Author Topic: Alternator wiring knowledge  (Read 1387 times)

John Fitzgerald

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Alternator wiring knowledge
« on: November 26, 2021, 04:41:55 PM »
On my 1995 Marquis, Cummins 8.2 I have been troubleshooting an inoperative tach.  I found that the tach is supplied the signal from the alternator AC.  I have the original wiring diagram from Gillig, which shows a wire from ignition to the IGN terminal, the tach to one of the three AC terminals, and a jumper from alternator + to the DUVAC terminal.  My coach had the tach wire on the DUVAC terminal and no jumper.  Any thoughts before I move ahead to 'correct' the wiring to the original drawing?  By the way the alternator has been replaced, and the install was rather shoddy with an incorrect bolt at the top, and dangerously loose output wire that was almost worn through and short to the block.  I see no original wire that would have been connected at the DUVAC terminal that would originate at the battery side of the isolator.

Thanks

Eric Maclean

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Re: Alternator wiring knowledge
« Reply #1 on: November 27, 2021, 03:17:08 AM »
John
Are you sure the wire connected to the DUVAC Terminal is the tach lead.
The DUVAC terminal is a remote voltage sensing wire for the alternators regulator the wire connected to it should have 12 volts +
And is connected to the alternator out put with a jumper wire or connected directly to the battery bank positive or to a voltage sense terminal on a DUVAC isolator setup for dual voltages.
If both your battery banks are 12 volts the DUVAC or S terminal on the alternator can be connected to the 12+ output of the alternator or extended to connect directly to the battery bank for a more accurate voltage control of that battery bank

The tach in your dash uses an AC signal to count RPM this wire should be connected to any one of the AC  stator taps on the alternator.

Hope this helps
Eric
1997 Patriot Yorktown
3126-B
2009 Chevy HHR
Roadmaster falcon tow bar
Demco Air Force one tow brake.

John Fitzgerald

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Re: Alternator wiring knowledge
« Reply #2 on: November 27, 2021, 12:43:20 PM »
I am fairly sure it is the tach wire, it is the correct color and the wire number matches the Gillig original diagram.  I will ring out the wire today with a meter.  It is one of those things where it appears someone replaced the alternator and didn't really understand the correct set up.  I did not understand the DUVAC idea until I did a fair amount of reading into the matter.  The alternator that I have in my coach seems to be a knockoff, and has no markings on it except + & - on the main lugs.  I agree with what you wrote, the reason I started down this road was that the tach has not worked since I bought the coach last May and the miss-wiring makes sense from that aspect.

thanks

Eric Maclean

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Re: Alternator wiring knowledge
« Reply #3 on: November 27, 2021, 02:52:09 PM »
John
Are the "S" and "I" terminals not  marked

Eric
1997 Patriot Yorktown
3126-B
2009 Chevy HHR
Roadmaster falcon tow bar
Demco Air Force one tow brake.

John Fitzgerald

  • Guest
Re: Alternator wiring knowledge
« Reply #4 on: November 27, 2021, 04:13:27 PM »
Thanks, it have it all set to go back together but it seems the upper mount needs a metric bolt.  I found that a 1/2" bolt fit the upper hole in the alternator, but now that I bought a long enough bolt to go through both upper mounting bracket holes it seems that won't go and the bracket on the engine measures 12mm hole diameter.  It looks like I am shopping for a new bolt.  The original bolt was a 3/8" that only was in the front most hole of the upper mount, the alternator was not installed correctly when I bought it.

thanks