I have a 1996 Tahoe with aprox. 209,000 miles on it. I purchased it with aprox. 203,000 and from the time I purchased it the transmission slipped slightly between 2nd and 3rd. Last winter I noticed that it would act as if it was in neutral when it shifted into overdrive. At first the overdrive problem was intermittent, and of course did not do it at all when I took it to the local transmission shop. They told me that the 4L60E transmission was probably worn out due to the mileage and quoted me a price to rebuild it. This summer I installed a 100,000 mile transmission and torque converter out of another Tahoe that had no transmission issues. When I took my Tahoe out for a test drive it does exactly same shifting behavior as it did with the old transmission. I have read many accounts about voltage issues, and my Tahoe will read 10.5 volts with all of its stock equipment turned on and continued to do this after I installed a rebuilt high capacity alternator. Do not know if the two have anything to do with one another and would appreciate any help I can get. Thanks.

Check the transmission connector, ignition switch, bus connections in the underhood electrical center, and main power distribution.

Blue Bowtie, thank you for your thoughts. I have heard that there could be problems with the ignition switch. All of the electrical plugs on the transmission came out hard and went back hard, although it seemed to be caused by the sealant GM put on the connections. The plugs them self looked ok. So I was wondering how is the best way to check the ignition switch, bus connection, and main power distribution.

See if you have power at the trans plug key on, is the first item to look at.