84 Johnson 115 tach problem

[aktrucker]

I have an 84 Johnson 115 that I bought used last year and wasn't able to use until this year. I have been working thru some issues....one that has me a little baffles is the tach (tach came with teh motor when I bought it). It works very strangely...once the tach gets to around 5500rpm, the needle goes backward until the reading is around 4500rpm. I can hear by the engine tone that it is increasing in rpm. I am trying to figure out if the tach itself is bad, or if there is something with the wiring that may be screwed up. Has anybody ever heard anything like this? One of the other issues I am having is a mid-range misfire...I replaced 2 coils with used ones, it got better but still didn't go away. Makes it kinda difficult to figure out what rpm range I'm getting the misfire with my tach all screwed up.

one other thing I noticed is when I use my motor trim switch it makes my tach decrease or increase by 200-300 rpm

 

[emdsapmgr]

Re: 84 Johnson 115 tach problem

Unlikely it is the tach. Erratic tach operation is usually a result of a bad rectifier. In addition, when the rectifier goes, usually it also quits recharging the battery. The tach issue should not be related to the miss, as the charging system is separate from the ignition system. When under load and the engine starts to miss-put an inductive timing light on each of the four plug wires and look at the flashes for missfires and no fires.

 

[countvlad]

Re: 84 Johnson 115 tach problem

if you have a 3 wire rectifier:
http://forums.iboats.com/showthread.php?t=190638


if you have a water cooled rectifier:

http://forums.iboats.com/showthread.php?t=190636

 

[aktrucker]

Re: 84 Johnson 115 tach problem

If my rectifier/regulator (I have the 35amp system) is bad, wouldn't it be fair to say that I could be causing damage or problems to another electrical component thus causing something in the ignition system to go bad or not operate properly? For instance if an alternator goes out on a car it will weaken the spark as the battery is drawn down.....so in theory if my charging system is not working properly it could be weakening my spark. Not that the bad rectifier is the direct cause of the misfire, but it could be a root cause of other electrical issues leading up to the misfire. I am getting a new rectifier/regulator, we'll see where it goes from there.

Thanks for the help, I would have never guessed the rectifier could cause the tach to work improperly.......

 

[countvlad]

Re: 84 Johnson 115 tach problem

If my rectifier/regulator (I have the 35amp system) is bad, wouldn't it be fair to say that I could be causing damage or problems to another electrical component thus causing something in the ignition system to go bad or not operate properly? For instance if an alternator goes out on a car it will weaken the spark as the battery is drawn down.....so in theory if my charging system is not working properly it could be weakening my spark. Not that the bad rectifier is the direct cause of the misfire, but it could be a root cause of other electrical issues leading up to the misfire. I am getting a new rectifier/regulator, we'll see where it goes from there.

Thanks for the help, I would have never guessed the rectifier could cause the tach to work improperly.......

i will not cause any problem other than the tach wont work and the battery wont charge... the boat ignition is on a diffrent circuit.... but if you have a water cooler rectifier i heard that they could catch on fire if their faulty... so i would change it as soon as you can...

 

[aktrucker]

Re: 84 Johnson 115 tach problem

Order has been placed.....I love electrical parts

OK, and I'm not arguing just trying to understand, if the ignition system is different than the charging system, does the ignition system run off of A/C, while the charging system charges D/C....so realistically the only reason the rectifier is put on it is so it will charge on a D/C current?

 

[aktrucker]

Re: 84 Johnson 115 tach problem

OK, finally got the new rectifier regulator in, and have it installed. Tach is finally working correctly, the battery is charging nicely, AND the miss fire I had in the ignition system is all but gone!! While the rectifier may not be directly linked to the ignition system I strongly believe it can (and did in my case) effect it under certain conditions......... My fuel consumption has even gone down drastically, so I can only believe the new rectifier regulator is what made the difference, I didn't change anything else....... Thanks for all teh help.