Optimax tach/rpm

[Claws641]

So I was out fishing today and my tachometer seems to have malfunctioned. I have a 208 with a 150 optimax. I typically run at 4200 rpms and am very familiar with the speed I get at that rpm and how the engine sounds at that rpm. Anyway, at one point I was cruise my at my typical speed and looked down and the tach was at 5200. The boat tops out around 5400 and I know for sure the engine wasn’t revving that high. By the time I got back to the dock it was too late to investigate. Any ideas? Boat running fine and no alarms.

Thanks-

 

[seasick]

Smartcraft Gauges?
Off the top of my head a few things come to mind: A low reading is possible if a sensor is bad but a high reading probably isn't. If the sensor were bad, I might expect the idle reading to be incorrect also
It could be a bad gauge, especially if it is an analog one.
The other thing that came to mind is a slipping prop but I think you might notice that behavior.
I have wiring diagrams for that motor but they don't really tell you a lot about what can cause your issue. I will do dome more research.

 

[seasick]

I found a few references to similar tach issues on Mercs. One thing that seemed to work in several cases was to operate the rotary pole selector switch on the back of the tack a few times to clean up the contacts. Note where it was before you start turning it! The high rpm issues were on analog gauges not the digital ones.

One boater just tapped the front of the gauge and the problem went away ( for how long I don't know)

 

[Claws641]

Seasick: thanks for the info! It’s an analog gauge. I’ll try the solution you suggest with the rotors selector on the back of the gauge and report back. I don’t think the prop is slipping because the boat (speed and engine sound) seemed perfectly normal. How would I know if the prop was slipping? Is there a way to test for that?

Thanks!

 

[DennisG01]

Rotating the selector switch on the back on the gauge is a definite option. I know for a fact that it works on stern drives as the selector switch allows you to select whether you have a 4cyl, 6cyl or 8cyl. I do not, however, know if this translates to outboards.

I agree with Seasick that you would definitely know it if your prop was slipping (failing hub). You'd absolutely notice it when getting on plane - it would be obvious. The RPM's would sky rocket.

 

[seasick]

Yes, the gauges have selectors for the number of poles (which may or may not equate to the number of cylinders) The poles are in effect the number of lobes or 'teeth' on the trigger coil. If the setting was incorrect, the rpms should be off across the range. If you are reading about 5200 when you think you are doing 4200, your idle revs should be reading higher by the same ratio. If your normal idle is 560, it would read about 690 now. If the top end reads wrong but the idle seems correct, I don't think it is a selector switch issue. The tach if analog (moving needle) it might have gone bad.

And yes as Dennis said, if that prop is slipping you would notice it under load and the highest load is when getting on plane.
Let us know how it turns out

 

[Claws641]

Thanks for the advice...I’ll post once I figure this out.

 

[Claws641]

I took the gauge out and twisted the adjustment knob on the back a few times and returned it to its original setting. Now the tach is reading like 1000k low! I think the unit is shot. Ive done a little research and it looks like a modern “smartcraft” gauge is the way to go....I currently have all analog. My engine is a 2007 150 optimax, and according to what I read online a smartcraft gauge and data cable should be plug and play on my year engine. Any experience or input?

Thanks!

 

[DennisG01]

"Original setting"... maybe the knob was not on the right setting to start with. Were the settings something like 4P, 6P, 8P? Your Opti is a V6, right? Make sure it's on the "6" (or the middle setting if my memory is incorrect on the actual labels). Yes, I believe you are correct that it should be a plug-n-play. But if you can get your current tach running, it's a lot less expensive!