Shifting problem (up/down) 94 Lesabre

I have a 94 LeSabre with 105,000 miles on it. On acceleration, it shifts very quickly, i.e. it will not let the RPM's build up. In addition, it will not "downshift" under heavy acceleration. As an example, if you pull out to pass, and put the pedal to the floor, it does not downshift, it stays in overdrive. This is consistent through all gears, and at all speeds. The check engine light did come on. I did replace one old/cracked vacuum line, but that did nothing. The car actually stalled on me pulling into the autozone parking lot. Would appreciate any help. Thank you.

For tran shifting issues I would do a scan and look for a bad TPS, if this checks good, a full scan with a trans function scanner will be required.

A '94 also still has a vacuum diaphragm for line pressure modulation. It would be good to verify the vacuum line is connected at both ends, and check teh throttle valve cable.

Pay no attention to the guy who posted the comment about the throttle valve cable. He's an idiot. There is no TV cable on your 4T60E, only a manual valve and vacuum modulator.

Thanks Blue, for the correction to the post concerning the TV cable. A few questions for the OP; is the cruise control & speedometer still working? Did you recently remove the battery cables for any reason? Have you noticed a substantial drop in fuel economy? Is the alternator output steady, or is it fluctuating? Have you noticed any drop in battery voltage? This particular vehicle/engine combination should get 17-18 mpg city driving, with a good tune & normal driving habits. Shifts are controlled by the ECM/PCM, using feedback from a network of sensors. 94-95 PCM's have a reputation for going bananas when subjected to voltage spikes. This may, or may not be the problem, however, if it is, the only option you have is to replace it.

... 94-95 PCM's have a reputation for going bananas when subjected to voltage spikes. This may, or may not be the problem, however, if it is, the only option you have is to replace it.

I can vouch for that. My '95 PCM lost the VSS scalar and outputs. The speedo would display 50-60 MPH constantly, cruise didn't function, and shifting defaulted to local (hydraulic) control, with very firm, MPH-based shifts instead of load-based shifts.

http://72.19.213.157/files/1995H-BodyPCMLocation.jpg

A failed diode in the alternator rectifier bridge imparted some AC voltage into the system, causing all sorts of problems, including the failed PCM.

If this happened to you, the "up-side" (if there is one) is that the transmission might be O.K., and a replacement PCM only cost me $65 (salvage yard) and another $50 for a re-flash at a local NAPA.

Everything else worked fine, and the car ran good to answer all your questions. BUT, I did just replace the alternator about 100 miles before this problem. I did notice the new alternator caused the voltage gage in the car to read higher than it was before the alternator died. I can't exactly remember where the needle was but it was not to far from the red mark (maybe 18 ??) This all sounds pretty complicated for my blood. I'm just not sure if I should take it to the auto repair shop, or a Buick dealer.