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Woodenhead

Floating tachometer on mercruiser i/o

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The tachs on my 1987 vintage 260hp mercruiser i/o motors tend to drift occasionally. I usually slow cruise around 1300 RPM, once in a while when I look down I'll see it drift up to about 2k, if I make a small throttle adjustment (less than 100RPM diff) it'll settle back down. Also, if I cruise at 3k I'll see them occasionally drift up and down a bit which is a bit of a nuisance when you're trying to synchronize motors. The motor RPM isn't changing, just the tach reading. They both do it occasionally (not usually at the same time) but they're are independent electrical systems. The STBD motor has a new coil, ignition pickup (in the distributor), cap and rotor so I can pretty much rule out those parts.

 

Could it just be the gauges are getting old and could use a good blowout or replacement?

 

I've googled my way through this but there's so much BS in some of these marine forums it's hard to get anything useful out of them.

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Old age and moisture are more likely the issue. Or you could have a voltage issue at the gauges, maybe even a bad ground. Do you have a hand held tach? You could verify the rpm at the engine and go from there.

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Old age and moisture are more likely the issue. Or you could have a voltage issue at the gauges, maybe even a bad ground. Do you have a hand held tach? You could verify the rpm at the engine and go from there.

You make a good point about checking the grounds at the gauges. The cabin voltage always seamed a bit low and I've chased all the hot wires down and made sure the connections were good. I did measure the ground resistance from the battery connection to the ground in my accessory plug up front and it was less than 1ohm but I should probably check the ground on the cluster.

 

I do have a handheld tach but it's an external unit that runs off a reflective target or with a cone on the center of a motor shaft, that would confirm the accuracy but not do much to help me diagnose it.

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I have a combo tach,oil,temp gauge on the Trophy and it does the same thing on occasion. I've renewed the connections to no avail, I'll probably look for a new gauge if it gets bad enough. I use the tach a lot.

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It's real easy to stop them from floating.......

 

take them out and throw them in the lake......

 

That will also give you space for a shiny new set.

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If I could actually narrow it down to the gauge itself I'd gladly replace it. I think I'll switch them to the 4 or 6 cylinder mode and see if they smooth out.

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If I could actually narrow it down to the gauge itself I'd gladly replace it. I think I'll switch them to the 4 or 6 cylinder mode and see if they smooth out.

Somehow I don't think that will help, but it might help speed you in your journey to buying new ones.

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Somehow I don't think that will help, but it might help speed you in your journey to buying new ones.

I think I read somewhere that if the tach is going south, switching it can change the circuits it's usining internally and help you figure that out. The reading will just be off for a while.

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make a jumper wire for the power and ground to temporary power the tack to see if it changes, my guess is the gauges because if you alter the rpm they correct themself's, it is like they are getting bored and there mind start to drift. much like a 16 year old boy

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make a jumper wire for the power and ground to temporary power the tack to see if it changes, my guess is the gauges because if you alter the rpm they correct themself's, it is like they are getting bored and there mind start to drift. much like a 16 year old boy

 

I'll try that, maybe run a big long piece of speaker wire straight from the batteries.

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that will work

 

I guess there's no reason to disconnect the existing power and ground from the gauge cluster since it'll just run in parallel anyway. I have a bunch of toggle switches laying around, maybe I'll connect one into the power wire so I can flip it on or off easily when I'm running to catch it when it's acting up and see if it makes a difference, maybe even narrow it down to the ground or hot wire if it ends up being power related. It would make more sense if it were the ground though since the port and starboard gauge clusters are powered separately.

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8 out of 10 electrical issues are the ground circuit or atleast that is what they told us in school.

Sent from my SM-G900R4 using Tapatalk

I remember a lecturer about that.

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A shameful update :D as I didn't get much accomplished yet. I did mess with the switching on the tach's a bit and ran them double speed on the 4cyl setting for a while, didn't seam to make much difference. Rigged up some speaker wire with some alligator clips for the battery, a fuse and a switch on the hot to run some temporary power to the cluster but still haven't hooked it up (this weekend).

 

I did order a sychronizer gauge and will hopefully install that in a few weeks. It'll run off the same signal wires that goes into each tach so if nothing else maybe it'll help figure out where the problem is coming from.

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It never ends Mike...

Always seems like when you fix one thing you get 2 more problems and no extra time to fix!!

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It never ends Mike...

Always seems like when you fix one thing you get 2 more problems and no extra time to fix!!

and if it aint broke fix it till it is!!! thats how I roll B)

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Amen to that fellas. But these are minor issues I can deal with as I get time. If I make it through the season with no major issues that stop me from enjoying the boat then I consider it a win :)

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Digging this up from the dead. I did try running the extra power cord and it didn’t fix anything. I also ran a new ground wire to the main ground at the cluster. Neither one of those changes the issue. 
 

A few other symptoms with the crazy tach. 
The port tach always reads correctly, the starboard motor/tach is erratic sometimes. 
 

When the STBD motor is idling the tach will read about 600RPM, then I’ll hear the tone of the engine change, it’ll idle up to about 900RPM but the tach will read 1800-2000, seems to be reading about double. The motor is definitely not reving that high. Then after a minute or so the tone will change again and the idle will drop back down a couple hundred RPM and the tach will read correctly.

 

The synchronizer gauge I put in reflects what the tach is doing and gets pegged to one side. It piggy back off the same signal wire the tach uses leading me to believe it’s something in the ignition signal, not the gauge. 

 

Over time I’ve replaced the cap, rotor, ignition sensor, ign wired, plugs, coil. 
 

For a while I was thinking the carb was screwing with the idle but maybe the thunderbolt ignition module is wigging out.

Edited by Woodenhead

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Would a worn distributor cause this type of issue?

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4 hours ago, C Paul Go said:

Would a worn distributor cause this type of issue?


Possibly but I’ve replaced the distributor. I think the tach signal comes from the primary side of the ignition coil which comes from the the ignition module and sensor under the distributor. It’s like something in that signal is doubling up at times. 

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6 hours ago, marksmith said:

Have you hooked the ‘normal’ tach up to the non normal side?


No, I was going to but then I installed the synchronizer gauge which reflected what the wacked out tach was showing so I figured it wasn’t the gauge. 
 

I did get a good buy on a Twinscan tach though so if I get it installed this winter it’ll replace both tachs. 
 

http://www.floscan.com/html/blue/seriesdetail.php?sid=15

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