Thanks in advance for the help. I tried a thorough search and didn't see anything related. I've been a Master ASE tech for years, but this is my first experience in the tuning world, so I'm trying to get a handle on some things.
The quick background: I swapped a stock '08 5.3 (LC9) into my '84 C10 pickup last fall. Everything has been working pretty well with minor tuning tweaks. I wanted the ECM to control the fans, so I wired the fans just like the 2008 setup. There are dual fans with the 3 control relays. They are working normally at high and low speeds for engine temperature. The problem comes with the a/c. I built some a/c lines and hooked everything up this Spring. I have the a/c high side pressure sensor wired in just like factory. The compressor is not engaged by the ECM because the factory setup uses the BCM to send a request over the CAN to the ECM for compressor engagement. I've left the compressor wired into the 1984 controls.
Here's the issue. The fans never turn on with the a/c. The a/c pressure signal in the ECM on my scan tool says 0 psi, 0 volts. I check the 5 volts, low reference ground, and signal circuits at the ECM and everything is good. The signal circuit at the sensor reads 1.2 volts and backprobing the ECM at the signal circuit shows the same 1.2 volts. So I'm a diagnostic tech and I'm thinking the computer must have a problem. We happen to have a 07 Denali (6.2L) in the parking lot with a E38 ECM, so I decide a quick "swap diagnostics" is in order. Now, I can't start my truck with the donor ECM, of course, but when I swap it in, I see 1.2 volts on the scan tool from the pressure sensor. I come to the conclusion that the computer has failed internally.
So, I buy a junkyard ECM, it's out of a Equinox, but the same E38. I pay for new credits and flash the new (used) ECM with the same tune from the original 2008 computer. The pressure sensor still reads 0 psi and 0 volts.
At this point, it must be in the tune. I've poured over all the options in the editor and don't really see anything. I'm wondering if the donor truck didn't have a/c at all, but I don't see any option to set that. Sorry for the long winded question, just hoping someone might have a tip or experience with this. Let me know if any more info would be helpful. I know I can run the fans off of a pressure switch and more relays, but if I can get it to work with the factory wiring, I'd rather go that route. Thanks again for the help.