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rocketmench · 02-18-2021, 12:41 PM

Hi all, I realize this is an older thread, but I thought I'd post an update regarding how those 2003+ Blue/Green P59 PCMs that do not accept analog 12V signal for A/C request can be modified to accept that signal. I'm going to be restoring a 1968 K20 and I want to have analog A/C with my LS swap also.

In summary, I obtained two PCM's to compare; one that had the IAC stepper control mosfets and A/C request circuitry, and one that did not. I traced out the circuit at pin C2 17, where the A/C request signal is sent. On the PCM that can accept this signal, there is a pull-down resistor and debounce (low-pass filter) resister/capacitor combo that is missing in non analog A/C request PCMs. I verified that when these components are soldered into the board, the PCM responds to analog A/C request. And fans 1 and 2 can be made to come on as well if using the Analog mode in the OS.

I posted the details of which resistors and capacitors to add in a post over at ls1tech.com:

https://ls1tech.com/forums/conversio...c-request.html

Hope this helps.

02-18-2021, 12:41 PM	#16
rocketmench Registered User Join Date: Jun 2010 Location: Granger, Indiana Posts: 3	Re: AC wiring Hi all, I realize this is an older thread, but I thought I'd post an update regarding how those 2003+ Blue/Green P59 PCMs that do not accept analog 12V signal for A/C request can be modified to accept that signal. I'm going to be restoring a 1968 K20 and I want to have analog A/C with my LS swap also. In summary, I obtained two PCM's to compare; one that had the IAC stepper control mosfets and A/C request circuitry, and one that did not. I traced out the circuit at pin C2 17, where the A/C request signal is sent. On the PCM that can accept this signal, there is a pull-down resistor and debounce (low-pass filter) resister/capacitor combo that is missing in non analog A/C request PCMs. I verified that when these components are soldered into the board, the PCM responds to analog A/C request. And fans 1 and 2 can be made to come on as well if using the Analog mode in the OS. I posted the details of which resistors and capacitors to add in a post over at ls1tech.com: https://ls1tech.com/forums/conversio...c-request.html Hope this helps.