My 1984 pontiac gran prix with the 305 engine and 200,000 plus miles wont downshift. This problem has been present for about a year now. I gave the car a tuneup recently and discovered that the "tach" wire from the HEI coil was burnt or corroded so bad that the wire broke when I tried to remove the coil from the old distributor cap. I replaced the coil and to my amazement the transmission started to downshift properly. After about 10 minutes the transmission went back to not downshifting. I have not looked at the new coil yet to see if the wire burnt through again. What does the "tach" wire from the coil have to do with the transmission? Could this be an ignition module problem that is causing the loss of downshift? All of this time I thought the transmission was bad so I was very surprised when the tranny started working after the tuneup and very disappointed when it did not last long. Any advice would be appreciated.