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Old 09-11-2006, 12:53   #1
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Alternator driven Tachometers, need help

The Tach on my diesel is alternator driven. It works fine after cranking the engine and letting it idle then if I rev it up the tach drops to zero. Its as though once the batteries get up to snuff the alternator stops charging and the Tach stops reading. This is just my analysis and this may NOT be what is actually happening. But I would like some help as to how to overcome this problem.
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Old 09-11-2006, 12:55   #2
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yep, that's what happens. Turn on some DC loads and your tach will return
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Old 09-11-2006, 12:57   #3
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Is there any way to make it work without turning on loads that are not needed??
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Old 09-11-2006, 12:58   #4
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altenators

This has been discussed in datail in this thread too.............enjoy

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Old 09-11-2006, 13:00   #5
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no. you're alternator seens no need to work at that time. You're getting free power so use it up. You cauld change to a different type of tach, one that would count flywheel teeth, or an optical sensor on the crank pulley. Really, just keep the system simple and turn on a cabin light or the always needed VHF
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Old 09-11-2006, 13:10   #6
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Tach response

I'll be a contrarian, and say it is certainly possible, but I think it is unlikely the cause is as you describe. And there is certainly not enough information in your post to decide one way or the other....

Some diagnostics for you:

Is the alternator internally regulated? (An alternator without a "smart" regulator will never drop its output so low that the tach wouldn't work)

If you rev the engine and the tack goes to zero, does it resume working if the rpms drop again? This would indicate that the belt is slipping under the heavier load of higher RPMS)

If you start the engine and rev it up within a few minutes, does the tach response fail? (output even from smart regulators is high for a default period right after start up.)

Finally, you can test your proposed cause by setting things up so the tach is not working, and then turning on something that draws current. This would force the alternaor to be energized. If the tack still stays off, you should look for the problem elsewhere.

I'd bet the problem is with the belt. Once they start slipping at higher rpms, they can lose "bite" on the pulley and keep slipping.

Bill
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Old 09-11-2006, 13:11   #7
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Thanks, that sounds good enough.

Now on the same subject, I have something to offer in return for the gems of whisdom I just recieved.

This tach was NOT made for this boat/engine/alternator. I picked it up at a used boat stuff shop because it was cheap ($19) and I needed one. It has a potentiometer on the back to adjust it. This pot is there to adjust for alternators with different numbers of poles..is my guess. There was not enough adjustment to make it work with my alternator which apparently has a rather small number of poles. So I removed it from the case and found that internaly the pot was wired as a variable resistor (one end tied to the wiper) and had an 18K resistor in series with it. The pot was variable from 0 to 50K so as a starting point I shorted out the 18K resistor and lo and behold the calibration point is now about 1/4 turn of the pot. Works great and a lot cheaper than $150 for a new tach.

Now a question. What is the easiest (read fewest fancy tools) way to calibrate the tach. Right now I have assumed the idle speed on my Perkins 4-108 to be about 600 rpm and have set it thusly but I'll bet the idle is slower than that.
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Old 09-11-2006, 13:24   #8
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I had the same issue when I had to calibrate my new tack. I went on ebay and bought an inexpensive optical laser tachometer. It has come in handy a number of times for other things.
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Old 09-11-2006, 13:55   #9
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I'll bet your idle is 600 if not as high as 900. If not it should be.
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Old 09-11-2006, 14:12   #10
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OK, then, I guess I will get a laser tach and get it right.

BUT...turning on loads did not help. It still will die when the engine is raised above idle. The belt is not slipping as it had recently been adjusted because of the cooling problems I was having.

I guess I either need a new Alternator or a new sensor for the rpm. Maybe both. I have been thinking of replacing the 60 am alternator with a 150 amp alternator anyhway so I can run an AC off an inverter.

This alternator has an external regulator that was added in Tampa by the mechanic when he was trouble shooting the lack of charge. The alt had NO regulator but according to the mechanic was supposed to have an external one...a small black plactic box bolted to the back of the alternator that has one wire coming out of it and connecting to a large terminal on the back of the alternator case..same terminal the tach is connected to.
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Old 09-11-2006, 14:43   #11
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Just bought one...$33..... now if I can just get the tach to read continously..#8-)
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Old 11-11-2006, 16:24   #12
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Got the Tach working...all the time...but still need to calibrate it.

The problem was that there was no AC terminal on the back of the alternator so the PO had connected it to another terminal on the regulator. So it worked SOME OF THE TIME. I got some advice from an alternator guru and soldered a wire to the end of one of the diodes that could be seen thru an opening in the back of the alternator and led that to the Tach. Works perfectly every time now!!! As soon as the pocket laser tach gets here I will be able to calibrate it.

Gonna do the same thing on the Volvo 2003M that is in my Albin 25 trawler. That will kill two birds with one stone. The Tach on the Albin has never worked.
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Old 11-11-2006, 17:57   #13
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That Volvo uses a SEV Marchal alternator. You're beter to swap it for a cuality one. There should already be an AC tap post on it too.
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Old 11-11-2006, 20:46   #14
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Quote:
Originally Posted by never monday
That Volvo uses a SEV Marchal alternator. You're beter to swap it for a cuality one. There should already be an AC tap post on it too.
Thanks for the information. I didnt know there was an AC post on it. I will look more closely and see if I can get it going.
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