Tachometers not working correctly

[woodguy81]

I have many small issues to sort out on my project here and my latest is non functional tachometers. I have a pair of 6.2 mpi mercruisers. one is a 2002 and one is a 2004. at least that is what the computers are now. These were built from a couple 5.0 mpis that I swapped the parts onto new 6.2 long blocks and got computers for. Back to the issue. The tachometers are way off. reading way high and inconsistantly. Where do I even start to figure this out? I did not look at the gauge setting yet as Im told they can be adjustable for 4, 6 , or 8 cylinders but this is a 30 year old boat that only ever had v8's in it so assuming that all good where else should I look? Is there a manual that covers this area of service? I have manual 31 currently but didnt see anything there.

 

[Bondo]

I did not look at the gauge setting yet as Im told they can be adjustable for 4, 6 , or 8 cylinders

Ayuh,...... Boat tachs live a tough life, exposed to weather, 'n splash, so are notoriously inaccurate,.....
Sometimes, just rotating that selector switch back, 'n forth a few times will bring 'em back to reasonable accuracy,..
Sometimes not,..... then you replace 'em,....

For truly accurate reading, I use my timing light, that has that function,...

 

[woodguy81]

No other ideas? Ill look into the gauges themselves but where do they read from on the motor side?

 

[Bondo]

No other ideas? Ill look into the gauges themselves but where do they read from on the motor side?

Ayuh,... The tach gets it's signal from the (-) post of the coil,....

 

[woodguy81]

Hey just an update on this one. Im not sure how they are related but I connected the pcv valves that i neglected to hook up when I reassembled everything on these engines and both tachometers started reading correctly. Anyone know how or why this is? No other changes were made to the engines so far.