I replaced the small speedo gear in my 1986 240 and when i plug my cluster back together and started the engine the tach was not functioning properly. It will rise when I rev up the engine but then it drops to below 0 when it is idling. When I drove it it would again rise upon acceleration and once at cruising speed it dropped back to 0. It is the large tach and I plugged the red/white wire back into the same post it came off. All the other gauges function perfectly and my speedo is working again. No wiring was changed. Any tips as to what might have happened?