why does my 4+3 transmission not engage

I have a 1984 corvette with the doug nash overdrive. I have a overheating issue on the car but I have noticed that my overdrive will not engage once the car gets to 190 degrees. If I stay in the 180-185 degree area it will engage just fine everyone I hit the switch but the minute I get to 190 and up it doesn't matter how many times I rock the switch back and forth or how long it's switched on the transmission remains in 4th gear normal. What could be the cause of this. How do I bypass the ecm so the overdrive is manually controlled. I have read lots of different things which is the correct way

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