Most likely you've got the tach pickup on the intermediate plate of the engine/transmission. If so the signal is routed through the EGR computer to the cluster. If you search a bit you can find how the EGR can be eliminated and the signal passed directly to the tach module in the cluster.

It should be an easy fix. If it's the EGR, it can be eliminated. If it's the sensor that's one bolt (and they very rarely go bad). If it's the wiring, that can easily be replaced. And if it's the tach module, that's like two screws to swap out. The EGR module gets its power from the OVP (over voltage protector) and it has a fuse on it...that would be the first thing to check. And sometimes the cable to the box gets corroded and needs to be cleaned up.

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