Inconsistent and inaccurate readings from tach

[capslock118]

Hello,

So this has been going on for about 2 years now, and this years first launch seemed the worst yet.

My tach does not give accurate readings, I know this because of the following:
-sound of the engine doesn't match what the rpm says
-sat. the reading at idle was 2k (after a clean carb and link and sync)
-sun. the reading at idle was 1k - same feeling to sound/vibration as sat.
-on sat. WOT was reading past 6k (dial didn't go any further) and my engine, according to mercury, cannot physically exceed 5500rpm and WOT
-WOT throttle sounded like it was at 5500rpm, not sounding like it was at the bleeding edge of failure.


Now, 2 years ago, my mechanic did some quick tests on the electrical on my engine for some other completely unrelated problem because fuel wasn't making it to the engine (end result was a bad primer bulb).

He said that my switchbox assembly was giving inconsistent readings, explaining the tach, and also stated that it's a sign that they could go bad, and it can happen at any time, and when they do go bad, bad things will happen to the engine.

I would like to plan on replacing the switchboxes next season (after my own personal tests) but they are $200 a piece OEM and I need two of them. I would have to go third-party, but definitely not right now.


My questions:
does this sound reasonable? or - is there other areas i can diagnose to try to get a better reading on my tach; hopefully as a cheaper (if not temporary) solution? Maybe it's just a bad tach? maybe something else?

 

[Silvertip]

Re: Inconsistent and inaccurate readings from tach

Your interpretation of maximum WOT is very wrong. WOT specs from engine manufactures is an indication of the maximum RPM that is NOT TO BE EXCEEDED. That does not mean it is physically impossible to achieve. Install the wrong prop and it will easily exceed 5500 RPM. Install one with too much pitch and it will not be able to reach the max recommended WOT rpm. Next -- your ear is not a measure (at least not a very accurate measure) of what 5500, 2000. 1000, or any other RPM is. The tach is driven by the pulses from the charging system, not the switch box.

 

[capslock118]

Re: Inconsistent and inaccurate readings from tach

hm, interesting. there would still have to be some physical limit to how fast the pistons could move i would guess - but you are clearly saying 5500 is not it.

ok - so let's go with that. and let's also go with the tach being driven by pulses from the charging system.

that makes it sound like if I have a bad charging system the tach could be off; or if I had a bad battery the charging system might be off.

Now I am questioning my battery this year here: http://forums.iboats.com/showthread.php?t=488522

as for the charging system - i replaced the regulator and rectifier last year, so unless there is something else going on...stator maybe?

but that leads me back to my original question: what should I be looking at/for?

 

[jjlove68]

Re: Inconsistent and inaccurate readings from tach

capslock, my 1988 Merc 100 on 1989 SeaRay Seville 16 and it is doing the same thing. If I smack my tack while the engine is running it will jump and stay at almost any random RPM no matter what the actual engine RPM is. The needle bounces smoothly so I don't think it is sticking inside the gauge. I am pretty sure I have a bad/intermittent connection to the tach from the engine and/or the coil in the tach itself that drives the needle has gone bad. At some point I will actually do something about it but the reality is as long as you can confirm that you have the correct size prop and you are not over revving at WOT it really doesn't matter.

 

[aayjay]

Re: Inconsistent and inaccurate readings from tach

My old SeaRay is doing a similar thing, sometimes it seems like the tach is jumping up to more revs thn I know i am giving with the throttle, and i do understand that after a number of hours you can get an "ear" for your revs.. yet i have not found the cause of this.. I have cleaned the contacts of the grey sending wire that comes from the regulator which triggers the tach and it smoothed out some, it seems the salt gets everywhere causing corrosion, and a good trigger pulse is essential to accurate tach. reading.

 

[capslock118]

Re: Inconsistent and inaccurate readings from tach

seems reasonable to assume salt.

still, i can't seem to figure out what this issue is or how to diagnose it.

 

[jjlove68]

Re: Inconsistent and inaccurate readings from tach

I have the Mercury shop manual for my motor. I should just look up the diagnostic procedure for this shouldn't I? I'll attempt to remember to have a look this weekend.

 

[j_martin]

Re: Inconsistent and inaccurate readings from tach

capslock,
If the battery is charging, which is indicated by voltage going up when the engine is running, the regulator is good. If it works at all, the tachometer signal is probably good. You could tap the tachometer lead into either yellow lead to test. It still depends on the rectifier being good, but if yer battery is charging, it has to be.

Power to the tachometer has to be clean. A dirty ignition switch, or wiring to the console could be suspect here.

What you're describing is a flaky connection in the tachometer itself. If there is a poles selection switch, that would be the most likely cause. Determine which position it is in , then rotate it through it's entire range several times, then put it back where you found it. It would be a screw slot driven switch on the back of the tach.

hope it helps
john

 

[capslock118]

Re: Inconsistent and inaccurate readings from tach

I should just look up the diagnostic procedure for this shouldn't I? I

I read through mine, I didn't find anything specific to an inaccurate tach reading but maybe I was either being stupid or the process was more subtle than I had thought. Guess I'll read through again.

