View Full Version : Tach doubles RMP

02-27-2013, 09:32 AM
What would cause a tach to roughly double the RPM? My Port tach is always fine but occasionally the Starboard one goes quirky and it seems to double (guesstimate) the RPM.

Boat is a 2008 with Merc 6.2 MPI's.

02-27-2013, 09:52 AM
Most tachs have a switch for 4-6-8 cylinder motors. Perhaps yours is Moving or worn out and malfunctioning?


02-27-2013, 10:32 AM
Yes I think the setting switch is off or broken from what I understand of how the tach works. the sender input is looking for a ground pulse coming from the ignition system coil so the tach is looking on a 4 cyl motor for 4 pulses each cyl firing 1 times or one revolution of the dist = 2 revolutions of the motor tdc of compression to tdc exhaust back to tdc compression a tach set for 8 cyl motor would be looking for 8 ground pulses in one rev of the dist = 2 revs of the motor not sure if this is considered 1 rev or not but the cyl travels to tdc twice

so a tach set for 4 on an 8 cyl will read double the revs.

an easy test might be to swap the sender leads from the 2 motors so the port tach is reading the starboard motor and vice versa .note if the leads are long enough or you may need to make a test pigtail.

if the problem stays with the tach it is a tach problem and if the problem moves to the other tach it is something in the sender wire or somewhere in the ignition system .

check your grnds and make sure the sender leads isnt shorted to grnd

I guess I should add this is how I understand an analog tach to work not sure if you merc's have digital input gauges that some mercs have.

02-27-2013, 12:13 PM
My boat is in the crossover years so the gauges are still analog. I'll check the switch and the grounds. Thanks!

02-27-2013, 01:22 PM
if it does it OCCASIONALLY where it DOUBLES, it's not the 4-6-8 selector switch.... it's a loose wire at a connection somewhere along the line...

02-27-2013, 02:58 PM
Sometimes all you need to do is rotate the switch a few times to clean its contacts.