Try re-setting the ECU by disconnecting the battery, (or Main ECU fuse) for a minute. COuldn't hurt.
The reason I asked about aftermarket is because it is acting like it is reading a Multi-Spark or High Energy ignition. They throw off stock tachs to read high from the extra pulse signals the tach reads.
If you got a Multi-Meter with a dwell tach function it could probably eliminate the coils and ECU. Kindda leaving you with the tach.
Does the car run well?
When did the tach start to mal-function?
Let me know and I will do some research and watch fro your reply.
Wednesday, August 24th, 2011 AT 10:52 PM