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the tach operates from the td signal that is produced by the ignition module.at idle that signal is about 9.5v and drops as rpm's rise.it also feeds to the injection computer and is the signal that keeps the fuel pump relay turned on once you release the key from the start position.pull out the cluster and you will see a two pin plug for the tach on the backside.pull it off and test with ignition on.you should find 12v on one pin.now start engine and test other pin for td signal.if signal there then instrument is bad.
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