How does the rpm gauge work on my 115 73 Johnson?

[teslaman]

After installing my fishfinder, I noticed that the RPM gauge didn't works. I have a feeling that I disconnected something when I was running cable.

How does the rpm gauge work exactly? What tests can I do to determine if the unit is damaged or if its simply a connection issue? Is there a sensor that could be bad?

 

[R.Johnson]

Re: How does the rpm gauge work on my 115 73 Johnson?

Check the gray tach wire going to the control box, along with it's ground wire. The tack sense's off the charging system.

 

[Silvertip]

Re: How does the rpm gauge work on my 115 73 Johnson?

There is no tach sensor -- but there is a tach "sense" line. Chances are you inadvertently disconnected it when you installed the locator. There are four wires on the tach. One feeds the internal light. the second wire feeds 12 volts to the tach when the key is on. The third wire is a ground wire. The fourth wire is the "sense" or tach feed from the engine. It originates at the voltage regulator/rectifier. Pulses on this line are counted by the electronics in the tach and converted to an RPM indication. Check just below the key switch on the throttle/shift control box. There is a connector there. The tach may be connected there and you just popped the connector loose. If there is nothing connected there, the tach is connected under the dash so you need to get down and under to check those connections.

 

[R.Johnson]

Re: How does the rpm gauge work on my 115 73 Johnson?

There is no quick tach connector on the 1973 control box. A guick check would be to check the voltage at the tach sensor lead, this should be gray wire, but the back of the tach will have the lead marked sensor. There should be about ten volts' at this lead. The meter should be set on DC volts, and connected between the sensor lead, and ground.

 

[teslaman]

Re: How does the rpm gauge work on my 115 73 Johnson?

Saturday I stuck my head under the console and traced the gray wire that you guys are talking about. The wire and its connections appeared to be ok at both the meter and the tach located in the console.

The other wires are hard to trace. Apparantly, the previous owner had a 10 year old do the wiring. At some point, I need to remove all the old wiring and start fresh. I'll get the vot meter out and check for the 10-12 volts.