75 Merc No Spark

[mercurymang]

So I finally got everything ready today and set out to the lake. What a disaster. It was too windy and couldn't get the boat started. Wind was blowing me into the dock as I couldn't get the boat started. I thought it was flooded but when I got home I found that I had no spark. I'm suspecting my switchbox but i'm not sure. I was looking at a troubleshooting doc on the cdi website for the switchbox and it's talking about the rectifier. On a '75 650, I was not aware that the rectifier had anything to do with the ignition system. Can anyone speak to the or give me any pointers on troubleshooting a switchbox on a '75 650 merc ?

 

[Laddies]

Re: 75 Merc No Spark

A bad rectifier will cause a voltage drop and the system will break down, here's a test from CDI

 

[mercurymang]

Re: 75 Merc No Spark

Thank you Laddies. I will run these tests tommorrow afternoon. I know my rectifier is bad but it was running with it bad. I believe it had one leg shorted. Perhaps their both bad now and that's why it failed. Fortunately, i had already ordered a rectifier/regulator. Should be here monday.

Question, when they refer to the trigger, is that coming from the distributor?

 

[Laddies]

Re: 75 Merc No Spark

Yes

 

[mercurymang]

Re: 75 Merc No Spark

Laddies, thanks for your help. This afternoon I set out with your check list in hand and started going through the list. I had rigged up a starter switch and was checking the voltage while starting at the +12v source on the switchbox. I observed that when I touched the vm lead to the 12v post on the switchbox, the motor tried to start. I checked further and found that I indeed had 12v on the lead from the harness but when I tested the post, no voltage was present.
The last thing I did yesterday before I left was I had cleaned all the connections on the switchbox. I pulled the connector off and realized that the harness did not make connection to the post on the switchbox by way of the base at the switchbox (which is what I had focused on, I think it's actually plastic) but rather the brass nut. One side of all the nuts on my motor have black paint or something on them - not sure if that is from the factory or if someone tried to paint it down the line. Anyway, I think I had turned the nut around and the paint (or whatever it is) blocked the connection.
After cleaning all the connector nuts up and redoing all the connections, it started right up.
Lesson from this is (as usual) to always consider the last thing you did before it broke.:joyous:

 

[oldman570]

Re: 75 Merc No Spark

The black coating is a rubber based paint coating that is put on the conections to help keep the crossion down. it can and will cause your trouble as you have found out. If you want to try to keep the corssion down after cleaning the connection of any electrical connection, Get a tube of Dielectric Gresse and use it on the connection. It can be bought at most all auto supply stores and can be used on all and any electrical system where air and mosture will be present. JMO
Oldman570

 

[mercurymang]

Re: 75 Merc No Spark

Yep, I put dialectric on everything. I'm a believer in that stuff. Sure am glad that was the problem.

edit: So that black stuff is from the factory. Still learning stuff.