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Author Topic:   Wild Tachometer Reading
Oldslowandugly posted 05-31-2015 09:27 AM ET (US)   Profile for Oldslowandugly  
Hello all. I have a 1993 Evinrude 48-SPL mounted on my 1985 Whaler SPORT. The one-year-old, normally-dependable tachometer began reading 3,000-RPM at idle and went off the scale at WOT, but battery charging was fine. As per the advice I read on this site I used a multimeter to check the electrical system and all was well.

Since the usual suspect is the rectifier, and I happened to have a brand new one on hand, I swapped in the new one. Problem solved. Oddly, when I did the rectifier diode test on both units, they tested the same: perfect. I am perplexed at how this could be and wonder if there is a better test for a questionable rectifier.

Years ago I had a very similar problem with an Evinrude 30-HP engine's stator. It was installed new by a dealer, charged well, but the tach went crazy, reading high, dropping to 0, jumping around. A new rectifier did not help. No one could explain why and I eventually swapped in an old used Johnson 35 hp stator. Problem solved. Again, both tested the same-perfect. What am I missing here?

jimh posted 05-31-2015 09:52 AM ET (US)     Profile for jimh  Send Email to jimh     
Your report a problem with tachometer readings, and replacement of the rectifier assembly resulted in a remedy. But when testing the original rectifier assembly in a static test using an Ohmmeter (I presume) to test the resistance of the diodes when forward-biased and reverse-biased, you don't measure much difference between the original and the replacement rectifier assemblies.

This can be interpreted in several ways. If we assume the static test of the rectifier assembly with an Ohmmeter was valid and there is actually no difference between the original and replacement rectifier assembly, then the remedy provided by the rectifier was just coincidental, and the real remedy was provided by some other component in the circuit. There is a reasonable basis for this to have occurred because in the process of installing the replacement rectifier assembly you may have disturbed other circuit elements by mechanically moving them, which might have restored a bad connection or other discontinuity.

You are now facing the same problem as David Bowman faced aboard the space craft DISCOVERY in the cinematic film 2001: A Space Odyssey when the AE-35 unit, which had been predicted to fail by the supervisory HAL-9000 computer, was found, on static test on the bench, to be in good working condition. For more details, see

I defer to the suggestion made by the HAL-9000: return the device under test, in your case the original rectifier assembly, back to service (on the outboard engine) and verify that it has failed by observing the return of the defect, erratic tachometer readings.

An alternative theory of why your static resistance test of the original rectifier assembly does not detect a defect can be made as follows: the defect is only manifested under real circuit operating conditions. The presence of heat or of higher voltages than those of the Ohmmeter or of higher currents than those of the Ohmmeter are necessary for the defect to appear, and your static test on the bench with the Ohmmeter cannot detect the failure.

jimh posted 05-31-2015 09:57 AM ET (US)     Profile for jimh  Send Email to jimh     
OLDSLOWANDUGLY alludes to prior advice given about resolving erratic readings of an outboard engine tachometer, but I believe he has overlooked the most likely cause: intermittent connections in the calibration switch of the tachometer. Refer to the many prior comments about this as mentioned in

Diagnosis of Tachometer Problems

Oldslowandugly posted 05-31-2015 07:49 PM ET (US)     Profile for Oldslowandugly    
JIM--Thanks for the reply. I did indeed read all I could about tachometer problems and the various tests involved in proper troubleshooting (posted by yourself and the others). I assure you the connections were sound, and I did not disturb any other components. What tipped me off to the rectifier was that I took the gray tachometer wire and connected it to a different yellow wire, the one with the blue stripe. The tachometer reading was different, not correct, but a different wrong reading. It would start at 3,000-RPM, rise a bit, then drop down and begin reading almost properly. Reconnecting to the yellow/grey stripe got the same-as-before bad reading. After the new rectifier was installed, switching the grey wire around resulted in a proper reading no matter which yellow it was connected to. As for temperature, the stone cold motor read just as badly as a warm motor. Thus I must assume to be true what you said about it testing OK yet failing under real world operating conditions. That must also apply to the stator I mentioned; it tested OK but failed in use. I also use liquid neoprene on all my electrical connections, and failures are very rare, just because I hate electrical problems so much!
jimh posted 06-01-2015 08:47 PM ET (US)     Profile for jimh  Send Email to jimh     
From your latest narrative I can infer the original rectifier assembly was defective in a manner that only manifested when in the actual circuit and was not detected by simple low-voltage, low-current resistance checks. Jettison the original rectifier assembly out the air lock and into deep space.
Oldslowandugly posted 06-01-2015 10:10 PM ET (US)     Profile for Oldslowandugly    
What? And create even more space junk like in Gravity. Anyway, thanks for the replies. I guess I just can't trust electrical stuff, and I will just replace when in doubt.
jimh posted 06-02-2015 09:10 AM ET (US)     Profile for jimh  Send Email to jimh     
I spoke metaphorically.

Re GRAVITY: the worst space movie ever made. It completely ignored the Laws of Astrophysics. It required preposterous suspensions of disbelief.

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