Checking out a tach

[Snoopy-2]

My tach is not working but when I turn on the key the tach goes to zero (from its normal 1000 shut off position) like it should. I was told to check my pulse wire to make sure I am getting signal from the engine. I have a 1998 johnson v4- 115 hp and was told that the grey wire on the back of the tach is the one to check for impulses. Wha is needed to check the pulse and what should the readings be on a volt meter?

 

[Snoopy-2]

I spoke with my neighbor who has more boating knowledge than me and he said if the voltage is low it could be a bad Volt-rectifier as the pulse for the tach comes from that. I took the cover off engine and looked at the Volt-rectifier. My unit has 5 wires-(2-yellow together,1 grey and 1-purple together and 1 heavy red). I looked up a new unit for this motor and it showed only 4 wires. Manufacture #586075. Why the difference in number of wires? The unit appears to have a rubber seal between the unit and block. Is the unit water cooled and does a gasket come with a new unit?

 

[Snoopy-2]

No Title

Installed unit has 5 wires the replacement has 4 WHY??

 

[Silvertip]

Why are you looking at rectifier/regulators when you haven't determined the existing one is bad. Look at the back of the tach and you will see a calibration dial. Using a small screwdriver, note the current setting and then rotate the dial back and forth several times (six or eight is fine) and then set it back to the original setting. Start the engine to see if the tach now works. Using a voltmeter with an AC (not DC) scale, check the voltage on the gray wire. it will be rather low. If there is any reading at all the tach is very likely seeing pulses. If there is no voltage, repeat the test at the engine. If no voltage on the gray wire tap into one of the yellow wires. If the tach still doesn't work it is likely toast but I suspect you will find that to not be the case.

 

[Snoopy-2]

My tach is brand new it replaced the original one that came with the boat. The old tach never worked and I replaced it with a new one. All wires were replaced on to the proper terminals and at least now the pointer moves. Pointer normally rests at about 1,000 rpm when you turn on the key it goes to zero but does not advance with the motor rpm's. I was told to remove the pulse wire and check the voltage with the motor running. What I wanted to know is what kind of a reading I should get if the signal from the rectifier is operating properly. I was told that if that unit is not operating properly it would effect the tach not working properly.

 

[UncleWillie]

Your engine most likely has a 12 pole alternator. It will produce 6 pulses per revolution.
At 1000 RPM you would expect to see 100 pulses per second on an Oscilloscope or a Frequency Counter.
A Volt meter should display ~ 5-10 volts AC. The Exact number is not important as long as it is NOT near Zero AC, or 14 volts DC.
The original Tachometer is probably good.