OT:Alternators on Deisels - Off-Road Forums & Discussion Groups
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post #1 of (permalink) Old 12-27-2001, 02:26 PM
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OT:Alternators on Deisels

I know that the tack on a deisel runs off of the alternator but how exactly? I just replaced the alternator on my Deisel XJ on Saturday and now the tach is acting goofy. Its all over the place, sometimes bouncing at much as 1000 rpms. I discovered today that if I have the parking lights on it stabilizes it. So what I am wondering is does the tach work on the output of the alternator, as in the amps, or does it have some way of tell the rpms of the alternator? This would be a great help in diagnosing the problem. Any of you guys here that know deisels have any ideas? Thanks for your help.


I do the same thing every weekend, its just the location where I do it that changes!
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post #2 of (permalink) Old 12-28-2001, 07:20 AM
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Re: OT:Alternators on Deisels

BTT, now body has any ideas about this??

I do the same thing every weekend, its just the location where I do it that changes!
post #3 of (permalink) Old 12-28-2001, 11:25 AM
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Re: OT:Alternators on Deisels

I don't know the specifics of the diesel XJ and how the connection is made. "Alternator driven" tachs usually sense the fields flashing in the alt. Alternators produce three fields per rev so (# of pulses/3 = alternator RPM). The alternator is not spinning at the same rpm as the engine, it is "overdriven" by the belts, so that needs to be addressed by an adjustment in the tach itself.

My aftermarket tach on my Cummins has a small coil that straps to the alternator. When the field flashes the EM pulse washes across the coil and generates current just like it is doing in the alternator windings. The intrument (tach) is measuring frequency (hertz) of that current, not amps, so load should not affect the readings.

I don't have a clue what is wrong with your system, I just thought I would post what little I know about the operation of diesel tachs. Probably doesn't help you out much

September 11, 2001- We will not forget! We will not forgive!
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post #4 of (permalink) Old 12-28-2001, 08:41 PM
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Re: OT:Alternators on Deisels

Utah is close...
You do have three sets of windings in the stator, and those do carry the AC current produced to the rectifier.
But they change polarity each time a pole face in the alternator passes them, and that induces another 'pulse' of current...
So, it's the number of pole faces times RPM, then divide by three...
Count how many times the positive side 'Pulses'.... (and Utah is correct about Hertz!)

If you change pulley size on the alternator or crankshaft, you will screw up the count.

If turning on your parking lights smooths out the tach, you probably have a bad capacitor (usually on the back of or inside the alternator) or someone didn't get the old capacitor into/ on the new alternator.
Take the alternator back where you got it and see what they say... (if it's one of those discount 'Auto-Jerks' places, you already know more than they will...)

In the early days of electric tachs in diesels, if the tach quit working, it was a sure sign the alternator was on it's way out...
(Still is with Chevy diesels. I just lost an alternator in the dually the other day, and the tach quit working.)

So many cats, so few recipes...
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