tach info

[rbigg]

I run a Johnson 115hp '77 yr. <br /><br />How does the tach on this engine get it's signal? One way would be for it to have a magnet on the flywheel for the pulses. There are other ways, too. <br /><br />Just curious how it's set up on this engine.

 

[Joe Reeves]

Re: tach info

There's a gray wire leading out of the instrument cable (engine to contol area). That gray wire is connected to the yellow/gray wire of the rectifier at the engine.<br /><br />That gray wire st the control area would connect to the sensor terminal of the tach (usually another gray wire).<br /><br />The above is assuming that you have an OMC tachometer.

 

[rbigg]

Re: tach info

Thanks for that info. And yes, it is an OMC tach. However, it quit working and I need to replace it. However, I'm on a very tight budget. I've found a 4/6/8 cyl automotive tach for less than 1/3 the price of the OMC that I think might work. The main thing is that both the OMC setup and the automotive setup seem to take a low voltage pulse as the trigger. Seems like it would work OK. I would need to read the 8 cyl scale to comp for the 2 cycle 4 cyl Johnson.

 

[Joe Reeves]

Re: tach info

The fact that the tach quit operating doesn't mean that the tach is at fault. You could have blown the rectifier on the engine. You can test the rectifier as folows:<br /><br />(Small Rectifier Test)<br />(J. Reeves)<br /> <br />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 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. <br /><br />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. <br /><br />Note that the reading obtained from the red rectifier wire will be lower then what is obtained from the other wires. <br /><br />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.

 

[rbigg]

Re: tach info

OK. You're right about the possibility of a bad rectifier. Know anywhere online where I can find a schematic/wiring diagram of my engine so I know approx where to look for the wires?

 

[Hooty]

Re: tach info

A quick test for the rectifier, (may not be 100% accurate) is check the battery voltage without the engine running and then check it while the engine is running. If the voltage doesn't check higher with the engine running, the rectifier is probably bad. I think NAPA can get ya one.<br /><br />c/6<br />Hooty