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.