'78 merc 1400 inline blowing rectifiers

[monk-monk]

My 1978 mercury 1400 keeps blowing rectifiers...probably explains my tach problems...what should i check for the cause of blowing rectifiers..the internal wiring harness is brand new...should i be looking at the MerControl harness for problems or could one of the ignition components be causing it to blow rectifiers, distributor, coil, switchbox etc...could these components cause the rectifier to fail...Any advice is much appreciated !!

 

[Maxz695]

Re: '78 merc 1400 inline blowing rectifiers

is your battery secure? can it touch any metal and cause a short? Any contact of a positive to negative can blow out the rectifier including hooking up a charger in the wrong direction.

 

[monk-monk]

Re: '78 merc 1400 inline blowing rectifiers

Thanks for the reply Maxz695...yes the battery is secure and (being a 30 year vet with AC/DC electicity) im sure all polarity is correct...Im trying to split this into 2 catagories...I have even followed the mercury manual instructions for testing the controller harness...using a Fluke digital meter, i didn't find a problem there...BUT i still suspect the main harness or key switch...So i have constructed my own harness (factory key switch with new wires) so i can eliminate the boat harness...Im going to connect a battery to the starter solenoid and the ground to its normal spot on the lower cowling, connect my test harness to the switchbox and get my keyswitch power from the posi on the starter solenoid...I guess my biggest question at this point is (because the rectifiers are too expensive to use as test dummies LOL..) Can any of the other ignition components cause rectifier failure or does it originate from short circuits...Thanks!

 

[Maxz695]

Re: '78 merc 1400 inline blowing rectifiers

A scraping flywheel against the stator can blow out a rectifier but would also more than likely blow out the switchbox as well. If you have a regulator it may be over producing causing this problem

 

[monk-monk]

Re: '78 merc 1400 inline blowing rectifiers

Max, i changed the flywheel too ! I rotated the flywheels to see if there was any wobble or interference and all is good...I really think it is the keyswitch or main harness and i will isolate that...i guess when i install my test harness i will find out, if it blows another rectifier, then it's within the ignition components of the engine...guess i can shuck-out another $30 to see...

 

[Maxz695]

Re: '78 merc 1400 inline blowing rectifiers

good luck Offered what I could I,m sure others here have input but seemingly a lack of that lately

 

[Maxz695]

Re: '78 merc 1400 inline blowing rectifiers

One more thing comes to mind that might be worth looking into. I,m no professional marine mechanic by far but I,ve been a damn good backyard mechanic scince I was 16 i,m going on 49. They have alot of problems with the oil alarms on Mercury engins and sometimes the fault is caused by Moisture getting into the alarm modial through carcked poly insulation that corrdeds and makes contact between two points on the schematic board of the unit causing it to short out. This only happens when it get moisture it the unit therfore It works for quite some time then Bang Done. If you have a Stator adaptor on your stator it also has this poly fill . I know I have one from a 4 cyl 40 HP 4 stroker. It has a crack in the poly on the underside of it. If water is getting inside of the stator adapter it will create a bridge and cause a temporary short which in fact could cause the rectifier to fail. I hope this helpsView attachment 107533LOOK CLOSE YOU WILL SE THE CRACKED INSULATION

 

[schematic]

Re: '78 merc 1400 inline blowing rectifiers

Monk
Are you sure you are connecting it up correctly?

Is there any continuity from the stator leads to ground?

 

[Chris1956]

Re: '78 merc 1400 inline blowing rectifiers

Monk, The rectifiers will blow if the DC side is disconnected. So if you have an intermittent ground or times when the connection from rectifer to battery is open, the diodes in the rectifer will blow. I would suspect the quick disconnect plug.

You can get around it by buying one of those 25A Radio Shack rectifiers. It won't fix the issue, but they ususally do not blow when disconnected, and they are < $5.

 

[monk-monk]

Re: '78 merc 1400 inline blowing rectifiers

An that is what i did chris...i went an got some of the radio shack ones, installed one and it is charging the battery...in all the testing trying to get my tach working, i could have inadvertantly disconnected something, at a time, when i shouldn't have and that might have blown the new rectifier i put in....SO now my battery is charging, BUT my tach still draws to the negative side of zero RPM and flutters wildly...EVEN after making my own wiring harness that eliminated the main boat harness...NOTHING but a battery, a key switch with some wire leads on it and the engine....and the tach still doesn't work...craziest thing i've ever seen...!!