1981 35hp Evinrude blowing rectifiers

[JMT11]

The engine was not charging so I tested the rectifier with an ohm meter and it showed it was bad. I put in a new rectifiers and there was no still no charging to the battery. Retested the rectifier and it showed bad. Bought a new one and was told that a short in the tach can cause is to damage the rectifier so I installed the new one but did not connect the tach wire. Same result, another rectifier wasted. Stator reads .3 ohms on one lead and .4 on the other. Factory manual calls for .27 +/-.05. Is the .4 out far enough to cause this problem? Can anyone give me some advice on this?

 

[JMT11]

Correction, the second rectifier is ok, the batteries were bad in the ohm meter. This first replacement rectifier may have been bad before putting it in. Bought as new on eBay but I could see lock washer marks indicating that it had been install Before. I have been informed that I will not see much voltage change if any at idle or low rpm. It needs to be in the 3500 rpm range to see it on a meter. He said this system is only designed to maintain the start battery at a low amperage and recommended a solar panel for the second gps/fish finder battery. I will run it this week end and see if in maintains the start battery. Any input would be appreciated whether confirming or contradicting what I was just told.

 

[chris.olson]

Your stator is just a simple single-phase AC generator. The resistance test on the stator is lead to lead, not lead to ground. The rectifier is a simple two-diode bridge that turns the AC power from the generator into DC to charge the battery. Since the AC power is RMS (Root Mean Square) the rectified DC voltage will be 1.414 times that of the AC RMS voltage.

Your person who told you that it has to be at 3,500 rpm or more to see it charging is correct. It is not a high output alternator. You can check the AC voltage from the stator by testing lead to lead with an AC volt meter. At 13VDC output you should see about 10.5VAC at the stator (there is 0.7v forward drop in the diodes in the rectifier, on each diode).