72 Johnson 100HP will not start

57 7.5

Petty Officer 3rd Class
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May 25, 2007
Messages
93
Ok guys this motor isn't going to win! I know someone on here can help me as I've had in the past. Ok I have a 1972 Johnson 100 hp Hydro-electric shift, model number 100ESL72. It has run on several occasions after purchasing. When I first got it the shift switch was broke (worked electrically but the little tab that rotated with the throttle lever was broke off) and couldn't get it to turn over. I did some troubleshooting and found the starter solenoid and shift blocking diode were bad. Replaced those and wired a toggle switch for the shift (per a post found on here). Got it running and took it to the lake and got out running was a little rough (carbs needed cleaning). Got that worked out and we took it out on the lake again and went about five miles from the ramp and fished for several hours went to start the motor and the starter wouldn't turn the motor over. Got towed back to the ramp. Took it home and put it on the charger and battery was dead (less than a year old). Charged the batter overnight, my step dad unhooked it at 4:30a.m. to take it back to where he worked. I got up at 6 a.m and head to the lake, get it in the water and it wouldn't turnover. Took the battery back to Wal Mart and they said it was dead and replaced it. Took it home and charged it and it started fine. Went out on the lake again 5 miles away and fished for several hours and tried to start it and nothing. Battery was dead again. Charged the battery over night again same thing step dad unhooked the charger at 4:30a.m. and I would get up at 6:00a.m. and go to the lake and nothing battery dead again. So hit the books and started troubleshooting and found I had no clipper circuit as described in book and the purple wire from the choke solenoid was connected to different spot on the terminal board, corrected that and found that the rectifier was bad, causing it to drain the battery and the white wire for the safety switch was not connected. So replaced the rectifier, shift switch, and got a clipper circuit. Everything seems to be electrically connected correct (except the shift switch was a three wire vice 5 wire so the two white wires are not connected(would like to know where should they be connected), everything test fine. Now it will not turn over at all. When testing the shift switch wasn't getting power to either the green or blue wire. Tested the purple wire (Shift blocking diode) at the switch no power, disconnected the purple wires and with switch on had battery voltage, checked he shift blocking diode and it was fine. Connected the purple wires back and changed the meter to a lower setting and got a little voltage. Checked the connections at the terminal board at #9 and had the same thing, little voltage. I also tested the shift switch while switch is in the start position and got voltage to the green wire, put switch in forward and got power to blue wire. I also got power at the white wire on the switch. So I don't have an ammeter for testing my meter is an old cheap analog meter with 10 and 250 ma settings. I tested the power pack per manual and if I did it right it maxes out my meter at 250ma. Anyone got any ideas? Trying to upload some pictures of the terminal board, rectifier, clipper circuit and col pack.
 

Bosunsmate

Admiral
Joined
Apr 7, 2012
Messages
6,129
Re: 72 Johnson 100HP will not start

What does the charging on that motor? The stator?
GO to cdielectronic.dom troubleshooter if you dont know. STart at where the battery charge is obtained from and work back to the battery
 

57 7.5

Petty Officer 3rd Class
Joined
May 25, 2007
Messages
93
Re: 72 Johnson 100HP will not start

Yes it is stator charged, I think it is bad though, but it cranked before, so not sure why it doesn't turn over and I don't hear any clicking in the starter solenoid.
 

57 7.5

Petty Officer 3rd Class
Joined
May 25, 2007
Messages
93
Re: 72 Johnson 100HP will not start

Should the two white wires that go to the correct shift switch be connected together or the one from the switch to the purple wire and the one to the safety switch to the green wire?
 

57 7.5

Petty Officer 3rd Class
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May 25, 2007
Messages
93
Re: 72 Johnson 100HP will not start

Ok I took the starter solenoid off and performed the bench test and the plunger clicked when connecting 12 volt supply across the little post and no ohms across the two big post. I also was able to jump start it straight from the battery and it ran fine. So I'm not getting power or enough power to the starter solenoid to engage the starter.
 

57 7.5

Petty Officer 3rd Class
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May 25, 2007
Messages
93
Re: 72 Johnson 100HP will not start

Ok some more troubleshooting done: checked 20 amp fuse and guess what I found, no blown fuse or a fuse at all, a ball of copper wire shoved in the fuse holder. So I went and got fuses and put a new one in. Still didn't turnover. I checked the safety switch per manaual, it had 12 volts not connected with ignition to start and again while attached. The white wire to the starter solenoid has 12 volts and so does the other white wire to the starter solenoid. When got this motor and started checking it out to get it running the white wire for the safety switch was connected to the motor (grounded). Hope all this info is helpful. Let me know if you need to know something else.
 

