calibrate tachometer

[Bobg]

Has anyone ever calibrated the tachometer, my boat has been slowing down, I purchased a hand held lazer rpm tester, put the reflective tape on the pully on the crankshaft, it reads a couple hundred rpms low on the tach. vrs. the lazer tester.  I don't know what would make the tach start to read low, anyway, I got all winter but if I take the tach off, does it have to be sent out? or can it be done on the boat,  thank you  Bob

[Craig Illman]

Start here: http://c34.org/wiki/index.php?title=Tachometer_Malfunction_Q%26A

then this one: http://www.c34.org/manuals/S024.pdf

[Ron Hill]

Bob : Don't think your tach is slowing down, it probably was always a bit off.   

I wrote a mainsheet tech note article recommending that each C34 owner should use a laser tach and calibrate the tach in the engine panel at a number of different RPM settings.  Think I used 1500, 2000, 2200 and 80% power RPM. 
Put the laser readings on a small cut sheet with the actual reading across from them. Then you can read the engine tach and interpolate if necessary to know actual RPM. 
It's much easier and more accurate than screwing around with the tachs' dip switch settings!!

A thought

[Stu Jackson]

Ron's advice is good.  It's essentially doing a deviation table for your tach.  For example, if your WOT is only 2800 instead of the 3000 it should be, then just deduct that percentage from all the rest of the readings.  1400 would be 1500 actually.

That's one way.  The other is to make sure your belt is properly tight, not too tight, and it isn't slipping all the time.  I've always asked myself when the last time I changed the belt was, and in my case it's about time I did again!!!     

[Roland Gendreau]

I calibrated my tach using the Teleflex document that Stu sent out.  The procedure is pretty straight forward.  My alternator had been changed out by the PO (from standard to high output)  and the tach was no longer accurate.    It was not difficult to determine the proper dip switch settings.   Afterwards,  the indicated speed at idle was a little below 1000 and the speed wide open in gear was about 3200; both seem about right to me.

By chance, was your alternator changed out?

[Bobg]

Thanks guys, yes i did change my alternator, went from the old motorola to a delco 90 amp, maybe that's what happened, seems like it happened gradualy tho, I have gotten used to my engine sound, when I rev it up to 2500 via the laser, sounds like my engine is racing, but I do get back to around 5.5 knots like it used to be.  When I bought the boat, there was a sticker that said I should limit the rpm to 2200, boat has never been fast, but was always around 5.5 knots.  Ron, when you say 80%, should I use the laser and run the boat at full speed in gear at the dock, then take 80% of that for my maximum cruising speed?

[KWKloeber]

it's about time I did again!!!     

And use a Dayco Top Cog construction for best results, least wear/dusting, and coolest running!! 

JTSO
kk

[Stu Jackson]

1.   When I bought the boat, there was a sticker that said I should limit the rpm to 2200

2.   should I use the laser and run the boat at full speed in gear at the dock, then take 80% of that for my maximum cruising speed?

Bob,

1.  Where was that sticker?

2.  WOT out of gear should be 3,000.  My engine does that both in and out of gear.

[Ron Hill]

Bob : I seriously doubt that changing the alternator itself is what made the difference.  The change was the different pulley on the new alternator!!!! 
If you understand the dip switch settings, you'll see that there isn't a straight line curve between RPM settings.  That's why I mentioned a few settings.  Check those settings I mentioned and you'll see that the isn't a constant difference

Why your boat had a sticker about 22oo rpm is unknown??  I believe that your engine is listed as max rpm at 3200 rpm. 
Most engine manuals state that diesels like to run at 80% power so that would be 2560 rpm for a max of 3200.

A few thoughts

[Roland Gendreau]

Ron

Certainly a difference in pulley size will affect the tach;  but alternators sometimes have different number of poles, which also affect the tach calibration. Since the new alternator is a higher capacity than the old one, it may also have a different number of poles. In addition to the pulley measurements, you need to know the number of poles in order to use the Teleflex calibration procedure.

[Bobg]

should have been more plain on that sticker, the sticker was put on by the po and put under the tach.  So, if I go by my laser hand held, I should registrar how fast my engine runs at peak rpm, multiply 80% of that, note what my tack says, and use that figure for my normal underway speed.  I believe my tack would read like 2700 or so when my handheld reads 2500, seems like a big difference, and the engine seems to be racing, but will have to get used to it.

[Ron Hill]

Roland : You are correct on the number of poles makes a bunch of difference.
 
That's why I wrote the Mainsheet tech note article in the 1990s on How to set/reset the dip switch settings on your Teleflex tach to match the alternator!!

[Ken Juul]

IIRC the location of the tach needle can be adjusted using a small screwdriver through a hole in the back of the tach. Pick an rpm, match the needle to the hand held tach, double the engine speed, readjust.  Took about three tries to get most needle positions close.  In the days before infared tachs, people used to use a florescent light instead.  Details are in the Tech Notes, wiki, or search the board.  Maybe all three.