Digital Automotive Tachometer Reading 2X

[jlstodd1]

Hi folks. My problem is that a digital tachometer I installed in a offroad vehicle is reading twice the rpm that it should. I am using a MSD ignition box and MSD tach adaptor. The engine is a 350 chevy with a HEI distributor. I have gone around and around with both manufactures and all the wiring should be correct and yet the problem remains. The tach has a cylinder selector switch on the back and when it is set to 4 cyl the unit reads 4x as fast 8cyl 2x. Anyway is there something I could put inline in the signal wire going to the tack to reduce the pules by half? It is supposed to be a square wave voltage signal. Thanks a lot for any help you might be able to supply.

 

[bychon]

Quick answer: YES

but it's digital. Not my best stuff. Somebody else will be right along and hook you up.

 

[jlstodd1]

Thank you sir.

 

[MikeMl]

It sounds like you have some ringing and it is double counting. Maybe a "filter", or dropping the pick-up signal amplitude? I'll be back later to help with it...

 

[PCBWING]

Is the number of LCD?

 

[MikeMl]

I would try a simple RC filter consisting of a series 10K 1/4W resistor connected between the distributor connection and the input to the tach cable, with a shunt 10nF 15V capacitor connected between the center-conductor and shield of the tach cable. The cable going from distributor to tach is a shielded coaxial cable, right? The tach cable shield is grounded near the distributor connection, right?