Bosunsmate

Admiral
Joined
Apr 7, 2012
Messages
6,129
Re: 72 Johnson 100HP will not start

Ok I took the starter solenoid off and performed the bench test and the plunger clicked when connecting 12 volt supply across the little post and no ohms across the two big post. I also was able to jump start it straight from the battery and it ran fine. So I'm not getting power or enough power to the starter solenoid to engage the starter.

No ohmns infinity? or low ohmns near zero?- you want it to be the second
 

57 7.5

Petty Officer 3rd Class
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Messages
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Re: 72 Johnson 100HP will not start

low ohms near zero.
 

57 7.5

Petty Officer 3rd Class
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May 25, 2007
Messages
93
Re: 72 Johnson 100HP will not start

I think I may be the safety switch but not sure, that is what the manual is pointing to. I adjusted it as close to the lever as I could get it and still nothing. I even removed it and pushed the plunger in with my finger and still nothing. I'm no sure if it is supposed to be all the way out or in?
 

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
36,047
Re: 72 Johnson 100HP will not start

If this is the safety switch mounted on the engine block, then what happens if an assistant removes the white wire and holds the wire to the block.-----------Will the motor crank now ?
 

57 7.5

Petty Officer 3rd Class
Joined
May 25, 2007
Messages
93
Re: 72 Johnson 100HP will not start

Yes it is mounted to the engine block. I'm not sure didn't try that, will try that tomorrow and let you know. Thanks everyone for the assistance. Hopefully we can figure it out.
 

57 7.5

Petty Officer 3rd Class
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Messages
93
Re: 72 Johnson 100HP will not start

Ok, couldn't wait till tomorrow after work, it starts but dies when you release the key, if you hold the key it keeps running or if you unground the white wire. So what's the prognosis?
 

ronward

Petty Officer 1st Class
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Jun 24, 2013
Messages
346
Re: 72 Johnson 100HP will not start

not real big on electrical but it sounds like a bad key switch. just a thought but, disconnect the main harness, manually choke it and jump the solenoid. I'm thinking if it starts and runs then you have a bad key switch.
 
Last edited:

57 7.5

Petty Officer 3rd Class
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May 25, 2007
Messages
93
Re: 72 Johnson 100HP will not start

Thanks, i'll give that a try tomorrow since it's gotten dark now.
 

racerone

Supreme Mariner
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Dec 28, 2013
Messages
36,047
Re: 72 Johnson 100HP will not start

-------Sorry, disconnecting the harness does not work on this engine !!!!!!!!----------That works on engines 1973 and newer with the controls that have a big red plug.--------Does not work on engines with the black plug.
 

57 7.5

Petty Officer 3rd Class
Joined
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Messages
93
Re: 72 Johnson 100HP will not start

Thanks. So do I replace the switch or keep checking other stuff? If the rain susides this afternoon i'm going to recheck the rectifier and shift blocking diode to make sure they haven't went bad during all this electrical testing, just to make sure.
 

57 7.5

Petty Officer 3rd Class
Joined
May 25, 2007
Messages
93
Re: 72 Johnson 100HP will not start

Everything still checks out with the rectifier and shift blocking diode. So is it the ignition switch or safety switch?
 

57 7.5

Petty Officer 3rd Class
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May 25, 2007
Messages
93
Re: 72 Johnson 100HP will not start

Can someone help, do I need new ignition switch or safety switch?
 

57 7.5

Petty Officer 3rd Class
Joined
May 25, 2007
Messages
93
Re: 72 Johnson 100HP will not start

Okay, got another switch and still have the same problem. It will not turn over unless I ground the white wire from safety switch. I'm also no sure about the safety switch, is the button on the switch supposed to be out or in when starting? I took the white wire off the safety switch and connected a volt meter to it and to ground, when you turn the ignition switch to start it get's 12.45 volts (which is what I get at the battery) with control lever in neutral, get the same thing with lever in forward and reverse. So I'm thinking the shift switch I have is bad. Was too dark to continue troubleshooting last night. Once it warms up a bit I'll go out and test the switch to ensure it's bad.
 

57 7.5

Petty Officer 3rd Class
Joined
May 25, 2007
Messages
93
Re: 72 Johnson 100HP will not start

ok the three wire shift switch doesn't seem to work like the 5 wire does. So I removed it and rigged my own toggle switch now it is in neutral as it should be. Now it still will not turn over unless the safety switch is grounded but it doesn't start just turns over. is the safety switch bad?
 
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