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 Post subject: Tachometer calabration?
PostPosted: 09 Jul 2007 15:14 
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Midshipman

Joined: 05 Feb 2007 22:32
Posts: 74
Location: Cheaspeake Bay - Herrington Harbour South
Looking for recommendations for a company to recalibrate the original tachometer, preferably in the Annapolis area?

Thanks,
Glenn

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 Post subject: Re: Tachometer calabration?
PostPosted: 01 Oct 2007 13:39 
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Midshipman

Joined: 30 Oct 2006 15:40
Posts: 63
Don't spend any money on this task. There is no calibration on a diesel tach. It is driven by electrical pulses from the alternator.

If the tach is wrong, it has to be the alternator is being driven at the wrong speed because someone has messed with the sizes of the pulleys. Or there is a possibility there is a wiring fault, but I think this would be an intermitent problem.

What are the symptoms that make you think it needs calibration?

Jerry
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 Post subject: Re: Tachometer calabration?
PostPosted: 01 Oct 2007 15:05 
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Midshipman

Joined: 05 Feb 2007 22:32
Posts: 74
Location: Cheaspeake Bay - Herrington Harbour South
At idle it's showing 1000 and stalls. We tested it with a hand held laser tach and it showed 600.

Anything over 1800 it swings rapidly between 0 & 4K, after an hour or two it'll settled down, but seems to be reading higher than the engine sounds to be running. Reading 3000 sounds about 2250-2500 to me and other experienced crew.

We've not have the opportunity to try the handheld other than at the dock, as it belongs to the Hunter user group. At dock with the tack swinging we had to run her allot higher than our normal to get it up to 3000 rpm, leading me to believe the 2250-2500 is about right.

If in fact the RPM is determined by the alternator, that may explain the incorrect reading, as it was updated with a high output Balmar by the PO. The swinging needle is probably a bad electrical connection. checking the wiring is already on the winter project list...

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 Post subject: Re: Tachometer calabration?
PostPosted: 01 Oct 2007 22:34 
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Able Bodied Seaman

Joined: 31 Oct 2006 19:43
Posts: 37
The following are 2 quotes from the internet but if you search yourself you will find the problem is widely discussed. Notice that Balmar uses 14 poles while the original uses 12 poles that means that the tach is sensing more pulses so it thinks the engine is running faster than it really is. Also some tachs are fitted with a means to "calibrate" the speed shown. On Windfall there are several dip switches on the back of the tach and there was a chart supplied that detailed the settings for different drive ratios and pulses.
I wll try to copy a second web quote that deals with the erradict needle movement. I didn't get it but the point was that some voltage regulators will shut down the alternator output so completely, that the tach gets no pulse untill the regulator starts the charge up again. from the web sites I looked at, it seems that some regulators are worse than others. Many diesels have a magnetic pickup off the flywheel for tach signal so the alternator would not be used.

Sam


from the internet --- Geoff Johnson

05-20-2003, 01:36 PM

Having just installed a Balmar alternator and regulator I can guarantee you that with a high output alternator the rpm's you are reading are not the rpm's you are running. Even with my original Delco OEM alternator, the tachometer would read 2200 rpm when the actual speed was 2000 rpm as measured by a handheld tachometer. Now the panel reads 3000 rpm's when the actual speed is 2350 as measured by a TinyTach (which measures fuel pulses through the injector tube.)


also from the web---

Tachometers

The alternator tachometer is energized by the pulse frequency generated by the alternator, which in turn is dependent on the alternator's rotor

speed. Large case BALMAR alternators (94,95,96, series) have 12 poles and the small case (90/91/912 series) have 14 poles. Model 96-24-

220 alternators, as well as all 97 and 98-Series alternators are configured for 16-pole operation. Most standard tachometers are engineered for

engines with alternator pulley to crankshaft ratios of 1.8 to 2.8 to 1 on alternators with 12 poles.

The WHITE wire or stator terminal may be used for electric tachometers not having their own sending unit. Tachometers will have to be adjusted

and calibrated as necessary. We suggest running the engine up to a cruise RPM (2000), marking the throttle position BEFORE the original

alternator is disconnected. After the installation is complete, run throttle up to mark and set the tachometer to its appropriate RPM. If you cannot

set your existing tachometer, a programmable tach like the Teleflex Model 82430P should be considered as a replacement. Should bouncing of

the tachometer be observed when the batteries are fully charged, increasing the DC loads will often cure the problem.


 
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