78 Mercury 70 hp O/B; Can I test starter/ignition with no rectifier?

[agrazela]

All,<br /><br />I have a 1978 Mercury 70 hp ("Model 700") O/B. The tach stopped reading intermittently, so I checked the rectifier according to SELOC manual; it was bad on the post leading to the tach (though the other post checked out OK).<br /><br />I am sending for a new rectifier, but it will take some time to arrive.<br /><br />I have another problem, presumably with my ignition switch, lockout switch, starter or starter solenoid (engine intermittently won't crank when I turn the key).<br /><br />Questions:<br />1) Is there any chance the bad rectifier was causing the intermittent no-start?<br /><br />2) Is it safe to test the other systems (i.e., crank the motor) with the rectifier completely removed?<br /><br />3) Can a bad rectifier "destroy" a tachometer? (the tach now stays at zero at all times...the needle used to "drift" up and rest at about 1000rpm when powered down...is it broken?)

 

[andrewkafp]

Re: 78 Mercury 70 hp O/B; Can I test starter/ignition with no rectifier?

I have had a similar situation on my '80 50hp with the intermittent ign switch.<br />Make sure your battery connections and all wires going to the starter are VERY clean, tight and not corroded. You may wanns check inside that round plug on the starboard side too.The motor needs a good current to crank.My rectifier is new and charges perfectly, so the crank problem is likely unrelated.

 

[Ken G]

Re: 78 Mercury 70 hp O/B; Can I test starter/ignition with no rectifier?

1) A bad rectifier will not cause an intermittent no-crank problem. The rectifier is not part of the ignition or starting systems, it's only function is in the charging system. It also relays an electrical signal to the tachometer from the stator. I recently replaced a bad rectifier in my 1977 Merc 500. I have no idea how long it was bad. The motor ran fine but I determined the battery was not being charged. There was no tach on that boat at the time so I don't know how that would have been affected.<br /><br />2) It is safe to test and even run the motor with a bad rectifier or with the rectifier removed. If it is removed isolate or insulate the leads from the stator to prevent any damage to it. Another motor I own is a 1970 Merc 1350. I recently got this motor running after it has been sitting for about 9 years. During the tank testing I have no wires connecting the stator to the rectifier. Other than not charging the battery the motor is running fine.<br /><br />3) I don't know if a bad rectifier can ruin a tachometer, I'd guess that it wouldn't. I guess you'll find out when you install the new rectifier. It is normal for a tach to rest above zero when at rest. They will drop to zero when the ignition switch is energized. <br /><br />The tach problem could be related to yourintermittent no-crank problem. Inside the control box there is a white wire that is connected to the ignition switch, choke button, and tachometer plug. A bad connection in this circuit could be affecting your cranking circuit and your tachometer.

 

[agrazela]

Re: 78 Mercury 70 hp O/B; Can I test starter/ignition with no rectifier?

KenG,<br /><br />Thanks for the info re: rectifier.<br /><br />I do not know what caused the rectifier to go...I assume some previous owner probably reversed polarity on his battery since it was baked and cracked when I bought the boat 6 months ago, but tested OK. Guess it was just on its last legs.<br /><br />I'll have to wait and see if the tach comes "back to life" with the new rectifier.<br /><br />I will go into the control box first to look for my no-crank problem. I have noticed that the engine will crank if I nudge the shifter just forward or backward from straight up-and-down...perhaps there is some "crud" buildup inside the neutral position/lockout notch or stuck to to the little wheel on the lockout switch lever.