ideas we have seen appear to be brute force methods with relays that ignore the subtle integration between the two units. Someone on FB claims to have cracked this nut though. He said "You can write code to make Speeduino do anything, we did a Subaru that controlled them apart, and also controlled the A/C clutch, checking for the desired temperatures and pressures" We have tried to contact him with no results.

Has anyone here tried this or has a similar firmware solution? How about a brute force method that will operate the A/C somewhat properly as designed e.g. A/C clutch control, temperature control and A/C pressure limitations. (The Subaru service manual has all types of "conditionals" for that clutch control.)

Anyone have code they will share or even success with brute force methods? Detail appreciated. Thanx for your time.
I have not done your exact goal, but there are several ways to approach it, depending on the desired results. IMO, "brute-force" is simple on-off switching based on input parameters, e.g., if a pressure or temperature is exceeding a threshold, then it does something. Alternatively, a much "smarter" control may be used to juggle inputs and outputs to varying degrees and with additional conditions, e.g., hysteresis.

My questions are what is minimally required, and what is ultimately desired. Conditions would have to be specified, and how that data is collected, along with the desired result. This could be approached with Aux I/O settings in Speeduino, or with a secondary µC (Nano or something), gathering data from both your AC sources and Speeduino, in order to drive outputs, either independently, or through Speeduino, etc.

If you want to pursue this, list the overall concepts and actions, along with inputs and outputs. We don't need pin numbers or anything yet, just what it does, when, why, and how it knows.

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