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Author Topic:   Johnson 70hp 84
Lake George Whaler posted 09-26-2002 06:38 AM ET (US)   Profile for Lake George Whaler   Send Email to Lake George Whaler  
The tach stopped working. The wires from the control box to the tach look ok, no breaks or loose connections. I,m not sure where on the engine the signal is picked up? Any help will be great.
Clark Roberts posted 09-26-2002 07:49 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
The tach signal emits from the rectifier, a small, black, matchbox size gadget that converts alternating current to direct current. Not sure of location but you may be able to trace wires from under flywheel directly to it and then check the tightness of terminal wires (loosness may be cause of tach failure). Rectifier may have failed and you can check this by running engine and noting via a voltmeter whether or not voltage goes up with engine rpm. If not, then rectifier is faulty! Replace and problem with tach should go away. I believe (could be wrong here) that your engine has an unregulated system (no voltage regulator) but if it does the rectifier may be built into the regulator... a shop manual would be a good idea as I'm not sure (even though I have owned lots of 3 cyl OMC's) of the electrical details of your engine. Others most likely will have good advice... Happy Whalin'... Clark... Spruce Creek Navy
Bigshot posted 09-26-2002 10:41 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Actually on that 84 it is probably a round silver thingy on the left side(facing engine from boat). They are about $20.
where2 posted 09-26-2002 12:57 PM ET (US)     Profile for where2  Send Email to where2     
I second Bigshot's comment about the round thingy about the size around of a quarter, and about as high as 8 quarters stacked up. Has 3 wires running into it, vaguely I seem to recall them as purple, yellow and black. The leads are short and go to a terminal strip not more than 6" away. Check your connections at the terminal strip, then check to see if your battery is charging when you run the engine using a volt meter. If the battery isn't charging, your rectifier is definitely toast. However, check continuity between the gauge and the purple wire at the terminal strip to verify sending wire integrity. If sending wire checks OK, Replace the rectifier, and if that doesn't fix it, replace the tach. (tach costs more than rectifier...)

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