Evinrude rectifier testing..

[Guvner..]

Anyone have any experience on testing the 3 wire 18-5708 (replacement number from Sierra) rectifiers on 1978 115 HP Evinrude?<br /><br />From my manual it says ground to either of the yellow wires and then reverse the leads to show diode action. I get nothing on one I have and a short on the other. I guess I'm wondering if my meter has a high enough ohm setting to be accurate for this test. It is digital and goes up to 40M (not K.. that's on a lower scale..) As stated both the yellow and yellow gray on the one I just bought off an ebayer show a complete short on any range when going to ground while my existing one shows nothing in either direction. (normal way of checking a diode.. I've done this before on diodes..) Just don't know what range I should be on with a meter or a digital is even usable for these.<br /><br />Thanks... Gary

 

[The Marine Doctor]

Re: Evinrude rectifier testing..

You will look for continuity one way only. I use an analog meter for the tests.<br /><br />Red-->=Yellow yes<br />Yellow-->Red No<br /><br />If you get continuity both ways..you need to replace the rectifier.<br /><br />TMD

 

[rickdb1boat]

Re: Evinrude rectifier testing..

 

[Paul Moir]

Re: Evinrude rectifier testing..

Just to straiten a few things out. Neither of those tests work with a digital meter. The forward voltage drop of a diode is significantly higher than the resistance test voltage of a digital meter. It only works with analog meters, since they use a much higher voltage to test resistance on non-VTVM meters. Since the higer voltage leads to errors in testing, it is not used with Digital Meters or VTVM's.<br />In order to allow you to test diodes, digital meters all have the 'Diode Test' function. It may be the 'pointy thing pointing at a line' setting on your meter. Like this <br />->|-<br /><br />Use it instead of the resistance setting. It tests the voltage drop of the diode with an (appx) 2 volt, current limited test voltage. Good working diodes will usually test between .4v and .7v on this setting when forward biased (read 'in one direction') and overrange when reverse biased. (read 'in the other.')<br />Diodes fail in two ways - open or shorted. If it's open the meter will display overrange in both directions. If it's shorted, it will display something very low, like .003, in both directions. Failing shorted is far more common than failing open - ie, the diode melts down, turns into a conductor and so stops heating, since it now has negligable voltage drop. It sounds to me like this is what is wrong with yours.<br />Good luck!

 

[Joe Reeves]

Re: Evinrude rectifier testing..

Using the K.I.S.S. method with a $8.95 volt/ohm meter, the following would tell the tale.<br /><br />Remove the rectifier wires from the terminal block. Using a ohm meter, connect the black lead of the ohm meter to the rectifier base (ground), then one by one, connect the red lead of the ohm meter to the yellow, yellow/gray, then the red wire (some rectifiers may also have a yellow/blue wire. If so connect to that also). Now, reverse the ohm meter leads and check those same wires again. You should get a reading in one direction, and none at all in the other direction. <br /><br />Now, connect the black lead of the ohm meter to the red wire. One by one, connect the red lead of the ohm meter to the yellow, yellow/gray, and if present, the yellow/blue wire. Then reverse the leads, checking the wires again. Once more, you should get a reading in one direction and none in the other. <br /><br />Note that the reading obtained from the red rectifier wire will be lower then what is obtained from the other wires. <br /><br />Any deviation from the "Reading", "No Reading" as above indicates a faulty rectifier.

 

[jimchere]

Re: Evinrude rectifier testing..

Most digital meters have a specific diode testing mode to get you around the problem stated above with digital meters. Its the selector postition on the dial that looks like a sideways triangle with a vertical bar at the point, and gives you a tone to boot (connectivety=beep, no connectivety no beep) so you don't even have to watch the readout and can do it by ear while digging through the wires.<br />Then again, with patience the analog never...never fails.

 

[Guvner..]

Re: Evinrude rectifier testing..

Well... Both my original (officially Open diode) an the Ebay one (offically shorted diode) have failed the test. I thank the two of you who pointed out the diode testing setting on my digital meter that I've managed to plain miss forever. It had the exact same readings in both directions on that one too.. .001 on the one yellow lead and .004 on the yellow with the gray strip in both directions.<br /><br />No more playing with this... Going to find a new one in the morning and hope it brings back my tachometer... <br /><br />All the best... I'm sure this is the kind of string that makes reading these forums a valuable resource...<br /><br />Thanks... Gary

 

[Guvner..]

Re: Evinrude rectifier testing..

Just in case anyone has been following this... I got the new rectifier today. It tested out fine on the meter using the diode symbol that is one of the selections on my digital meter as stated by two posters before this. I knew that since it tested out with good results that it was probably going to be the reason for my tachometer failing to run. Once installed the tachometer indeed came back to life... <br /><br />Thanks again... Gary