 

[jjlove68]

Re: Inconsistent and inaccurate readings from tach

I read through mine, I didn't find anything specific to an inaccurate tach reading but maybe I was either being stupid or the process was more subtle than I had thought. Guess I'll read through again.

J_martin has a good point and sounds like he knows what he talking about.

I was thinking of looking at the diagnostic procedure for a non functioning tach and maybe it would provide some insight. You might be able to read the frequency of the pulse in the tach signal wire if you have multimeter capable of frequency readings. It could be blown or almost blown rectifier or something. In either case, the first step to any diagnostic procedure like this is to thoroughly inspect and clean all power and signal connections. Odds are that will fix both our problems.

 

[capslock118]

Re: Inconsistent and inaccurate readings from tach

It could be blown or almost blown rectifier or something.

Sure hope not, i just replaced the rectifier last year.

In either case, the first step to any diagnostic procedure like this is to thoroughly inspect and clean all power and signal connections.

Will have to try that.


So now for the last few weeks my tach has been completely dead. It just stays at 0 no matter what, though at WOT it does try to move, but it only does as much as 'jiggle'.

My ignition switch is going now, more good news. It was just yesterday when it started to happen, it acts as if the key is loose. When in the on position the electronics keep getting shut off and turned back on, the beeper keeps signaling that it was just shut off, just turned on. Doesn't tuen the engine off.

It's definitely a bad ignition switch, but I noticed that the tach seems to be a bit more responsive when the ignition switch keeps bumping around, so it very well could just be a power problem, though the issue is isolated to the tach.

 

[capslock118]

Re: Inconsistent and inaccurate readings from tach

The ignition issue turned out to be a loose fuse.

The tach is still dead. At this point most of the time it is at zero and sometimes.it's skiddish but never accurate.

I replaced the tach with no change in behavior with a spare I had.


What else could I look at?

 

[jjlove68]

Re: Inconsistent and inaccurate readings from tach

Now my motor has completely died... I finally pulled it out of storage a couple weeks ago. It fired right up but would only idle on one cylinder instead of two. I had spark to all four cylinders so I pulled my carbs and cleaned everything up. No evidence of any blockage in the carbs so I pulled the upper reed blocks. Again no sign of a problem. While I was picking up the read block gaskets at the local Mercury dealer I decided to buy new spark plugs. I was confused when he gave me standard plugs instead of surface gap and he told me that they changed the spec to normal plugs a long time ago. Whatever, I put them in and fired it up and she purred like perfection in my driveway. Fast forward one week later, I'm at the boat ramp and... nothing... not even a hint of spark... This is the first time my engine has completely let me down. After throwing a few things I calmed down and started doing some basic checks.

The key switch and other safety switches in line definitely break contact to ground like they are supposed to on the "kill" wire and the tach seems accurate when I crank the motor. The fact that the tack is getting signal tells me something is working. Now I am in the middle of building a DVA so I can continue to diagnose things. Radioshack sucks and doesn't have any capacitors rated to 400V+ so I need to track one down at work. I hope it is not the switch box because I just replaced it two years ago along with one of the coils. I also hope it is not the stator because then I'll need to buy the tools to pull the flywheel... I will know as soon as I take another look at the wiring diagram but I am assuming the tach signal is coming from the trigger coils so I can see how the tach could work when the stator is dead.

 

[capslock118]

Re: Inconsistent and inaccurate readings from tach

...

well, right, i'm under the impression this issue is with the switch boxes not sending the signal to the tach - however, i'm no mechanic, but i would guess if the switch boxes were dead then I wouldn't get an electric signal out to the plugs to create a spark. so i'm hoping someone might be able to chime in to what to look at next.

The switch boxes are $300 a piece and I have two of them!

 

[jjlove68]

Re: Inconsistent and inaccurate readings from tach

I hope not... My switch box is only a couple years old and I have a tach signal but no spark... My DVA is built but now I have Irene aftermath to deal with. The more I think about it my specific problem with the tach is probably internal to the gauge itself. Hmmm I just started wondering if a short in the tach could kill the spark... Just like shorting the black-yellow "kill" wire to ground.

 

[ufm82]

Re: Inconsistent and inaccurate readings from tach

Off the original topic but there IS a physical limit as to how fast the pistons can move within the engine. It's called metal and when you zing the engine to the moon and the metal gives up the ghost, bad things happen. Unlike a 4 stroke engine that will experience valve-train issues prior to self-destruction sometimes, a 2 stroke engine has no valves to "float" and will run at higher and higher rpms until something lets go. That usually means extra holes in your engine. As for your main issue- I got nothing.