If your talking about the 85 model, you may want to check your egr computer. If that or the egr relay is not working properly, then your tach will not work. I have spent countless hours chasing down a non functioning tach when it ended up being the egr computer. The contacts that plug into the harness get a green fluffy corrosion and it flake the tach out. This is only on the 85 w123. The other years do not run the tach through the EGR and the Klima relay.

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