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  1990 Evinrude 88SPL: Tachometer Accuracy; Alternator Volts

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Author Topic:   1990 Evinrude 88SPL: Tachometer Accuracy; Alternator Volts
Mr T posted 05-02-2012 03:49 PM ET (US)   Profile for Mr T   Send Email to Mr T  
While out running yesterday, I noted two [unusual things] with [a 1990 Evinrude 88SPL outboard engine]. The [tachometer] is WAY off, [indicating] from 1,800 to 2,400-RPM at idle, even though I know [the actual engine speed is] around 700-RPM. At full-throttle the tachometer indicated [more than] 8,000-RPM, but I know [the 1990 Evinrude 88SPL outboard engine] is propped for and runs at or around 5,500-RPM.

A little later, the Voltmeter started showing 14.7 to 16.0-Volts while running on plane. [The voltage] dropped with engine speed, but it worried me. Did I lose a [rectifier]?

jimh posted 05-02-2012 07:57 PM ET (US)     Profile for jimh  Send Email to jimh     
I don't think a rectifier has been lost.
Teak Oil posted 05-02-2012 09:57 PM ET (US)     Profile for Teak Oil  Send Email to Teak Oil     
The charging system on a V4 carb motor has no regulator, so it's normal to go over 15-Volts at planing speeds. Since there are only a few Amperes of output this will not hurt your battery. Also most electronics on your boat can handle 16-Volts without a problem.

My 90-HP V4 on my Montauk did the same thing

jimh posted 05-03-2012 08:22 AM ET (US)     Profile for jimh  Send Email to jimh     
Perform the procedure I described in

Mr T posted 05-03-2012 02:07 PM ET (US)     Profile for Mr T  Send Email to Mr T     
I have already pulled the tach and checked the pole position switch- I also moved to another position and set back to the original position with no change noted. In regards to the overcharging, the motor has been running in the boat for over 4 years without ever showing the voltage levels as high as the 15.9-16.0 volt range. What I find puzzling is the sudden change from a typical charging voltage around 14.2-14.5 volts to the current levels.
Teak Oil posted 05-03-2012 07:43 PM ET (US)     Profile for Teak Oil  Send Email to Teak Oil     
I would have someone check that voltage with a multimeter to verify the accuracy of the gauge while underway before you get too concerned
jimh posted 05-03-2012 08:20 PM ET (US)     Profile for jimh  Send Email to jimh     
In an unregulated battery charging system, it is often the battery itself that acts as the regulator. If the battery is in great condition it will tend to hold down the charging voltage as it absorbs the charging current. As the battery condition deteriorates there is a tendency for the terminal voltage during charging to rise. The internal resistance of the battery is probably higher than it was in the past.
Mr T posted 05-04-2012 10:44 AM ET (US)     Profile for Mr T  Send Email to Mr T     
Good advice, and thank you. I have already verified the voltage accuracy using a hand held mulitmter, and that number, measured at the battery terminal is correct. I feel there may be a [problem] with the two-year-old battery and have replaced it. I will perform another on the water test today and report back with the findings.
Mr T posted 05-08-2012 01:17 PM ET (US)     Profile for Mr T  Send Email to Mr T     
Update: I removed the suspect battery and had it tested at a local shop. It was deemed "worn out," as the tech stated. I replaced it with a new deep cycle and trolling battery. I am aware that [a deep-cycle batery] is not what should be in there for a starting battery, but that is what was in there and the battery may find additional use as an electric trolling motor battery in the near future. I also cleaned and checked the terminals and found that while not terribly dirty, they did need to have some oxidation removed. Water testing since the repair has shown the tachometer readings to be correct, and the voltage closer to the 14.3 to 14.4-VDC level I typically see while running.
jimh posted 05-09-2012 08:34 AM ET (US)     Profile for jimh  Send Email to jimh     
Good repair. The existing battery probably would have left you stranded somewhere this season.

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