Charging and tach issue. Voltage rectifier upgrade to 35amp?

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Bman2895

Petty Officer 2nd Class
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Feb 11, 2016
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So my outboard is a 1978 Johnson 140. Came with the smaller standard rectifier. Well in my figuring out the tach issue I found my rectifier basically exploded. I swapped on one from my buddies 90 and it did fix the tach, but when running I didnt see much if any charging voltage?
And if I wanted could I toss on a 35amp rectifier?I will just buy the same part and replace if not, But I do have the a 35amp rectifier from a 1984 parts 140 in the garage. The kind that bolts to the top of the block. Can I just swap it on with my current stator? Is it possible it would work, but just not put out the full the 35amp output? I saw one other post of someone having done this and said it worked for them but I wanted to ask if anyone else had done it.
 
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Chris1956

Honorary Supreme Mariner
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Mar 25, 2004
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29,163
You can buy a new rectifier for a few dollars that has much more capacity then the OEM one. i.e. you can get a 25A rectifier that the engine cannot fry. Take a look on Amazon for such an item.
 
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Crosbyman

Admiral
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Nov 5, 2006
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these should work just hook up the 2 yellows (AC) from the stator ground the negative pin and the B+ to solenoid main post with the B+ from the battery ...and you will have spares ! $20/8
1789820363238.png

 
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Bman2895

Petty Officer 2nd Class
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198
This is the one I have off the parts engine.
 

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Crosbyman

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basic rectifier tan is for the tach most likely the prts look up does not show that rectifier . model you show . Maybe someone installed it later ???
1789847686259.png
in any event the square ones will rectify but won't provide a tach wire.

up to you to test it
 
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Bman2895

Petty Officer 2nd Class
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Ah well the tach is most important anyway. I would have figured the grey wire off the square one was tach? Seeing as it was grey. I might try it out. Looks like id have to add a key on power wire back to my little bus bar.

I have one of the little 3 wire ones on the way.
 
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Bman2895

Petty Officer 2nd Class
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Feb 11, 2016
Messages
198
Crosbyman said:
basic rectifier tan is for the tach most likely the prts look up does not show that rectifier . model you show . Maybe someone installed it later ???
View attachment 419834
in any event the square ones will rectify but won't provide a tach wire.

up to you to test it
Click to expand...
So it turned out the opposite. The larger rectifier provided a fine tach output, but I didn't get any charging voltage. Though I'm not ready to blame the rectifier for that yet as even borrowing my buddies know good 3 wire also didn't provide any charging voltage.

what should I see for charging voltage on these? At idle.

I did pop that fuse out and bypassed for a moment to see if it helped. It didn't. Gonna replace with a modern 2 blade fuse. What was the correct amp for the fuse? Couldn't see one on the little glass fuse.

Could it be a bad stator causing no charging?
 
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Crosbyman

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Nov 5, 2006
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DC volts/charge current come from rectified AC from stator so check if you have AC across yellows and stator coils are not grounded.
no AC=>NO DC !

measure battery voltage at idle then run up the RPMS and battery should show an increase possibly up to 14-14.5V


fuse size well chat gpt says...
For the 1978 Johnson 140 hp (models such as 140ML78, 140TL78, 140TXL78), the factory ignition-system fuse is:

20 amp fuse​

 
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boscoe99

Commander
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Aug 22, 2013
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2,388
It is the coil within the stator and the magnets that are creating the electrical power (amperes times voltage). Not the rectifier.

Rectifier is only converting AC voltage to DC voltage.
 
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saltchuckmatt

Captain
Joined
Jul 19, 2019
Messages
3,374
All answers above are correct, but in case you don't know, voltage increase is slow. Alligator clip your probes on, fast idle and go look at it in 5 minutes, should slowly increase if working correctly....otherwise pull the flywheel and see Dr Ohms
 
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