Tach is all over the place

[boatingnva]

I have a '99 Bayliner 2050, with 5.0L Merc I/O. For some time now the tach needle will bounce all over the place, never reading right. Just a little increase in rpms can cause it to peg, then it will come back down, and do it again. No real rhyme or reason to what it reads. I've checked the connections in the back of the tach, seems ok. Any suggestions?

 

[270sundancer]

Re: Tach is all over the place

My 98 SeaRay Sundancer 270, 454 7.4L, Bravo III Mercruiser does the SAME THING. Has for a while. I can "tap" hard on the gauge face and it usually makes it stop and gives the correct reading for a while. Probably a bad gauge from what everyone has told me.

 

[Dadicus]

Re: Tach is all over the place

Are the other gauges working okay? Because of the way my Mercruiser/Searay gauges are wired the failure of the temp gauge caused the tach to give erratic readings.

 

[boatingnva]

Re: Tach is all over the place

Are the other gauges working okay? Because of the way my Mercruiser/Searay gauges are wired the failure of the temp gauge caused the tach to give erratic readings.

All other gauges are working fine, only the tach is acting erratic.

Also, is there any way to test the leads at the tach to make sure it's the tach is bad?

 

[270sundancer]

Re: Tach is all over the place

All of my other gauges work fine too. Sometimes my speedo doesn't work, picks up junk in the sending tube, but that's what GPS is for, right? LOL Sometimes my tach will work fine for days, weeks, and then ERRATIC for days, weeks. Usually if I tap the glass though, it calms down and reads what it should be reading. I have pretty much become "tuned" in to the engine sound and throttle position to know when I am at cruising RPM (around 3400-3800 and 34-38 MPH) I have a pilot's license and believe it or not, my boat runs kind of like an airplane...34 squared..38 squared..(engine rpms to prop rpms on an adjustable pitch prop plane when landing). This is probably just a coincidence that my boat is "squared" rpms to mph's. I try to think of it like this...RPM's match the MPH pretty closely in my boat at normal cruising speed, planed out..

 

[Dadicus]

Re: Tach is all over the place

Also, is there any way to test the leads at the tach to make sure it's the tach is bad?

I posted this same question a few days ago and got no reply, so here's what I did. If your tach is the same as mine (a positive wire, a ground, and a signal wire) then there are limited options on what can be wrong. Either the tach is bad, or the power or ground is bad, or the signal/signal wire is bad. I'd start with the basics. Rig a long jumper wire and connect the tach to a good ground (probably on the engine). Then make sure you have 12 volts between the hot wire and the ground. The problem I had with mine was that the ground was bad because of a failed water temp gauge. Anyway, the signal wire, as I understand it, comes off of the coil or electronic ignition. You can probably trace it from the gauge back to it's source. As far as I know it sends pulses to the tach that allow it to measure the rpm. There's actually a setting on non-factory tachs that allow you to set them for 4, 6, or 8 cylinder engines. That would suggest that the tach "counts" the pulses in order to register the rpm. The most likely culprit in this set up (aside from a bad tach) is a bad connection either at the tach, or at the coil/elec ig, or at the main wiring harness connection. I'd check each of these connections separately

Sorry I can't be more positive about this. I'm pretty ignorant and learning all this stuff myself. Good luck.

Oh, and 270 Sundancer, the idea that your tach reacts to being tapped on suggests that it really is a bad tach. It sounds almost like a mechanical (sticking) problem with the needle.

 

[flabum]

Re: Tach is all over the place

before you go through all of that, run the adjustment screw on the back of the tach back an forth through it's travel several times. This adjustment screw has contaccts inside the tach that can get corroded and lose contact. By turning the screw back and forth, you can clean it. See if that works.