David Stotz

O.K. brothers, I've been chasing this one for awhile and I'm out of ideas. OEM tachometer, MSD ignition, Mercruiser side mount OEM throttle/shifter. At idle tach works fine, increase throttle, tach moves up as expected. Hit the trim-up button needle on tach goes crazy but returns to original reading once released. Cruising at about 3500 RPM turn on the bilge blower motor tach goes to Zero, turn motor off tach goes back to previous reading. I thought the problem was in the factory wiring harness so I ran a new Trigger wire directly from the MSD box to the tach and a new ground wire from the tach directly to the battery, problem did not go away. I've also checked the MSD box and it seems to be OK as per them. (reading the voltage at the tach output) Any Ideas are always appreciated!
Thanks, Dave

GOODT

sounds like a ground issue at a grounding block

David Stotz

GOODT, that's what I thought, but I also thought the new ground wire from the tach directly to the battery would solve that? I should also add that all the other guages work fine.

mtocrs1

Run a temporary power wire directly from the battery to tach and see where that gets you.

David Stotz

Motocrs1, I'll try that next. I didn't realize there was a seperate power wire to the tach. I assumed the power came from the ignition box!
Thanks, Dave

canajun

I have an MSD Blaster 2 coil with a Prestolite electronic distributor on my old Nova, and I have some tach gremlins too! I was idling in the other day and the tach was saying 1200. I tapped the tach, and it returned to normal. I knew there was no way (not loud enough). I also noticed that occasionally when I crank the motor that the tach goes nuts as well.

I think it is reading low as well, because in order for the boat to go as fast as my GPS reads, the prop would only have like 2% slip. That would be one hell of a claim for Stiletto props if it was true!