I had an '85 Firebird whose transmission (in warm weather only) would constantly go in and out of torque converter lockup when climbing the slightest grade. I took it to several dealers and was told I needed a transmission overhaul- "...they all do that when they need to be repaired.."
The transmission eventually failed and I had rebuilt. The rebuilder told me that he did not believe that the rebuild--although needed, I had lost high gear-- would take care of the problem that I described. And he was right. The rebuild occurred in October, and months later, when the weather warmed up, there I was-- in and out of lockup again!
I returned to the rebuilder and he road tested the vehicle, and it went in and out of lockup for him, too. He said that he believed he knew what the problem was. He noticed that the speedometer needle tended to flutter at speeds under 40 mph. He also told me that the car's computer received a lot of information--including vehicle speed from the speedometer head-- and that it fed signals to the transmission. Since the indicated speed was varying wildly, the computer was telling the transmission "Its O.K. to lock up, no it isn't, yes it is, no it isn't, etc." I replaced the speedometer cable (the entire assembly, not just the core), and the problem was immediately resolved.
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