1978 115 johnson charging and tach problems

[Bill Alexander]

First thing, I'm new to the group, and want to say thanks for this site and Hello to everyone. Now as for the problem I'm having is my battery doesn't want to stay charged when i run my boat. I'm not sure how to check it,and also My tach starts to drop once I start to run wot after just a little while. Can someone please help me with this? I can do mechanic work on just about anything, but this is my first boat. It's all Greek to me as of now.

 

[durban]

Re: 1978 115 johnson charging and tach problems

with a tester check to see how many amps your rectifier is delivering & what it should be then check what amps are required for your battery to charge it from your supplier they should be more or less on par because to many amps going into a battery eventually kills it .

 

[metriccrescentwrench]

Re: 1978 115 johnson charging and tach problems

Bill, more than likely the rectifier is bad. From the parts diagram on shop2.evinrude.com it looks like your rectifier is the single phase one. It will have a red wire and two yellow or yellow/grey wires. You can test it using a multimeter with the diode test function.

Disconnect all the rectifier wires. In the diode test position on the multimeter take a measurement between one of the yellow/gray wires and the red wire. Then reverse the test leads and take another measurement. In one direction the reading should be real high, and in the other direction it should be real low. Now do the same with the other yellow/gray wire and red, again in one direction the reading should be high and in the other it should be low. Now do the same between each yellow/gray wire and ground, same deal - reading should be high in one direction and low in the other. If any single test doesn't check out the rectifier needs to be replaced.

edit to add: Your tach gets it's signal from the rectifier. It's pretty much a dead giveaway that the rectifier is bad when both the tach and charging system are acting up.