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QT 16 Got A Dead Short


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#1 chieffan ONLINE  

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Posted July 22, 2016 - 05:02 PM

Put the Onan motor in the QT 16 yesterday.  Fired it up and got it running real decent.  Drove it about 30 yd up the drive and back, no seat or fenders yet.  Put the fenders, side panels and hood on this morning.  After dinner I took it out to do a little clean up after a new AC install.  Went about 20 yds and it started to cut out and back on again.  Continued this till I got the trailer hook up and back up to the shop.  There it died.  When I turn the key to on the Amp gauge shows a dead short.  When I unplug the reg/rect. the short goes away.  Same if I unplug the alternator from the engine.  Tried to start it with the alternator unplugged but all it would do is click.  Solenoid check good.  Question is what ohm reading should one get between the two yellow wires from the alternator?  Also what reading between either yellow wire and ground?

 

Thanks much for the help.  The Service manual don't go into much detail on checking the electrical system.



#2 KennyP ONLINE  

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Posted July 22, 2016 - 05:05 PM

Which Onan is that?



#3 chieffan ONLINE  

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Posted July 22, 2016 - 06:54 PM

16hp BF-MS


Edited by chieffan, July 22, 2016 - 07:05 PM.


#4 DougT ONLINE  

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Posted July 22, 2016 - 08:08 PM

I don't think you should have any continuity between The yellow wires and ground. Not sure on an actual ohm reading between the 2 but they should show continuity. They should put out 28v AC with the engine running. Not to be a smart a$$ but I'd take a real close look at the panels you installed and make sure there isn't a wire caught under one.


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#5 chieffan ONLINE  

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Posted July 22, 2016 - 08:52 PM

I don't think you should have any continuity between The yellow wires and ground. Not sure on an actual ohm reading between the 2 but they should show continuity. They should put out 28v AC with the engine running. Not to be a smart a$$ but I'd take a real close look at the panels you installed and make sure there isn't a wire caught under one.

I thought of that too and removed the right panel where the wiring goes..  Wondering now if possibly the alternator wires got pinched with the engine going in.  Real close under and around the starter.


Edited by chieffan, July 23, 2016 - 05:36 PM.


#6 chieffan ONLINE  

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Posted July 23, 2016 - 11:45 AM

Think I found the problem.  B+ in the rectifier/regulator is shorted.  Have 27.9 V AC coming out of the engine charging system.  Removed the B+ wire from the plug at the rectifier/regulator and put a empty terminal in the plug so I could get a probe to it.  Have nothing coming out and the short is gone with that wire unplugged.  I have another one that was on the Suburban but I think it is bad also.  Has melted spot on the back side..  Will find out later.


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#7 DougT ONLINE  

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Posted July 23, 2016 - 11:50 AM

Sounds like you are on the right track. The reg shorted to ground internal.  One other thing, you won't get voltage out of the center wire without the battery supply there.


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#8 chieffan ONLINE  

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Posted July 23, 2016 - 02:14 PM

Sounds like you are on the right track. The reg shorted to ground internal.  One other thing, you won't get voltage out of the center wire without the battery supply there.

Doug, not sure what you mean by center wire?  Two yellow wires from the alternator, full throttle shows 27.9 V AC.  Those two go into the Rect/ Reg.  The two AC in wires are side by side.  The one below (or above, depending which way it is turned) is the B+ out.  Should have about 13-14 V DC with engine running.  Both of my rectifier/regulator show a dead short between B+ and the one AC input terminal.  Same terminal on both units.  Internal short.  Have a new one ordered.  The thing that puzzles me is the Suburban tractor that I got the Onan out of was running with the rect/reg hooked up.  PO said it would not keep the battery charged.  Hum-m-m-m-m-m ?



#9 DougT ONLINE  

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Posted July 23, 2016 - 02:24 PM

OK, then the battery positive should have voltage on it to make it charge. For it to be a dead short, I'd think the bat + would have to be shorted to the case of the regulator. If you reverse the leads of the ohm meter do you still have continuity or does the continuity go to the other wire? As far as I know, all the 3 wire regs work the same. There are just diodes in there that separate the high pulses of the AC and send them to the battery without the backfeed. I found a diagram of one a few years ago when I was having trouble. If I remember there were 3 diodes to do the job.



#10 chieffan ONLINE  

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Posted July 23, 2016 - 03:21 PM

OK, I follow you now.  I did not reverse the probes to see if anything changed.  Will do some more checking and probe reversing and see what happens.  The unit off the suburban has a soft melted spot in the epoxy that covers the components on the back side.  Probable why it don't work.  Will check some more later when it cools down.  Just looked and it shows 101 in the shade one one thermometer.  Weather stations shows 93 and it is out in the open.



#11 chieffan ONLINE  

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Posted July 23, 2016 - 03:55 PM

Checked it again.  Unit is out of the tractor.  Show short between left AC and B+.  Same when probes are reversed.  With engine running and B+ removed from the circuit, everything is fine except no charge.  As soon as I touch the wire to the B+ terminal it shorts out and kicks the breaker out.   Has to be the regulator/rectifier unit as far as I can tell.



#12 KennyP ONLINE  

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Posted July 23, 2016 - 05:04 PM

Sounds like it to me!



#13 chieffan ONLINE  

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Posted July 23, 2016 - 05:39 PM

Have a replacement ordered.  Be here Tuesday morning.  I get the day off tomorrow, for a while anyway.  No parts to work on the tractor.  My start sanding on the sheet metal and get it ready for painting - when this humidity gets out of here.






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