Gauges on a 77 115hp?

[emilime75]

Can a temp gauge be installed to monitor the temperature of a 77 Merc 115 hp outboard? Are there provisions for this from the factory, or would it need to be a custom fabrication?

Also, what is the best way to hook up a tach? And what type of tach works best with this engine?

 

[daveswaves]

Re: Gauges on a 77 115hp?

Can a temp gauge be installed to monitor the temperature of a 77 Merc 115 hp outboard? Are there provisions for this from the factory, or would it need to be a custom fabrication?

Also, what is the best way to hook up a tach? And what type of tach works best with this engine?

Just about any tach will do, just connect the tach to one side of the rectifier. The connection shown in the wiring diagrams with the tach connection on the switch box often does not work, the rectifier always works.
A temperature gauge is not useable on this engine, the engine runs very cool as there is no thermostat, if the temperature gauge all of a sudden read hot it would be too late for your inline. The best indicator is the pee stream. If it is there you have water pressure, if it is not, or weak at idle you have a water pump issue.

 

[Chris1956]

Re: Gauges on a 77 115hp?

To elaborate on what Dave said. The wiring change for a standard tach will already have been made on that motor. One side of the rectifier will have the yellow stator wire connected to the brown tach sender wire. The three prong connector on the MerControl is to power the tach and the other gauges.

As Dave said, a water temp gauge is useless. I like to install a water pressure gauge. Get one that reads 0-15PSI. Install a brass tee into the hole that houses the teltale elbow. Now install the gauge fitting and the telltale elbow onto the tee, and connect the gauge hose and the telltale hose. I think they are all 1/8 NPT.

 

[emilime75]

Re: Gauges on a 77 115hp?

OK. I understand the temp gauge would be useless on this motor. As for the three prong connector on the mercontrol, I assume the prongs are positive, negative, and rpm signal from the motor? Or is each of the three prongs used for a different signal from the motor for different gauges, like tach and voltage gauge maybe?

I will look into the pressure gauge as well. What is the desired pressure at idle and at cruising speeds?

 

[Chris1956]

Re: Gauges on a 77 115hp?

3 prong connector is +12VDC, ground for all gauges, plus tach signal