Tachometrs for use with APC and Ford Lehman

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Tachometrs for use with APC and Ford Lehman

Postby johnrosendale on Tue Feb 16, 2010 3:23 pm

I'd like to know what tachs (digital or analog) may be used on the APC alternator to replace the original ones that came with my 1979 Ford Lehman 120?
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Re: Tachometrs for use with APC and Ford Lehman

Postby Coulomb on Wed Feb 17, 2010 11:30 am

Running a tach off the alternator requires adjustment to account for the sheave ratio of main to alternator.

From the Ample Power Primer: http://www.amplepower.com/primer/tach/index.html
"The frequency of signals from the alternator depends on the number of magnetic poles in the alternator. Ample Power alternators have seven poles in the small frame units and six poles in the large frames. These produce seven and six signals per revolution. "

If the alternator is spinning 3 times the engine, there will be 21 and 18 signals per engine revolution for small/large frame alternators.

Armed with the sheave ratio, and the signals per revolution, a tach supplier should be able to recommend a unit.
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Re: Tachometrs for use with APC and Ford Lehman

Postby johnrosendale on Wed Feb 17, 2010 12:08 pm

Thank you, that is good information.
I am troubleshooting why both my Tachs that are tied into an APC alternator failed at the same time. I had been running for about 3 hours, slowed down to an idle, and then noticed that my tachs had quit.
I have tested the alternator "field" and it shows 12.5 volts with the ignition on. I then tested voltage at the battery lug on the alternator and it showed the same amount. After starting the engine the voltage at the lug only came up about 1/2 a volt.
I have no idea how to test the tachometer terminal (next to the field terminal) for output. I did check the wires to the tachs from the alternator and they all seem fine.
Perplexed. Any input would be welcome.
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Re: Tachometrs for use with APC and Ford Lehman

Postby Coulomb on Wed Feb 17, 2010 12:24 pm

You may not be getting any charge. The tachs won't work if the alternator is not charging.
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Re: Tachometrs for use with APC and Ford Lehman

Postby johnrosendale on Wed Feb 17, 2010 1:01 pm

I think we're onto something... so, how would I know whether I am getting a charge or not? Does the extra 1/2 volt when the alternator is working count as a charge? How much charge should I be seeing at the battery lug on the alternator. If I'm not getting a charge is there anything I can do to get one? Do you service these alternators?

If I have to get a new alternator, what would you recommend given I'm on a tight budget. My typical boating experience is underway 4-5 hours between points at anchor. I may be at anchor for 2-3 days. One thing I would like to do is to bring my batteries back up to charge (about 1200 amp hours in batteries or 600 amp hours of charge) while at or near an idle. This would be the case if I decide to stay at anchor for over three days and need to recharge my batteries with my engine (Lehman 120).
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Re: Tachometrs for use with APC and Ford Lehman

Postby johnrosendale on Wed Feb 17, 2010 1:01 pm

I think we're onto something... so, how would I know whether I am getting a charge or not? Does the extra 1/2 volt when the alternator is working count as a charge? How much charge should I be seeing at the battery lug on the alternator. If I'm not getting a charge is there anything I can do to get one? Do you service these alternators?

If I have to get a new alternator, what would you recommend given I'm on a tight budget. My typical boating experience is underway 4-5 hours between points at anchor. I may be at anchor for 2-3 days. One thing I would like to do is to bring my batteries back up to charge (about 1200 amp hours in batteries or 600 amp hours of charge) while at or near an idle. This would be the case if I decide to stay at anchor for over three days and need to recharge my batteries with my engine (Lehman 120).
johnrosendale
 
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Re: Tachometrs for use with APC and Ford Lehman

Postby Coulomb on Wed Feb 17, 2010 3:06 pm

You need an ammeter in series with the alternator output to know how much you're charging. The half volt may indicate charging, but not how much.

There are quite a few Grand Banks using the Ford Lehman engine with high output alternators and Smart Alternator Regulators, and generator time has been significantly reduced in usage similar to yours.
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Re: Tachometrs for use with APC and Ford Lehman

Postby johnrosendale on Thu Feb 18, 2010 8:32 am

I did some more trouble shooting on my tachometer problem last night (as you know, they had quit working) and found my voltage at the field terminal on the alternator was 14. I figured this is why the tachs don't work; the alternator isn't charging becuase the batteries don't require it and subsequently the tachs don't get any juice. So I started up the enginge and tested the battery lug, expecting no charge, and instead found the alternator to be charging at 17 volts - that seems high. And the tachs were still were not showing anything. Do you think I have a problem at the alternator with 17 volts showing? Could this be a problem with my 3-step voltage regulator? Could I have burnt the tachs out with that much voltage? I remain perplexed.
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Re: Tachometrs for use with APC and Ford Lehman

Postby Coulomb on Thu Feb 18, 2010 11:40 am

You have more than one problem. The voltage is too high, pointing to the regulator, but with that voltage the tachs should be getting a strong signal.

Run the Trouble Shooting Guide on the regulator. http://www.amplepower.com/trouble/index.html
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