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Sounds like a loose or corroded connection either at the gauge or the engine. 

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I believe it feeds off the negative side of the coil.... You could also pull the tach and move the selector knob back and forth a few times. It's helped others with similar issues 

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7 hours ago, Water Dawg 6 said:

Anyone know anything about the RPM gauge?  One of mine seems to work when it wants to or bounces up and down. 

Switch the head unit gang plug from one tach to the other. If the problem transfers, then check the harness to the ECM. If the problem goes away, replace the head unit. W

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On back of tach is a rotary switch. That is the selector for the number of cylinders. Make note of present switch position. With a small screwdriver, gently move switch all the way from max position to min position a few time and reset to original position.

There is a wiper in there that will crap up over time. Moving it cleans it. 

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Hi wingnut - can you elaborate on this instruction in novice terms? I have a 2005 ssi 190 and not that knowledgeable on some terms you are using.  Boat only has one tachometer.  What is an ECM?  What is the head unit? 

Switch the head unit gang plug from one tach to the other. If the problem transfers, then check the harness to the ECM. If the problem goes away, replace the head unit. W

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9 hours ago, tcartwri said:

Hi wingnut - can you elaborate on this instruction in novice terms? I have a 2005 ssi 190 and not that knowledgeable on some terms you are using.  Boat only has one tachometer.  What is an ECM?  What is the head unit? 

Switch the head unit gang plug from one tach to the other. If the problem transfers, then check the harness to the ECM. If the problem goes away, replace the head unit. W

The OP has two motors, two tachometers. 

Since the OP has two, the suggestion was to move the connection (wiring) from one tachometer to the other in order to confirm where the problem originates (tachometer or wiring). This confirmation step saves time initially by avoiding removing, purchasing, reinstalling and returning a new tachometer before knowing what is wrong (if return is even allowed; often electrical stuff is not returnable and tachometers can be expensive depending on which type).

If the problem moves to the other, the problem is within the wiring. The wiring goes between the tachometer and ECM. The ECM is the engine control module, i.e. the motor’s brain if you will.

If the problem doesn't move, the issue is with the tachometer. To rectify requires replacement or as another poster suggested, moving the cylinder switch a few times to see if that clears the problem. (Many tachometers are designed to be used with a variety of engines, therefore on the back or side is a switch allowing the number of cylinders the engine has to be selected.)

The “head unit” in the post you reference is the tachometer.

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9 hours ago, tcartwri said:

Hi wingnut - can you elaborate on this instruction in novice terms? I have a 2005 ssi 190 and not that knowledgeable on some terms you are using.  Boat only has one tachometer.  What is an ECM?  What is the head unit? 

Switch the head unit gang plug from one tach to the other. If the problem transfers, then check the harness to the ECM. If the problem goes away, replace the head unit. W

Thank you for the clarification - since my 190 ssi only has one tach I’ll move the cylinder switch on it  to see if that solves the problem.  If not, believe my next test is to replace the tach.  If that does not work, then my problem is wiring or ECM.  Thanks

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11 minutes ago, tcartwri said:

Thank you for the clarification - since my 190 ssi only has one tach I’ll move the cylinder switch on it  to see if that solves the problem.  If not, believe my next test is to replace the tach.  If that does not work, then my problem is wiring or ECM.  Thanks

A suggestion.

If the switch doesn’t fix it, check the wiring before replacing. Won’t take but 5 or 10 minutes. Pull the plug, clean both sides, reinstall; follow the wires with your hand and feel for cuts, sharp bends, breaks, chaffing; confirm ground is clean and tight; etc. If you’re handy, use a multimeter.

99.999999999% it’s not the ECM. There’d be other issues if a bad ECM.

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The tachs can be very pricey $$$. If your boat is around 2010 or newer, you may be able to tap into the ECM and read the RPMs from your GPS. 

 

Cost for me was 175 to tap into it or 700 for a new tach. 

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1 hour ago, Iggy said:

The tachs can be very pricey $$$. If your boat is around 2010 or newer, you may be able to tap into the ECM and read the RPMs from your GPS. 

 

Cost for me was 175 to tap into it or 700 for a new tach. 

Agreed!  I even went with a VesselView 4 for less than a new tach.

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2 hours ago, Iggy said:

The tachs can be very pricey $$$. If your boat is around 2010 or newer,

+1 and my Smartcraft tach does not have the selector switch on the back to select 4, 6, 8 cylinder. I use the tach from the ECM

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Just now, ColdOne said:

+1 and my Smartcraft tach does not have the selector switch on the back to select 4, 6, 8 cylinder. I use the tach from the ECM

Right on, but this is an ‘05. Might not have SmartCraft or the Volvo Penta “equivalent”. Engine and particulars weren’t posted, so we’re all shooting in the dark. If I recall correctly, SmartCraft was introduced in ‘04 as an option?

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Thanks to all for the guidance. Blown away by the highly collaborative chap community.  My chap is a 2005 ssi 190 with the mercruiser alpha 6 cylinder carburetor - about 120 hours and gently used by me.  I’ll check the cylinder switch if equipped with one and also the wiring.  Tapping in to the ECM will require some additional skill so will research further.  My understanding is the temp is the critical gauge to watch for overheating - if the rpm guage is not working is there any risk to not repairing/replacing and living with it?

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I run my boat by feel... I can tell if I'm running the right RPM without looking at the gauge. 

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2 hours ago, tcartwri said:

Thanks to all for the guidance. Blown away by the highly collaborative chap community.  My chap is a 2005 ssi 190 with the mercruiser alpha 6 cylinder carburetor - about 120 hours and gently used by me.  I’ll check the cylinder switch if equipped with one and also the wiring.  Tapping in to the ECM will require some additional skill so will research further.  My understanding is the temp is the critical gauge to watch for overheating - if the rpm guage is not working is there any risk to not repairing/replacing and living with it?

No harm. The important gauges are temperature, speed, volts and trim position. You’ve had your boat for awhile and know at about “x” speed the motor is turning “y” RPM’s. About 30 MPH is about 3,000 RPM on many, but not all.

Get your head and hand up under the dash and see if the switch is there. If it is, move it a few times.

Always helpful is a post with a picture, or if you remove it, the gauge’s model number. I’m sure someone here will be able to identify it for you. Hard to know if expensive or not until the exact gauge is determined.

An alternative, surf eBay or Great Lakes Skipper. Often these sites are helpful. A call to Cecil Marine with the boat’s year, model and serial number might do it as well.

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Thanks again - I’m about two weeks out from seeing my boat.  Keep her at lake house.  I’ll check it out, try the suggestion on switch, and post pictures in two weeks  if not able to resolve.  You are correct - typically runs 3000 rpm at 30mph.   I will be able to tell by feel as per Phillbos post if not running right.

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Thanks to all.  I'll investigate this weekend.  The RPM gauge isn't important to me nor is most of the gauges besides Temp and Voltage.  Everything else I get from my Garmin Head Units except the Trim, but I know the feel of my boat and can tell by the wake where my outdrives are in the water.  Just trying to get everything working because I'm thinking about selling her after the fourth of July weekend. 

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Thanks all for the help with the tachometer.  I cycled the cylinder setting on the back and solved my issue with the needle jumping sporadically.  Worked like a champ.  Attached a photo for future reference to others.  My chap 190 ssi has the 6 cylinder carb engine so my setting is 2.  Used a small screwdriver and moved the setting back and forth a few times.  Tachometer now works as normal.  About 3000 RPMs at 30 MPH.  Thanks again to all for the assistance.Noname.jpg.38b9525d2b6780005fa501ce5fc17a2f.jpg

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