The big question here is this...what other than a transmission can cause overdrive to malfunction and only work unpredictably.
I've got a 1995 Bonneville that has always had a tranny problem. When I first bought it, I soon noticed that I had to get in the upper 3000's before it would shift into overdrive. The problem got worse over 20 months. Just in the last few months, I had to do the following to get it into "OD". I had to get it up to 3900 while in "D", take my foot off the gas and quickly shift into "OD" manually. The tranny seemed to have an easier time getting into "OD" if my foot was off the gas at the time I made the shift. If I gassed it, the "OD" wouldn't engage half the time. Strange, but someone knows what this is. The tranny got worse and regular "D" sometimes wouldn't even engage.. I assumed that the tranny was going out soon so I had a used one with low miles installed. Cost me a total of $1200. I just got it back yesterday. Guess what…the problem was not solved.
The car still has the same “OD” issue, exactly the same. The other gears work fine now, but like the old tranny, there are times where overdrive just doesn't engage. Also, If It does shift into "OD" at say about 50/55, the rpms are only about 1200-1300 and the car feels bogged down and the acceleration is also noticably slow just after the shift. This may also be important. I came home from work yesterday and overdrive didn't engage. I got home, turned the car off and sat for 5 minutes, then took it back onto the highway. The overdrive worked just fine. IS IT ELECTRICAL? There are a few other issues with the car that may play a part in the problem. One issue is the RPM. The car idles at about 600-650. If I slow down and bring the car to a stop, the car has an initial idle of about 850-900 rpm. The car idles at this rate for about 15-30 seconds, then the idle just drops to 600-650. The car begins to shake and I feel like I’m being rocked forwards and backwards in the driver seat. At this point, the check engine light comes on. I took it to a mechanic a week ago and he replaced the upper/lower manifold gaskets. He determined that the manifold was sucking air so we replaced those gaskets. This did not fix the problem. Somewhere in the upper right corner where the manifold sits, there’s a leak. We just can’t seem to locate it . I hope these issues are all tied together somehow. The tranny model is a 4t60e if anybody wants to know.