1984 115 inline 6 - Tach/Rectifier Question

[Opus]

Hello all....

I have been able to get all gauges on my boat working due largely to advice from here...bless you all. Now I am going after the tach...I did quite a bit of searching and downloaded some wiring diagrams and view of the rear of the tach but not yet gone out to play with it.

From what I am reading, the suspected problems are usually the rectifier but if I understand correctly, a bod rectifier would also stop my starter battery from charging. It is charging fine.

I don;t want to arbitrarily replace the rectifier but do understand if I have to, I can pick up the equal at RadioShack...can anyone give me the specifics from RS I will need to go pick one up specific to this 115????

 

[Chris1956]

Re: 1984 115 inline 6 - Tach/Rectifier Question

The signal for the tachometer originates as one of the ac inputs to the rectifier. It is possible for one side of the rectifier to be shorted to ground. If your battery charges, at least one side of te rectifier is good. Do a continuity test to se that the tach is connected to the rectifier. If that is good, switch the tach input to the other yellow AC wire and see if the tach works. If so, you can get a 25A full wave (Bridge rectifier) at RS fir a few $. You will need to change to spade terminals to use it.

 

[Opus]

Re: 1984 115 inline 6 - Tach/Rectifier Question

Chris...can you do me a favor if you have a moment? I am not understanding how an outboard can produce alternating current. I know enough about 110/120 to change a light switch and wall outlet in the house and thought I understood...but just not getting this AC coming off of an outboard thing.

I appreciate your help..only answer my thirst for knowledge if you have the time!!!!

 

[75TowerOfPower]

Re: 1984 115 inline 6 - Tach/Rectifier Question

I have a non working Tach, also and need to see if I have continuity between power and the tach and the rectifier and the tach. If I remember how the tach looked I have more than 2 wires connected to it. And whats the way to check the rectifier?

And if I understand how the rectifier/tach combo works the tach reads the pulses through the rectifier from the stator, right?

 

[Chris1956]

Re: 1984 115 inline 6 - Tach/Rectifier Question

opus, The flywheel magnets swing past a ring of coils, that are inside the stator. As they pass each coil a voltage is induced in that coil. This "spike" of voltage is more like a pulsating DC signal than an AC voltage, but is unsuitable for charging a battery. The rectifier converts this signal to pure DC, and the battery acts as a load to drop the voltage to just above 13VDC.

75TOP - The tach will ususally have a three wire hook up. You need +12VDC, ground and the AC signal from the rectifier or stator. The tach will indeed count the AC pulses, and convert to RPM.