My 89 56sel with 164,000 miles is not downshifting properly under full throttle acceleration. Under normal driving conditions it accelerates and shifts beautifully. The problem is evident when driving in the highway gear (I am assuming 4th gear) and there is the need for maximum acceleration as in a passing situation. I have replaced the kickdown switch under the gas pedal so that does not seem to be the problem. I have read where some have suggested the O2 sensor or catalytic converter could be the problem. Others had suggested transmission fluid and filter change. I prefer to avoid trying one thing after another without knowing what is the actual problem. I am thinking someone out there knows a lot more about this problem than I do. Any help is greatly appreciated.

Correction: Should read "560SEL". Forgot to mention that I have also replaced the kickdown solenoid on the passenger side of the transmission.

If you squeeze into the throttle while simultaneously pulling the selector back to "3", is there a delay or does the downshift occur promptly?

I have not tried that Frank but I will in the next day or two and let you know what happens. I am in the process of changing transmission fluid and filter. When I drained the fluid, it smelled and looked like new fluid. Filter was not clogged and there was no metal and no sludge in the bottom of the pan. The part of the transmission that I could see with pan and filter removed looked very clean. I bought the car several years ago without any service records so I wanted to see what the filter and bottom of the pan looked like in hopes that it would give me some idea of whether the transmission itself is good. I have only driven the car a couple of hundred miles since I purchased it. It drives and shifts great other than the passing gear/kickdown problem.

Problem seems to be solved with transmission fluid and filter replacement. Now downshifting into passing gear nicely.