Tach is fixed, but why?

oldslowandugly

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Last year I launched a 1985 Whaler 15 foot Sport, with a 1993 Evinrude 48spl. No problems at all. This year it tested great in a barrel so I launch again. Works fine so I go fishing the next day and the tach is suddenly idling at 3000 rpm and going off the scale at WOT, very odd since the prop usually holds rpms at 5100. So I research and find it probably could be the rectifier, but also the stator, battery, connections, pulse switch, lots of things. I take my multimeter and begin testing and it all checks out OK. Charging is great, but tach is wacky still. So I had a spare brand new rectifier and I figure what the heck and swap it in. FIXED! Retest the old rectifier and it still checks out OK. The only thing that may be amiss is I read that the red wire should show a lower value than the yellow wires. This one had a higher value, but all the diodes were functioning properly, no shorts to ground, no opens. Could that be it, and why? I am so confused.
 

gm280

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oldslowandugly, you tested the diodes in a static mode with only the dc voltage from the meter itself and not operational. Electronic parts can test good lots of times. But when you apply voltage they break down under load. Not really unusual if you worked around circuits for a living... One or more of the packaged diode in that rectifier is breaking down under load. And if you really want to find which one, apply some AC to the input(s) and see what you read on the output... JMHO!
 

oldslowandugly

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Hey gm280 that makes a lot of sense. I was only going by what the factory OMC manual said to do for a test. But that would explain why it tested good, but acted bad. I'm no electrical wizard but I'm not afraid to dive in when needed. These old motors are pretty simple but this would not be the first time I was stumped by an electrical problem. Thank's!
 

GA_Boater

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You may have had a loose or corroded connection on the rectifier and when the new one was installed, magic it works now. I had the very same problem on my Merc. I think Merc and OMC electrons are the same. :rolleyes:

You could put the old one back to see if it works now. You might have a spare.
 

oldslowandugly

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No, I checked that first thing. Plus, I always use a liberal coating of liquid neoprene on all electrical connections. For the actual boat wiring I solder, seal, shrinkwrap, seal, electric tape, then seal once more with the neoprene. I have never had an electrical problem on the actual boat, but I can't do that on the motor because things need to be serviceable. So I at least coat all connections with the neoprene, as the service manual suggests.
 

Silvertip

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The calibration dial on the back can cause this issue as well, especially after a long layup period. If that's the problem, just rotate the dial back and forth a few times and reset it to the original setting. The internal contacts can get corroded a bit causing the erroneous reading. Rotating the dial burnishes the contacts to clean them up.
 

oldslowandugly

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Can a stator do this also? The reason I ask is that before the 48spl I was using a 1985 30hp tiller that had a brand new stator installed to charge the battery. It did that well but the tach was crazy too. As it revved it would zoom up to about 3000 rpm then drop down to nothing, then zoom again, always about 2000 rpm higher than actual. No one could explain why, my dealer, the Teleflex rep, my cousin the electrical engineer. Changing the rectifier did nothing. But when I swapped in an older 1980 35hp stator, problem solved, tach worked perfectly. I was afraid this would require the same fix, luckily it did not.
 
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JoLin

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The calibration dial on the back can cause this issue as well, especially after a long layup period. If that's the problem, just rotate the dial back and forth a few times and reset it to the original setting. The internal contacts can get corroded a bit causing the erroneous reading. Rotating the dial burnishes the contacts to clean them up.

That's done it for me- twice.Exercising the selector on the back 'fixed' two tachs that gave wacky readings.
 

oldslowandugly

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That's never worked for me. I mean that I do switch the dial around, with no improvement. I even keep a brand new tach handy just for testing the boat's tach. It always registers the same defect as the boat tach. It would seem that even if an electrical component passes a cursory test, it might not work in actual use. No wonder I hate electrical problems so much.
 

Fed

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Retest the old rectifier and it still checks out OK. The only thing that may be amiss is I read that the red wire should show a lower value than the yellow wires. This one had a higher value, but all the diodes were functioning properly, no shorts to ground, no opens. Could that be it, and why? I am so confused.
Your testing has me a little confused too, are you sure you're testing it right?
 

oldslowandugly

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Fed, here's what I did. I removed the rectifier from the motor. On the bench I used an ohm meter to test all the yellow wires one at a time against the case (ground). What you need to see is a value in one direction, then reverse the leads and retest, and get infinite(nothing) in the other direction. Then you test the red 12volt output wire against the yellow wires, first one direction, then reverse direction, again looking for a value one way and nothing the other way. The rectifier is actually a set of diodes that only pass current one way, converting AC to DC. You are testing for that one way current, any flow in the other direction, is a defective unit. Then I read that the values found from the red wire to yellows should be lower than those found from the yellow wires to the case. In fact since then, testing of my bad one and several new ones indicate the opposite, a higher value from the red wire, so maybe the poster had it backwards. Either way, I have 4 rectifiers on hand and only the really old one that I already knew was shot tested defective. All the rest, both new and suspect, tested the same- fine. However, on the running motor, the suspect rectifier with the grey tach wire hooked to the yellow/grey stripe wire gave the crazy readings I posted. When I switched the grey tach wire to the yellow/blue stripe wire, the tach still was wrong, but differently wrong. That was my tip off. Once the new rectifier was operating, switching the grey tach wire to any yellow wire at all, resulted in a correct tach reading. So I believe gm280 when he says the unit may bench test OK, but fail in the real world when operating.
 
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Fed

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OK it sounds like you have it down pat, what threw me was when you said "no shorts to ground & no opens" which is exactly what you're looking for when you do the tests. ie, circuit one way but not the other.

I'd expect all the diodes should test roughly the same, it would make no sense to use diodes of different values in a bridge.
 

oldslowandugly

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My mistake, I was testing the red wire for a short to ground. THAT would be bad. I've read stories that rectifiers shorting the 12 volt red wire to ground can burn up a motor and boat. That wire is hooked to battery positive unless you have a main battery switch to disconnect it when you are gone.
 

dingbat

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On the bench I used an ohm meter to test all the yellow wires one at a time against the case (ground). What you need to see is a value in one direction, then reverse the leads and retest, and get infinite(nothing) in the other direction. Then you test the red 12volt output wire against the yellow wires, first one direction, then reverse direction, again looking for a value one way and nothing the other way. The rectifier is actually a set of diodes that only pass current one way, converting AC to DC. You are testing for that one way current, any flow in the other direction, is a defective unit.
not exactly.

A DC bridge is two sets of opposing diodes configured in a triangle. The bridge will show low resistance in one direction and a high resistance in another depending on which side of the bridge your reading. If you get nothing in one of the "circuits", you have a bad diode and your "leaking" AC past the bridge which could cause a tachometer to read incorrectly

Test procedure

http://www.cdielectronics.com/wp-con...3/153-1778.pdf

diodes-and-zener-diodes-html-6e70b981.gif
 
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oldslowandugly

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I went back and re-tested all, got the same thing- infinite one way, resistance the other way. Even on a brand new one.
 

Fed

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Should be infinite one way and almost no resistance the other way.
(Just having some fun oldslowbloke, I know you know):)
Could have simply been that low value red wire reading going even lower when the pressure was on.
 

dingbat

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I went back and re-tested all, got the same thing- infinite one way, resistance the other way. Even on a brand new one.

"Infinity" is simply reading outside the scaling of the meter.

Start with 1K scaling then switch to 10K to take your infinity reading. Use the new one as reference. They should all read the same.
 
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