I wired up a tach from a 05-13 FLH touring on my 2008 Street Bob. The tach is working, but showing the wrong speed. It indicates 350 RPM at idle & the rev limiter kicks in at an indicated 5000 RPM. Is it a simple resistor for me to calibrate the tach? A little concerned that the error is not linear. What is the idle speed and rev limiter speed anyway?

 

Idle should be around 1000, give or take. I have mine set at 960. The rev limiter with the stock tune is something like 5800, if I recall. The limit can be raised with a tuner to about 6200.
I have no idea how to calibrate a tach or if that's even possible, but it could also be that's it's not wired right.

 

I now think that there is a consistent offset of 750 RPM. At an indicated 1000 RPM in 6th I am moving at 48 mph, where the engine has to be at 1750 RPM. adding 750 to the indicated idle of 350 brings it up to 1100 actual (which although high is in the ballpark) and adding 750 the the indicated rev limiter rpm brings it up to 5750.

Still do not know hos to fix this offset problem. Maybe it is an error in my wiring - don't know.

 

I know on some of the earlier tachs there's a little break plug on the back of the tach when removed gives access to a an adjustable potentiometer for calibrating the teach... Might want to give it a try...

 

Your error should be linear as the rpm sensor in the transmission is just not at the pulse timing that the tach is programmed to see. Most Harley tachs don't have internal parameters to set them. Most people put on a piggy back module to alter the transmission sensor signal or get programmable tachs and speedometers to correct the pulse to get accurate info. Unfortunately, I don't think you can correct the FLH tach being off without making the speedometer also be off as they use the same signal.

 

Some piggy back or 2nd party ECUs also allow you to change the speedometer and tach calculations coming from the transmission sensor as well. That could be a solution.

 

Are you saying that the tachometer runs from a signal off the transmission??

 

Hell no... It runs off the ignition trigger...

 

**** didn't spell check... Supposed to be"Brass"

 

My bad, I was thinking of the speedometer. That would mean the tach being off would not be linear as well. Not much experience adjusting the tach so don't listen to me.