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  Tachometer, Thermometer Calibration

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Author Topic:   Tachometer, Thermometer Calibration
gusgus posted 08-04-2012 12:50 AM ET (US)   Profile for gusgus   Send Email to gusgus  
I recently rewired my 1987 Outrage 18 and replaced some of the electrical components.
The tach was one such replacement.
Now at slow speeds it all works as advertised, the Tach shows good idle speeds and the temperature gage operates well, but at "on step" speeds the tach is buried at high scale, the temperature gage looks like it has just died and yet they still work perfectly at slow speeds.

What a pain.

Any ideas?


Thanks in advance.

Tom W Clark posted 08-04-2012 01:25 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
What is the voltage being supplied to the gauges while "on step"?

A defective voltage regulator can allow the voltage to the instruments to spike causing false readings.

seahorse posted 08-04-2012 07:40 AM ET (US)     Profile for seahorse  Send Email to seahorse     

How about a make, model, and year of your outboard as there are different electronic circuits on various outboard models that generate tachometer signals.

Of course the quickest check is to substitute a known good tach to find out if it is an intrument problem or an engine problem.

L H G posted 08-04-2012 11:37 AM ET (US)     Profile for L H G    
I would check for the correct (6) pole setting on the new tach. If your tach is set correctly, the problem could be a defective voltage regulator/rectifier component, about $175.

Mercs have a poppit valve that causes warmer idle speed temps, clearly indicated on a temp gauge. With good cooling water, sometimes the temp gage will barely show a reading as the valve opens up for greater cooling at planing speeds. Also be sure the temp gauge has the lower scale for outboards (usually simply cool-hot), as opposed to the higher temp range used for I'O's.

gusgus posted 08-04-2012 11:44 PM ET (US)     Profile for gusgus  Send Email to gusgus     
Thank you everyone for the advise. [Changes topic to begin a discussion of the selection of a title for this thread--please, let's continue the discussio on the topic and not begin a new discussion about what the TOPIC line ought to be.--jimh]

I replaced the poppet valve about 2 weeks ago. The temperature guage operates normally when at low RPM.
The voltage is 13.8 at idle and at low power and 14.4 on step. Is this normal for my 1989 v150 ?

All the wires are new, less than 10 hours on the boat since replacement.
The Quicksilver tach was junk and the West marine bought tach was wired the same as the Quicksilver unit. It is and was switched for the 6 cyl motor.

I am stumped.

Phil T posted 08-05-2012 10:20 AM ET (US)     Profile for Phil T  Send Email to Phil T     
The "6" on the tach is not representative of the number of cylinders rather the number of poles.
gusgus posted 08-06-2012 02:16 AM ET (US)     Profile for gusgus  Send Email to gusgus     
Thanks Phil, I do understand the pole theory of the tach. The new tach had the same switch choices as the Quicksilver unit did.

Voltage seems right, but something is askew for sure.

Tom W Clark posted 08-06-2012 12:40 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Yes, the voltage is normal so I susepet the tachometer is on the wrong setting. If the old tach was junk, how do you know it was set on the right setting?

jimh posted 08-07-2012 08:32 AM ET (US)     Profile for jimh  Send Email to jimh     
The tachometer calibration using the pole setting switch causes a linear change. At low seeds the error is small. For example, the engine might be running at 600-RPM and the tachometer indicates 800-RPM. The resolution of the dial on the tachometer is about the same as this 200-RPM error, so an observer might not even notice the error. Thus, "tachometer works good at low speed" is an entirely feasible observation.

The same error at 5,500-RPM causes the tachometer to read 7,315-RPM, which is a much more noticeable error. The error is still in the same proportion, but now we could easily see the error.

The operating temperature of outboard engines often varies significantly depending on engine speed, and, as L H G has observed, there is typically a significant decrease in operating temperature when the engine speed is raised due to a very big increase in cooling water supply. When I installed a temperature gauge on my outboard engine, a very experienced mechanic said to me, "What'd you do that for; you're just going to watch it go up and down all the time."

L H G posted 08-07-2012 04:39 PM ET (US)     Profile for L H G    
With my V-6 Mercs, the temp gauge is essential. When I see planing speed temps gradually going up, I know it's getting to be time for a new impeller. And if a bad impeller situation occurs while underway, I use the gauge to keep the engine at running speeds where it won't overheat, set off the alarm and shut the engine down.
Tom W Clark posted 08-11-2012 10:55 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
How's the tachometer working now?
jimh posted 08-12-2012 10:31 AM ET (US)     Profile for jimh  Send Email to jimh     
ASIDE to L H G: With your Mercury engines, the time for a new impeller is every Spring. Mercury recommends an annual change of the water pump, which, as far as I can tell, is a sort of Mechanic's Annuity program for their dealers.

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