tach problem

[mikegj]

Hi all
Iv'e been running a 1979 johnson 100 hp javelin W/ faria 7000 rpm tach all summer with no problems. Last week I changed to a 1980 johnson 115 and have changed nothing with the tach or wiring and tach says its idling at approx 1500 rpm. I borrowed a infrared tach and it says its idling at 673 rpm. So what gives with the tach? do I need to switch something on the back of the tach and if so to what?

 

[Joe Reeves]

Re: tach problem

The charging system on both engines are identical pertaining to output (6 pulse system). If there's a dial on the back of that tachometer, it should be set to number six (6).

 

[mikegj]

Re: tach problem

Thanks Joe
I dont have a #6 switch on the tach. The Label reads as follows
SWITCH POLE
#1 4 pole
#2 6 pole
#3 8 pole
#4 10 pole
#5 12 pole
so witch one is correct for my current motor?

 

[Joe Reeves]

Re: tach problem

I'm not familiar with that setup but if memory serves me correctly, that charging system would be a 12 pole (not sure). Another member will jump in here shortly with the proper advice I'm sure.

 

[gyoung]

Re: tach problem

Set it on #5. Your system is 12-pole which produces 6 pulses per revolution.
Your results indicate it is probably set on #2 for a 6-pole system that only produces 3 pulses per revolution. Thus the 6 pulses per revolution would would make the tach think the engine is turning twice as fast as it actually is.