[C320-list] Tachometer Problem - Ring Gear/Sensor Gap

[Robert E. Sloat]

Scott-You are right.  The clearance is adjustable and Yanmar manual says the 
gap between the tachometer sensor and the ring gear should be 1.3 mm (0.0511 
in.).  It also says the sensor must not be loose fitted, so if it moves 
around that may cause problems.

There is a test of the sensor using an oscilloscope which is difficult to 
explain.

One easily measured spec. for the sensor is its internal resistance at rest 
which is 1.6 k ohms + or - 0.1 k ohms.  If the resistance across the two 
pins on the sensor which connected to the wires going to the tachometer is 
not that then the manual remedy is to replace the sensor.

Bob Sloat
Savannah 894
----- Original Message ----- 
From: "Scott Thompson" <surprise at thompson87.com>
To: <C320-List at Catalina320.com>
Sent: Saturday, July 11, 2009 8:32 AM
Subject: Re: [C320-list] Tachometer Problem


>I don't have my manual handy.  Is the sending unit adjustable?  It occurs 
>to me that clearances between the flywheel gear teeth and sensor might be 
>off on my boat if it is.  I've got erratic and low tach readings that I 
>haven't been able to track down.
>
>
> Robert E. Sloat wrote:
>> Larry-Good pictures and what you have concluded matches the Yammer 
>> Service Manual for the 3GM30F.  The number of teeth on the ring gear for 
>> the 3GM30F is 97.  For the 3HM35 version of this engine which I think is 
>> the sail drive model, the number of teeth is 114.  According to the 
>> Manual, the basic tachometers and the sending units for for either ring 
>> gear are the same so the black dial looks like it is used for switching 
>> to either ring gear probably.  This is not shown in the Service Manual 
>> and it doesn't mention the calibration screw.
>>
>>> From your pictures it is clear how to dial in a working the tachometer 
>>> if it
>> is off from actual RPMs measured at the crank shaft pulley.
>>
>> Bob Sloat
>> Savannah 894
>>
>> ----- Original Message ----- From: "Melissa&Larry Leibman" 
>> <theleibmans at verizon.net>
>> To: <c320-list at lists.catalina320.com>
>> Sent: Friday, July 10, 2009 8:14 PM
>> Subject: [C320-list] Tachometer Problem
>>
>>
>> Jamie,
>>
>> You may have already rectified the tach issue but if not, I have been 
>> having the same behavior for several months. I finally got around to 
>> checking into it beginning at the tach sender on the instrument housing 
>> and working up to the tach itself. I've uploaded a few pictures at:
>>
>> http://www.catalina320.com/mediagallery/album.php?aid=214&page=1
>>
>> It appears at least on Easy Going that the problem was at the tachometer. 
>> If you check out the picture at the link below, there is a calibration 
>> adjustment on the left side of the picture but on the right side is a 
>> kind of blurry black dial adjustment. I believe that adjustment allows 
>> you to set the number of teeth in the pinion gear. From the labeling 
>> around the dial I expected it to have 3 positions. However, it just turns 
>> continuously with no obvious detents. As the engine was running I 
>> adjusted this dial and at certain points the tach would jump much like I 
>> would observe while running. I adjusted within the first range/position 
>> until the tach appeared to give a stable reading. I figured I would also 
>> need to adjust the calibration screw but when I rechecked the tach with a 
>> laser tach, the readings appeared to match within the precision of the 
>> tach in the cockpit. I then ran the engine at several rpm settings and 
>> all seemed to agree well with the
>> laser tach. Previously my laser tach gave readings 100-400 rpm lower than 
>> the cockpit tach depending on the speed.
>>
>> http://www.catalina320.com/mediagallery/media.php?f=0&sort=0&s=2009071017503760 I 
>> haven't had a chance to take the boat for a spin so I'm not certain if 
>> the problem is really solved but so far so good.
>>
>> Larry
>
> -- 
> Scott Thompson
> Surprise, #653