trying to get my autogauge digital tacho woring on a perkins diesel engine

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HELP ! can anyone tell me if you have to fit some kind of special kind of sensor to a diesel engine to get the tacho working correctly there is a terminal on the alternator but when i connect this the tacho reads 5100 when the engine is idling! its a 4 cylinder engine and the switch on the gauge is on the 4 any ideas PLEASE keefryan2002@yahoo.co.uk 07966276250