90 HP Johnson intermittent tachometer readings

[bmwalk]

I have a 1986 90 HP Johnson on a 1997 boat with a Faria Tachometer (includes system check). The tach will periodically fall to 0 or read a slightly lower value with the needle trying to jump up to the correct value. When the needle drops to 0, it is not on the peg but instead slightly above the peg. This is the position it starts at after turning on the key. Readings get a little more reliable after running awhile (15 min plus).<br /><br />I have tried a second tach - same behavior. I am assuming the problem is with the source of the signal on the engine. Can anyone suggest where to start checking on the engine, or any speculations on the root cause? <br /><br />The engine runs smoothly at all speeds all the time. The only problem is the tach reading. Thanks!

 

[superman]

Re: 90 HP Johnson intermittent tachometer readings

Hi Paddleman: Check your wiring back to the motor including the stator wire connections to the rectifier. I think on that one that is fed from the pulses from the stator. Also it might connect thru a plug on the back of the control. Good Luck

 

[bmwalk]

Re: 90 HP Johnson intermittent tachometer readings

Thanks superman, I'll start going through the wiring. <br /><br />Your reply would lead me to assume that there isn't any piece of electronics that conditions the signal to the tach. To say it another way, if the motor is running fine, then chances are I have a wiring issue, and not a broken electronic part (since there isn't one?). Is that an accurate conclusion?<br />Thanks again for the suggestions!

 

[G-Daddy]

Re: 90 HP Johnson intermittent tachometer readings

Your problem is most probably with the rectifier itself. It is probably fried and needs to be replaced.

 

[workinoffsho]

Re: 90 HP Johnson intermittent tachometer readings

I had the same problem with my 81 90hp. I ended up changing the rectifier and that solved the problem. It wans't all that expensive and you nly have two wires to deal with so changeout is easy and a 5-minute job. Hope this helps.

 

[bmwalk]

Re: 90 HP Johnson intermittent tachometer readings

Thank you. I'll check through the wires this weekend and also test the rectifier with an ohm meter. I would have expected a battery charging problem if the rectifier was shot however if it's intermittent, perhaps not. The tach does work more often than not once it warms up.<br /><br />For other's reference, I've found both the teleflex website (www.tflx.com) and the Faria website (www.faria-instruments.com) to have very good documents on troubleshooting tach and guage problems.

 

[bmwalk]

Re: 90 HP Johnson intermittent tachometer readings

While I didn't get a chance to do a thorough test of all connections and the rectifier, I was able to determine that I have an integrated rectifier/regulator that bolts onto the top of the engine (6 bolts) and is partially under the flywheel. My first call to the local OMC dealer yielded a $230 price so I'm going to take the time to be as sure as possible that this is my problem before buying one. If I have to replace it, the flywheel will have to come off. Anyone have suggestions where to get a flywheel puller that will work with this engine? (90 HP Johnson, model J90TLCDC, s/n J7137506).<br />Thank you!

 

[Joe Reeves]

Re: 90 HP Johnson intermittent tachometer readings

Paddleman..... Sorry to say, but your explanation of the tachometer problem is related to a faulty water cooled regulator/rectifier assy, such as the expensive item you mentioned. Check around for a better price... third party suppliers such as Sierra etc.<br /><br />A flywheel puller should be available at most auto parts stores or tool depots. Just be sure to purchase one that has slots for three bolts to engage the three threaded holes surrounding the flywheel nut.<br /><br />The flywheel bolt torque is 105 foot pounds. Be sure to torque it!