Tachometer question 79 Johnson 115

papachaz

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the tach isn't working on the boat I just traded for. I followed Joe Reeves suggestions for the quick diagnostics and learned two things, which I'd like some opinions on please. I'm still awaiting the arrival of my service manual, maybe once I get it I can quit buggin y'all, LOL.

First thing: wiring diagram decal on the breather silencer shows top to bottom for the wires to be connected in this order - yellow, 2nd yellow with gray, 3rd gray, 4th red, 5th purple. The yellow and yellow with gray are reversed. I swapped them but it made no difference for the tach to work. Also doesn't seem to make a difference in the way the motor starts and runs. Should I leave them this way, attached according to the diagram, or just hope that whatever mechanic that worked on it last had a good reason to reverse them?

2nd thing: I left the yellow and yellow with gray wires attached according to the decal above. I took the gray wire loose, attached it to the yellow with gray (which I thought was the right one according to Joes post) and it did nothing. I then attached it to the yellow and the tach started working. Is it ok to leave this wired this way until I can get a new rectifier? I won't be long to get it, I just have to decide if I'm going to make 40 mile trip to the nearest marine parts dealer, or order them from here. I do need a few other things, so I might just make the trip over there and maybe get to pick the brain of one of the service techs a little. They don't seem to mind answering a FEW questions from time to time.

I've managed to get the motor to start and stay running, and now that the tach is working, I have it idling about 1200 on the muffs. Was going to take it to the lake and put it in the water for some adjustments, but it's supposed to storm this afternoon, and as impatient as I can be sometimes, I know I'll be better off to wait for the manual to arrive.
 

airman1094

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Re: Tachometer question 79 Johnson 115

Do you have a multimeter? Check your voltage at your battery...if it is registering under 13 volts while running your rectifier is most likely blown...the tach reads the rectifier pulses to establish rpms...other than that check connections and the like. If there is voltage above 13 at battery while engine is running and connections are good disconnect the tach and check the tach wire for voltage, if no voltage check the wire, if voltage consider replacing the tach....
 

papachaz

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Re: Tachometer question 79 Johnson 115

thanks airman, yes I have a multimeter, will do just that. I don't think the tach is bad though, because it's working while it's connected with the yellow wire.

also, the voltmeter shows 12 volts when it's running, should that be higher? is that a sufficient indicator or do I need to get out the multimeter?
 

airman1094

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Re: Tachometer question 79 Johnson 115

I would say the voltmeter would be sufficient...I'm quite sure something in the charging system is amiss...should be reading more when engine is running....I'm going to attach an excerpt from a maintenance manual to check your stator and rectifier. I have a 1975 115 Johnson and it has the 6 amp rectifier...yours should be the same...
 

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papachaz

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Re: Tachometer question 79 Johnson 115

awesome! thank you, I will get this checked tomorrow. It's started raining here and supposed to be some severe storms this evening. I am going to check this tomorrow, and then go pick up the rebuild kits for the carbs and water pump tomorrow, if I need a new rectifier after this test, then I'll get one of those too. Very very much appreciated airman!

hopefully my service manual will be here saturday. I plan on taking the carbs down and soaking them tomorrow night and hopefully getting them back together saturday.
 

airman1094

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Re: Tachometer question 79 Johnson 115

If you need anymore help let me know! Feel free to PM if needed! Good luck with the work!
 

Joe Reeves

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Re: Tachometer question 79 Johnson 115

(Small Rectifier Test)
(J. Reeves)

Remove the rectifier wires from the terminal block. Using a ohm meter, connect the black lead of the ohm meter to the rectifier base (ground), then one by one, connect the red lead of the ohm meter to the yellow, yellow/gray, then the red wire (some rectifiers may also have a fourth yellow/blue wire. If so connect to that also). Now, reverse the ohm meter leads and check those same wires again. You should get a reading in one direction, and none at all in the other direction.

Now, connect the black lead of the ohm meter to the red wire. One by one, connect the red lead of the ohm meter to the yellow, yellow/gray, and if present, the yellow/blue wire. Then reverse the leads, checking the wires again. Once more, you should get a reading in one direction and none in the other.

Note that the reading obtained from the red rectifier wire will be lower then what is obtained from the other wires.

Any deviation from the "Reading", "No Reading" as above indicates a faulty rectifier. Note that a rectifier will not tolerate reverse polarity. Simply touching the battery with the cables in the reverse order or hooking up a battery charger backwards will blow the diodes in the rectifier assy immediately.
 

papachaz

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Re: Tachometer question 79 Johnson 115

thank you Joe! I've read your stickies, very helpful and informative. What's your opinion on the yellow/yellow gray striped wire being swapped in my OP? should I put them back as they were, or since I changed them to match the decal/diagram, leave them that way? All help is greatly appreciated!
 

Joe Reeves

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Re: Tachometer question 79 Johnson 115

thank you Joe!What's your opinion on the yellow/yellow gray striped wire being swapped in my OP? should I put them back as they were, or since I changed them to match the decal/diagram, leave them that way? All help is greatly appreciated!

I just checked my manual rather than to rely on memory. Leave them as you changed them to match the diagram. Make sure that metal jumper is in the right place!

1 - O <- Yellow

2 - O <- Yellow/Gray
.....| <-------------- Jumper between 2 & 3
3 - O <- Gray

4 - O <- Red

5 - O <- Purple
 

F_R

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Re: Tachometer question 79 Johnson 115

The yellow and yellow/grey wires are AC and interchangeable. The grey tachometer one can be connected to either one. Yours is probably working with the wires interchanged because the rectifier has one bad diode in it (out of four, total). But that one bad diode will keep it from charging the battery---the reason you only have 12V while running.
 

papachaz

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Re: Tachometer question 79 Johnson 115

ok thanks guys! I have the rectifier off, after testing it the yellow w/gray had continuity in both directions, the other wires checked out fine. Since I'm going after carb and water pump kits, I'll go ahead and pick one up while i'm there. Thanks again!
 

Joe Reeves

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Re: Tachometer question 79 Johnson 115

At that small vertical terminal strip.... make sure that, before you start installing that new rectifier, that the polarity at that red wire leading out of the engine's wiring harness which connects to that terminal strip is Battery Positice + voltage.

I've seen a few batteries hooked up backwards, and also seen batteries hooked up normally BUT were charged backwards (it happens).

Bottom line.... reverse polarity will blow that new rectifier instantly!

Also, when all is as it should be, I'd recommend disconnecting the cables/wires from one of the battery terminals to avoid accidental shorts (that happens too).
 
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