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.
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.
Most tachs have a switch for 4-6-8 cylinder motors. Perhaps yours is Moving or worn out and malfunctioning?
Matt
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.
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My boat is in the crossover years so the gauges are still analog. I'll check the switch and the grounds. Thanks!
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...
Sometimes all you need to do is rotate the switch a few times to clean its contacts.
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