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Author Topic:   OMC Tachometer Problem
50lbStriper posted 11-19-2005 05:29 PM ET (US)   Profile for 50lbStriper   Send Email to 50lbStriper  
I recently purchased a 1992 90 hp Johnson on a 17 Montauk. The tachometer did not work from day one. We have performed series of tests and have been unsuccessful in correcting the problem. Most recently we changed the rectifier due to the fact that we couldn't obtain any reading on the Gray wire from the terminal block. To ensure that the tachometer still worked, we connected it to the yellow wire on the terminal board and it worked just fine After installing a new rectifier we still do not get a pulse out of the gray wire. I am using a Radio Shack Multimeter - on the AC scale - read 7.5 to 11V on either of the Yellow Yellow/Gray wires - The meter seems to work fine - On the gray wire from the rectifier I get no reading - Zero - I'm assuming I should read 3.5 to 5V on the AC Scale - The tach does not work - Also on the purple wire coming from the Rectifier I do not read any voltage AC or DC - One other test I did with the engine running and red wire removed from the rectifier I get 5 Volts DC from the red wire coming from the rectifier. It connects to the terminal board to a red wire that already has 12V DC live - Does that sound right ? What is really strange to me is that the purple wire has no voltage and yet at the gauges is has 12 volts. Is it possible that the purple wire feeds the rectifier, rather than the rectifier being the source of current ?

Any of your thoughts would be greatly appreciated - Thanks

Chris

jimh posted 11-19-2005 05:58 PM ET (US)     Profile for jimh  Send Email to jimh     
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jimh posted 11-19-2005 06:15 PM ET (US)     Profile for jimh  Send Email to jimh     
The VIOLET wire is a switched 12-volts which has voltage when the ignition key is in RUN or START position.

I am not familiar with the 90-HP OMC engine, and I don't have the service manual, so I cannot be of too much help.

If the rectifier/regulator is working properly you should obtain a voltage reading on its output (red wire) of 14 to 15 volts output depending on engine speed.

The battery charging output and the tachometer pulse output are derived from the AC voltage present in the STATOR winding associated with the YELLOW and YELLOW/GRAY lead. Disconnect these leads and check for:

--any continuity to ground; there should be none.

--continuity between YELLOW and YELLOW/GRAY; should be continuity. I don't know the exact resistance; on the V6 stator it is 0.16-ohm. The V4 stator is 0.7-ohm on the 125-HP motor.

If there is a problem with the battery charger winding of the stator, the rest of the engine will still operate. It runs off different windings. However you charger output will be bad and often the tachometer will be affected. This may be your situation.

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