Join Date: Feb 2010
Location: Grass Lake, MI
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Hmmmm... not a problem I've seen before. There is no direct connection (that I know of) between A/C usage and trans shift pattern. My first guess would be that when A/C is on, you have to tip deeper into the throttle (to counteract the compressor load), and maybe you're running right on the edge of the 5-6 shift point. So at heavier throttle (A/C on) you are still running in the 5th gear zone, but when you turn A/C off and let up on the throttle slightly, that lowers the 5-6 shift point and puts you in the 6th gear zone.
Or maybe something is screwing up your throttle position sensor reading when A/C is on (making it show a higher throttle than actual). A dealer or shop with a scan tool could monitor the throttle signal while driving (at constant throttle) and see if it jumps around when A/C is turned on or off.
One other thing to consider is that the 68RFE will not shift into 6th gear until the Torque Converter Clutch (TCC) is engaged. Again, there should be no direct relationship between A/C and TCC engagement, but your scan-tool-equipped tech can also monitor TCC state and confirm whether the TCC is engaging or not (when A/C is on).
If everything looks "normal" when monitored with a scan tool, check this: Hold the throttle at 50% open as you accelerate through the gears, and see when the 5-6 upshift occurs. At 50% (45 degrees) throttle, the 5-6 shift should occur at roughly 2540 output RPM (assuming TCC is engaged). See if you get that same shift point with A/C on or off.