87 merc 115 tachometer troubleshooting

[bigdavech]

Thanks to the help on this board I now have my outboard running! The new problem is I now have an inoperative tach; the needle just sits on zero. It was working fine just the other day. The first thing I need to know is where does the signal originate from the motor? Even with the service manual I can't seem to find the source. I'd at least like to be sure there is a signal going through the wiring harness before I try to access the impossible to get at rat's nest where the tach itself resides.

 

[jorenac1]

Re: 87 merc 115 tachometer troubleshooting

I have the same problem, with my 90HP 78 6 cyl in line,except I found out the signal originates at the rectifier; specifically to one of the two yellow or yellow red wires coming out from the alternator stator. I have installed one of those aftermarket tachs with adjustable pointer. I will be working on it tomorrow. www.tflx.com offers some info. Good luck<br />Jorge

 

[Trent]

Re: 87 merc 115 tachometer troubleshooting

First off you need to check and see if your battery is being charged. With a voltmeter check the voltage at the battery. Then crank the engine and at fast idle re-check the voltage. Does it go up? It should. If it does not, most likely your problem is a bad rectifier. It is connected electrically to the tach. Take a look at this pic and see the gray wire in the middle of the pic. This wire goes to the tach and gets its signal from the rectifier. This is a common problem and the tach is usually the first thing people see.<br /><br />I believe this is your wiring diagram....<br /><br /> 115 Wiring Diagram