1973 50 hp johnson

[Birdseye12]

Alright guys I have a 50 hp Johnson that I can't get any spark on. The cylinder compression tests both came out at 145. I have rebuilt the carbs and replaced the power pack and I still can't get a spark on either cylinder. Does this sound like the next step would be the stator? Or is there something I am overlooking?

 

[wrench 3]

YES. You're over looking something. Diagnosing the system to find out what's wrong.
Go to http://boatinfo.no/lib/evinrude/manuals/1971-1989johnsonevinrude.html#/204. You'll find trouble shooting on page 205.

 

[boobie]

You can also go to cdielectronics.com for more info.

 

[F_R]

Easy first step: Disconnect the kill wire (black w/yellow stripe) from the power pack and recheck for spark. If you have spark, trouble is in kill circuit.

 

[Birdseye12]

Wow thank you wrench! I have been trying to get my hands on that through the local library for months, but the person who has it won't return it. I have disconnected the kill wire and rechecked to no avail. I've gone through the ohm testing on the cdi troubleshooting page and stuff checks out ok. and also I have replaced the power pack with a new one from CDI so hoping that they sent a good pack. I'm getting parts together to make a DVA adapter, I didn't have one of them. This motor has sat for about 20 years. Ignition parts from a visible inspection look to be no major cracks or leaking. My next thought is stator due to not getting spark on either coil, but ill finish the DVA adapter and get the peak voltage readings see what that tells me. This is an electric start engine and the starter engages and spins the flywheel great would that rule out any possible issue with the rectifier?

 

[F_R]

The rectifier serves one purpose and one purpose only--to change the AC output from the alternator to DC for charging the battery. It has absolutely nothing to do with the starter spinning the flywheel. Well of course except if it were a motor that had been running and the battery went dead from not being recharged. Obviously not the case here.

 

[wrench 3]

A shorted rectifier that's letting AC through will make short work of a power pack. It's definitely worth the time to test it.

 

[F_R]

A shorted rectifier that's letting AC through will make short work of a power pack. It's definitely worth the time to test it.

Never too old to learn, I'd like to hear a technical explanation of how that can happen.

 

[wrench 3]

Never too old to learn, I'd like to hear a technical explanation of how that can happen.

Electronic components like a nice steady DC currant within a fairly narrow voltage range. A malfunctioning rectifier can produce voltage spikes high enough to fry the electronics. It can also produce reverse voltage spikes that will have the some negative affects as hooking up a battery backwards. This damage can come from the ground side, even if they don't share the same positive system.

 

[Birdseye12]

Thank you very much for the responses. I was in the process of popping the flywheel off when I wrote the first post. Left the puller sit overnight so I went ahead and took it off to get a better visual of stator. Looked ok but did have a little leakage from one charge coil. Ran ohms test on it. Yellow/yellow gray was 1.2 ohms, brown to ground was 836 ohms. Got to looking a little closer at that rectifier and disconnected it from the terminal box. The red wire looked terrible, it about crumbled in my hands in two spots, so goes without saying I will be ordering a new rectifier. Should the stator also be replaced considering the leakage coming from charge coil or is that still salvageable?

 

[F_R]

That stator will give you nothing but grief. Replace it.

 

[Keyboardman]

Agreed, replace that stator. Also, I found the original factory service manual for my '74 50 Johnson at DIYrepairmanuals for $35. Well worth the money.

 

[wrench 3]

Just curios but did you check the resistance between the two black/white wires?
Also agree, if the coil is sweating it's probably been overheating. Not a good sign.

 

[oldboat1]

Electronic components like a nice steady DC currant within a fairly narrow voltage range. A malfunctioning rectifier can produce voltage spikes high enough to fry the electronics. It can also produce reverse voltage spikes that will have the some negative affects as hooking up a battery backwards. This damage can come from the ground side, even if they don't share the same positive system.

Wrench -- Interesting. I've always thought that rectifiers either function or completely fail (as when polarity is reversed and a diode burns out). I'm not sure how spiking occurs, or how a failed rectifier can result in stator current running through the negative side to the power pack. Any additional explanation?

 

[Birdseye12]

I checked the resistance on the black/white wires, 296 ohms..... ugh.

 

[Birdseye12]

That's connected to power pack.

 

[wrench 3]

Wrench -- Interesting. I've always thought that rectifiers either function or completely fail (as when polarity is reversed and a diode burns out). I'm not sure how spiking occurs, or how a failed rectifier can result in stator current running through the negative side to the power pack. Any additional explanation?

Reversed polarity will burn the diodes out but, under normal operation, it's not uncommon for them to short out and start transmitting currant in both directions.
I don't know for sure how it affects a semi isolated circuit but I have seen it happen. I'm assuming that the of AC voltage charges up the capacitors in an electronic unit and then, when the voltage reverses, it draws it back out the same way it went in. The excessive voltage would fry things along the way.

 

[oldboat1]

Thanks, Wrench. I know the '80s 35 amp stators could overheat and melt down coils that charge up the power pack, and water-cooled regulator/rectifiers could burn up in their own right. Likewise I remember seeing that unregulated rectifiers that fail could potentially cause stator overheating -- so makes sense that a power pack could be damaged at the end of a chain of events. Appreciate the info.