TACH PROBLEM ON 1988 MERCURY 35HP OUTBOARD.

LOREN SILVA

Recruit
Joined
Jun 25, 2014
Messages
2
THE TACH ON MY 1988 BASS TRACKER HAS STOPPED WORKING. WHEN THE IGNITION IS OFF THE TACH READS 200 RPM. WHEN I TURN THE KEY ON , IT DROPS TO ZERO AND WHEN I START THE ENGINE THE TACH STAYS AT ZERO. ANY IDEAS?
THANKS,
LOREN
 

lmuss53

Lieutenant Junior Grade
Joined
Sep 9, 2008
Messages
1,227
Take a reading on the battery posts with a voltmeter and see what the voltage is with the engine not running. Then start the motor and take a second reading to see if the charging system is working, The reading with the motor running should be something more than it was without the engine running, let's say one volt more, a little over a volt is OK, and with smaller unregulated outboards 14 or more volts is not unusual.

If the reading does not go up or goes down, you have a charging issue, the failed tach is a first indicator of this because the tach runs off the AC output of the stator, which provides the AC to the rectifier, which changes it to DC to charge the battery.

My Johnson motors pick up the AC for the tach, before the rectifier or rectifier regulator comes into play, so a failed tach with no charging would suggest a stator problem on those motors, your Merc might work differently though.
 
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