Wacky Tachy

[nola mike]

Been having an issue with a floating needle on my tach. Seems more pronounced when hot or at high rpm. I've had the problem before, I thought the issue in the past was some internal corrosion in the tach. It seemed to resolve last time when I worked the selector screw back and forth.
This time, I've cleaned all the grounds, engine, coil, tach. Also tried running dedicated ground to the tach, no change. Ran dedicated sender wire to the coil, no change.
I do have some decreased positive voltage. That's been there to some extent since I've had the boat, but don't know if it's causing an issue now.
I took the DVM to the switch before I took a good look at the wiring diagram, but from what I remember I had battery voltage at "I", which dropped to 10v when I turned the switch on. Purple wire was also at around 10v. This jibes with what my voltage gauge has been telling me for years.
Looks like yel/red at "C" only gets 12v when cranking, right?

Troubleshooting plan:
1. Run 12v from battery to switch, attach to "I". See if I still get voltage drop when key on. (I don't know what the significance of that would be though)
2. See if I get 12v to purple at that point. If so, I have a problem with my main 12v feed.
3. If I still get a drop in purple, but none at "I", I have a bad ignition switch?
4. My gut tells me that my tach is bad, so I guess next step would be get a new cheap tach and see if that the problem persists with that.

Other suggestions?

 

[tank1949]

Hook up multi meter and compare readings. If major difference, buy or borrow another tack and repeat. Salt water does a number on gauges. Trust me!!!!!

 

[achris]

Any drop just from turning the key to ON indicates a bad connection between the battery and the key... Track it down using a 'voltage drop' testing regime. Could be anywhere, in either (or both) the positive or negative circuits. Ground connections to the engine block need to be shiny clean, both the cable terminal and the block surface. And don't forget to check the big black instrument harness connector...

Chris.......

 

[nola mike]

Found a couple of drops. Cleaned connections and am now dropping 0.25v across the ignition switch and 0.5v overall. This is much better than the 1.5v that I've always lost since I've had the boat. Anyway, I'm getting 14v to the tach under way, and still having the issue. So can I assume that the tach is the problem? Unrelated, tide went out while beached and made getting the boat in... Challenging. Lost my speedometer pitot, so now I'm driving by ear to see how fast I'm going and how hard the boats working.

 

[achris]

Found a couple of drops. Cleaned connections and am now dropping 0.25v across the ignition switch and 0.5v overall. This is much better than the 1.5v that I've always lost since I've had the boat. Anyway, I'm getting 14v to the tach under way, and still having the issue. So can I assume that the tach is the problem?

I would...

Unrelated, tide went out while beached and made getting the boat in... Challenging. Lost my speedometer pitot, so now I'm driving by ear to see how fast I'm going and how hard the boats working.

Download a GPS app to your phone (there's dozens of them!) and just have that sitting on the dash for your speedo until you get the pitot fixed. (and it'll be 10x more accurate, especially at low speeds)...

Chris.........

 

[wellcraft-classic210]

Does you tach have a setting switch for engines with different cylinders -- if so try moving it repeatedly to other positions and then back to its original position -- if those contacts get corroded it can cause a intermittent high reading.

 

[nola mike]

wellcraft-classic210 It does, and I tried that. I seem to remember that helping a few years ago

achris , using GPS, pitot coming today but I need to take the boat out and wait for the sealant to set up. Annoyed about the tach, I'll have to replace all the gauges and it will be tough to find one that look right...

 

[achris]

.... Annoyed about the tach, I'll have to replace all the gauges and it will be tough to find one that look right...

It's not a Merc gauge? (They are all still available)

Chris.......

 

[nola mike]

It's an 80s vintage teleflex. I can find a gauge that would work,but it wouldn't look the same as the others. Last resort will be to cut open the gauge and clean it, but I wouldn't want to bet on my odds of success.