Forums

Since this has turned out to be more complicated than I initially thought, I think I'm going to just go with custom made AC lines. I can use both stock 92 Cutlass lines from the evaporator, the drier, and condenser. Then all I need is to cut and splice the Impala compressor-end hoses to the Cutlass condenser+drier ends. Two hoses, 4 crimps, no problem!

I had a fun chat with ChatGPT and learned some possible solutions. One is to just use an aftermarket pressure switch, wired inbetween the Cutlass HVAC module AC Rquest and clutch relay. Another option is an 02-03 GTP PCM (also 00-02 Regal GS and 02-03 Bonne SSEi are potentials). It's an L67 with an 4T65-EHD, plus is has a discrete AC Signal request input instead of a Class2 signal from a BCM. I'll just have to repin a few wires at the PCM connectors. The GTP is probably a great choice because it has full support in HP Tuners, which I already own. The 04 Impala PCM is supported in HPTuners also but I don't think it supports all features.

I'm definitely not looking to go with a manual switch. Let me see if I understand how the system works in the 04 Impala... The HVAC Control Module has an AC Request Switch, when driver wants the AC on, it outputs a Class 2 Serial signal to the BCM. The BCM doesn't appear to have any other direct inputs related to the system, so it might just pass that along to the PCM. The manual does specifically state that the AC Request signal comes from the BCM, not from the HVAC module. In any case, the PCM needs a Class 2 Serial input containing the AC Request On signal. Then the PCM decides along with the other inputs if it should engage the clutch. If it doesn't get this signal, the PCM will never trigger the clutch relay to engage. So if that's all correct, I think I have three options. 1) Find out if there's a PCM that will run the L67 but doesn't use a Class II signal for the AC Request Signal. 2) Simulate a Class II Serial signal with an Arduino including an AC Request On message. Other forums have decoded the signals, further research is needed to determine if this is feasible. 3) Bypass the PCM entirely, again with an Arduino. Take the AC Request signal from the Cutlass HVAC module (which I think is actually just 12v switched?), the AC Pressure Switch, optionally Power Steering pressure switch, throttle position, and coolant temp sensors, all into an Arduino programmed to power the clutch relay under the right conditions. None of these options sound fun! What is different about my swap that makes this neccessary? Tons of people have put 3800's in W-bodies and I've not seen anyone talking about AC issues.

I doubt that any of those PCMs would do what you’re wanting them too. I don’t know about the Lumina, but even 97 2nd gen cars would have used class 2 data from the BCM to receive the AC request signal. The Lumina PCM being from an NA car would also pose a different set of challenges altogether though.    GM was famous for changing things like the crank/cam correlation, crank sensor patterns, sensor types etc….on the same engine over the years, so an older PCM for a series 2 L67 may not even work with a newer one. Also take into account when those write ups were written. In the mid-latter 2000s, using a 2004 as a donor car wouldn’t have been common as they were still too new, which might be why the references are to older ones.    Assuming this isn’t a daily driver, I’d almost consider going the manual compressor switch route. The power steering and WOT compressor cutouts aren’t overly critical, and the low and high pressure refrigerant switches should be easy enough to integrate into a manual AC compressor switch.     

I would use an older PCM from a different 3800, which is what every swap guide I read suggested in the first place. I used the engine-matching PCM because it ended up being easier, but there must be reasons previous swappers used late 90's PCMs instead. I also have an 2000 GTP and 98 Lumina LTZ PCM. Both of them tested to start and run the engine, but both had different transmissions and the LTZ is N/A. Both would need programming, but one of them might have a compatible switched ground AC request signal. Surely I'm not the first person to have AC in their L67 swap!?