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Author Topic:   Battery and Tachometer
EJO posted 05-20-2015 01:40 PM ET (US)   Profile for EJO   Send Email to EJO  
Finally took boat out for shake down before the Memorial weekend. [The engine] wouldn't start even after I fully charged the battery over the previous days. I always have a jumper pack, so [the engine] started fine. The battery is old. I just will replace it with a new one.

The other problem I noticed was that my tachometer was 800 to a 1300-RPM off, indicating too high, that is, my idle speed showed 1,800-RPM on my Mercury tachometer.

I checked my speed with GPS and [at full throtlle the boat speed was greater han] 34-MPH, but [the engine speed] showed completely maxed out on the tachometer. The [engine] normally runs 5,900 to 6,000-RPM at full throtle [with a boat speed of] 35-MPH. Thus I knew [the engine speed] was fine. My Mercury 60-HP Bigfoot maxes out at 6,000-RPM, so I know it wasn't running [at] 7,500-RPM. I would guess as my tachometer only goes to 6000-RPM.

I asked couple of guys from the local marina what could [cause] my tach to be off so much. They said you can calibrate the tachometer. Can a bad battery influence my tachometer? My engine alternator was putting out 14.1-Volts during charging. If you can calibrate the tachometer, how do you do it?

ANDREW PITCHFORD posted 05-20-2015 05:01 PM ET (US)     Profile for ANDREW PITCHFORD    
I have seen this happen, when [the ignition key switch is moved from the] off position to start with no pause. Next time, turn to run position, pause [until a] beep is finished, then start. [This procedure] works every time for me. Always bothered me when the tach was off 1000, [until] I realized what was happening. When [the power to the tachometer is cut the dial reading] stays at about 800-RPM When [power is applied when the ignition key is turned] to RUN position and the beep ends, the tachometer will go to 0-RPM
EJO posted 05-21-2015 03:03 PM ET (US)     Profile for EJO  Send Email to EJO     
I did notice what you are talking about, but it didn't seem to solve my problem. I just bought a new [Group] 24 marine starting battery. I will clean all the tachometer contacts. The battery contacts were cleaned when installing after the winter. Next time when in the water will turn the key slower. I wish it was this weekend but [the weather] will be cold and wet above the water here in SW Michigan. Typical: another Michigan long-weekend ruined by weather.
jimh posted 05-22-2015 07:34 AM ET (US)     Profile for jimh  Send Email to jimh     
Generally I do not think that a tachometer reading would be affected by the battery voltage. A tachometer measures the frequency of pulses of pulsating direct-current rectified in the alternator. It does not measure the voltage.

Some tachometers designed for after-market replacement applications include an adjustment to set the calibration of the tachometer to match the frequency range of pulses from an engine. The calibration control is typically located on the rear panel of the tachometer. The control usually is in the form of a small rotary switch with detents for several settings. The settings correspond to the number of pulses from the alternator per revolution of the crankshaft.

If a tachometer has such an adjustment as I describe above, it is very common that the reading of the tachometer will go out of calibration due to slight corrosion or other electrical circuit problems in the selector switch.

In a prior article I describe a procedure for diagnosis of problems with the tachometer. Please see

Diagnosis of Engine Tachometer Problem
http://continuouswave.com/ubb/Forum6/HTML/003695.html

and perform the procedure I describe in that article.

It is common for most tachometer dial pointers to not point to the 0-scale on the dial face when there is no power applied to the tachometer. I do not believe that this is part of the problem described in the initial article in which an error in tachometer reading at high engine speeds is reported. Also, I do not believe that adding a time delay in the operating of the ignition key turning from OFF to RUN to START will affect the calibration of the tachometer. The calibration of the tachometer is fixed to correspond to the number of pulses per revolution of the crankshaft, and the amount of time that the engine receives power when not running to when the engine is cranked over for starting has no effect of that calibration.

EJO posted 05-22-2015 09:54 AM ET (US)     Profile for EJO  Send Email to EJO     
Jim as always thank you for the detailed reply which is what I was looking for.

I did install a Group-24 battery last night, which gets harder and harder with my size and the small center console door. I never understood why Boston Whaler didn't have a larger access door at the front of their center console or even an hinged option.

I used contact cleaner on the tachometer and checked the connections to make sure I had good contact. I [connected] the water hose and started the engine. Tachometer was still not reading correct. I knocked a couple of times on the cover and she sprang back into place. I agree with you: the switching ON-OFF or slow turning the key doesn't influence it, as I always wait for the beep to finish before cranking.

I'll see if I can get my head through the door to see the calibration screw and move it back and forth.

Looks like you'll have a little better weather on the east side of the state. Enjoy your Memorial weekend.

jimh posted 05-22-2015 11:08 AM ET (US)     Profile for jimh  Send Email to jimh     
Re tapping on the tachometer restored calibration: this suggests the calibration switch exercise I recommended may help.

ASIDE re Memorial Day--we seldom--if ever--go boating on Memorial Day due to the increase in traffic on the water and at the boat ramps. I have to work on Friday and Sunday. No plans here. I still have the boat in storage, and probably won't get it out until the first week of June, nor in the water until the second week of June.

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