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Dear AVL940T,
Hope you're well. The tachometers in 940s seem rarely to fail. When cars are 30+ years old, a circuit may be disrupted by a coating of corrosion only a few molecules thick, and so invisible to the unaided eye.
Starting with the instrument cluster, I presume the other instruments - fuel gauge, speedometer, and coolant temperature - work correctly. If so, it is not likely that the tachometer has failed. It is likely that the cluster's ground (behind the plastic panel on the door-side of the driver's footwell) is in good order.
Even so, I'd first remove the instrument cluster, and use an aerosol oxidation remove on the wiring harness connectors and - using a cotton-tipped swab - on the cluster's male wiring contacts. Deoxit has served me well.
Presuming that this makes no difference, I'd apply Deoxit to the wiring harness connecter for the crank position sensor (CPS).
If this does not restore tachometer reading, I'd get a Bougicord CPS and replace the new, in-service unit, which may be defective.
I'd be surprised if one or more of these measures does not restore smooth operation. When the CPS signal is lost, the engine computer does not allow the fuel pump to work after its initial "spin-up". This is a safety measure: should an accident cut a fuel line, that will stop the engine. When the engine stops, the loss of the CPS signal stops the fuel pump, preventing the delivery of fuel to an engine compartment, and so preventing a fire.
Hope this helps.
Yours faithfully,
Spook
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