High voltage affecting tach?

[sammith]

Hi all, last year my tach wasn't working and I realised my voltage rectifier was done. Replaced it and now my tach works, but it seems to be reading too high (still need to confirm this). I realized though that my voltage regulator must also be bad since the voltage will start off good, but go up to ~18V after a few minutes of boating.

I have a new regulator on the way, but I was wondering if the high voltage could be causing the high RPM reading. For reference, the motor is an older Mercury V150 and the gauge is a Faria gauge. I already turned the selector at the back of the gauge like I read in other forums and it should be at the right setting.

 

[Scott Danforth]

could be the tack is now fried. keep in mind, most tachs are a $1 air core motor with $0.25 digital to analog converter wrapped in $3 worth of decal, housing and glass (then sold to the boat manufacturer for $10 and to the general public for $100)

doesnt take much to damage the tach if the incoming signal was out of spec

 

[gm280]

Could also be that the high reading regular is allowing some A/C to flow through and that could cause the tach to read questionably. Before changing the tach, wait until you receive the new regular and see how that works.

 

[sammith]

Would the tach still read "fluidly" if damaged? It doesn't jump or change readings all of a sudden or anything like that. It reads surprisingly well, just the readings seem high. At idle it'll read around 1000RPM, but by the time I hit 30MPH, it's redlining. I know there are many factors that could impact this (prop pitch, etc) which I've looked into last year when I suspected the tach was accurate, but now I'm starting to wonder if the engine is actually not over revving and that it's just the tach that's out to lunch.

Ultimately, I really need to get out there one day and measure the revs on the flywheel, but now that I know my voltage regulator is broken, I was curious to know if that could impact the readings of the tach.

 

[sammith]

Just noticed I may have ordered the wrong regulator. The one I ordered is a 6 wire, but I see others with only 5 wires (1 red wire less) but they both cross reference to the same OEM part number (815279T). Looking at my wiring diagram, I think I would need the 5 wire regulator, but would the 6 wire one work also by either putting the 2 red wires together or not connecting one of them (the sense wire)?

 

[sammith]

On some models, the 2 red wires seem to go to the same place (on the starter solenoid), so I'm guessing it should work if I wire them together. Does that sound right?

Here's one example I found online: https://images.app.goo.gl/a2FZJo4yr1Dr11Ma6