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Author Topic:   Tachometer: Diagnosing Malfunction
wbullwin posted 04-27-2009 11:08 AM ET (US)   Profile for wbullwin   Send Email to wbullwin  
[Occassionally] [the indicator on] my tachometer starts [moving upward] for no reason, [i.e, there is no change in engine speed]. I have the throttle set, going along on smooth water, and all of a sudden, [the reading on the tachomoter] starts going from 3,600-RPM to 4,250-RPM, to 4,000, to 4,200, and eventually settles back down to 3,600. The engine noise does not change so I know the [speed] of the engine is not changing, just the reading. The engine is a 2003 Mercury 90-HP two-cycle on a 2003 170 Montauk. Is [the cause of this problem the] tachometer or the sender? Has anyone ever seent this problem?
Tohsgib posted 04-27-2009 11:38 AM ET (US)     Profile for Tohsgib  Send Email to Tohsgib     
If an OMC I would say your regulator or rectifier is starting to go. Not sure the same about Mercs. If starts to get real
Buckda posted 04-27-2009 11:56 AM ET (US)     Profile for Buckda  Send Email to Buckda     
On my 1985 Merc 150, this was a symptom that was resolved by replacing the rectifier - a $30 part that attaches via 2-3 screws directly to the cooling jacket on the powerhead. Unplug the old one, plug in the new one, re-install the screws and you're back in business.

Does anyone have a procedure for actually testing if the rectifier is bad? I believe that you'd see excessive voltage on a meter, but your battery may be absorbing that.

Tohsgib posted 04-27-2009 12:13 PM ET (US)     Profile for Tohsgib  Send Email to Tohsgib     
Yes your volts will be high.
jimh posted 04-27-2009 09:10 PM ET (US)     Profile for jimh  Send Email to jimh     
I have seen this problem several times, once or twice in person, and often reported here in the discussion areas.

Try this remedy:

--Locate the adjustment control on the rear face of the tachometer. It will usually be marked in several steps with notations like 2,3,4,5,6, which are numbers that refer to the poles in the alternator.

--Make a note of the current position.

--Using a screwdriver of the proper size, move the adjustment or calibration control back and forth through its range of adjustment. Do this at least five times.

--Return the calibration control to the original setting noted above.

Report the situation with the tachometer reading stability after performing this procedure; I will make additional suggestions.

ASIDE: This topic was originally titled "Floating Tachometer." I though the use of floating in regard to a boat was particularly confusing. Most tachometers will float, briefly, but I would not expect them to retain buoyancy for long.

wbullwin posted 07-21-2009 04:14 PM ET (US)     Profile for wbullwin  Send Email to wbullwin     
[Moved from a new thread begun in a different discussion--jimh]

I finally had time to look into this last week and saw that the tachometer was set between settings. There are various positions that the screw can be turned to, but mine is not exactly aligned to one of the numbers. Are the tachometers tuned to each engine? Or, is mine just not set right? I didn't touch anything until I got more information.

number9 posted 07-21-2009 07:28 PM ET (US)     Profile for number9  Send Email to number9     
The switch position depends on motor manufacturer and model. Sometimes the marks don't line up but usually the switch points closer to the mark for your application.
jimh posted 07-21-2009 10:55 PM ET (US)     Profile for jimh  Send Email to jimh     
ASIDE: I moved the latest replies back to this thread. They were posted as a new thread in a new discussion area. Please use your original discussion thread to post a follow up.

The calibration controls are switches with detents. They are not continuously variable. Perform the procedure I recommended above. This ought to provide a remedy if the problem is with the calibration switch contacts having become corroded or intermittent.

Marlin posted 07-22-2009 06:38 AM ET (US)     Profile for Marlin  Send Email to Marlin     
I had this same problem with my tach (on my 2003 Mercury 115 4-stroke) briefly last year, when it just went away, then again this year at the Manhattan rendezvous. I wondered why my engine was running so hard, but I just put it down to a heavy 5-person load and a full tank of gas. Not until the next outing did I realize it was the tachometer.

I used Jim's technique exactly, and the problem was immediately resolved.


jimh posted 07-22-2009 08:28 AM ET (US)     Profile for jimh  Send Email to jimh     
Bob--Glad to hear of your success!

Regarding testing of a rectifier--The best way to test a rectifier is to remove it from the circuit. This is not possible if the rectifier is part of an assembly with the regulator, as is often seen on OMC motors. On smaller motors, particularly Mercury motors, the rectifier is a separate component and can be easily removed from the circuit for testing. You have to remove it to test it because otherwise the low resistance of the stator windings and the bias voltage of the connected battery will make measurements difficult and confusing.

Most rectifiers are FULL WAVE BRIDGE configurations. This is a four-terminal device:

Inside the rectifier there are four diodes. By using a multi-meter each diode can be tested more or less independently of the other three, if all diodes are working properly. A shorted diode can affect the other diodes.

To test, the diodes from the A and C terminals to the (+) terminal should show forward bias with a positive voltage at A or C. The diodes from A and C terminals to the (-) terminal should show forward bias with a positive voltage at (-). It's harder to explain in a narrative manner than to actually test. Generally, if someone understands how diodes work, they'll know how to test a full-wave bridge rectifier.

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