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Author Topic: No 12V Accessory power in front Junction box MCI 102A2  (Read 1986 times)
TNT
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« on: October 07, 2009, 11:32:47 AM »

  I have no 12V power in my front junction box. and hence no windshield washer .or accessory 12v power  from what I read in the book (spec 102 section 7 ) it should be on stud 55 but no power  so I traced it back to the ac J/b.. I have  12v coming from battery  it goes to the relay 12v relay.  when you turn on the master switch on it doesn;t energize the relay to put the 12v on the out going stud but if it use a piece of wire and bridge it from stud #10 ( that is where it is suppose to be fed from and is 24v)   to #2 on the relay, it energizes the relay and closes the contact. but still no juice on stud 55 in the F J/b...

  Now if it take yet another "jumper" and go from the out side of relay  to stud 6 in the AC  J/B then it all works. I have 12v Power on 55 Fnt J/B .. My problem is why would this not be already working? and could I hurt/burn  up  anything by using these jumpers.  There are other wires on stud 6 AC J/B.

I hope someone can help. Thanks Rob
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Ob1quixote
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« Reply #1 on: October 07, 2009, 03:27:36 PM »

I have no experience with bus electrical, but judging by your description, you have a 24V control circuit controlling the 12V accessories, you have 2 problems. 1st, you have a bad relay. 2nd, you have a problem somewhere in the output side of the master switch.

Solve the master switch output problem {or work around it} and replace the relay. If you want to tap into another circuit, test its normal load with an ammeter, and test the circuit you want to add and see if the additional load wont overload that circuit.
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bevans6
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« Reply #2 on: October 07, 2009, 03:56:53 PM »

on my earlier MCI, the 12 volt feed goes from the "lower" battery connected to the Vanner, through a fuse and up to the stud in the front panel directly (via the AC junction box, but that's just a connection).  The result, not that it's good or bad, is that 12 volts is always energised as long as the battery is connected.  throwing the main disconnect in the front bay disconnects the +24 volt feed, but does nothing to interrupt the +12 volt feed.  Result - the 12 volt feed is unswitched, and there can be trace losses that discharge the lower battery, like the new radio I hooked up that drained the battery...

So having the 12 volt feed go through a solenoid that is controlled by the 24 volt main switch is probably an enhancement to what I have.  If you bypass the solenoid, effectively you have exactly my system, stock in 1980.  Your choice...

Brian
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1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
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TNT
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« Reply #3 on: October 07, 2009, 06:27:08 PM »

 Yes I believe that the 24v powering the relay is exactly like Brian said to make it so when the main power(24v) is switched off it kills the accessory (12v) too. 

The relay is good because when I use the jumper it works perfect . ( and I have a spare..lol. I thought that at first too) and the master switch seem good as well . because if i jump it from stud 10 ( the "feed" to the relay ) it works and it also kills the power to 10 if you shut off the master switch.  I'm not trying to bypass the relay , it works with the jumpers.

It looks like all I need to do is run 2 jumpers .. but I am afraid of burning something up. and I am not sure why it stopped working. We have only had the bus about 6 weeks.. Thanks for the suggestions!
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Dreamscape
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« Reply #4 on: October 08, 2009, 03:23:16 AM »

Run the jumper with an appropriate fuse feeding your device until you can figure it out.

Paul
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Ob1quixote
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« Reply #5 on: October 08, 2009, 03:38:43 AM »

Well, if I understand that you have 12V coming into the relay and no 12V coming out when the relay is energized with a jumper, sounds like a bad relay to me!
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bevans6
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« Reply #6 on: October 08, 2009, 05:44:53 AM »

What he's saying is that  he doesn't have 24 vdc signal on the relay to tell it to turn on the 12 vdc.  so  an open circuit from the 24 volt switched supply to the relay.

Brian
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1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
Spicer 8844 4 speed Zen meditation device
Vintage race cars -
1978 Lola T440 Formula Ford
1972 NTM MK-4 B/SR
TNT
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« Reply #7 on: October 08, 2009, 11:41:01 AM »

What he's saying is that  he doesn't have 24 vdc signal on the relay to tell it to turn on the 12 vdc.  so  an open circuit from the 24 volt switched supply to the relay.

Brian

EXACTLY!! Man, I wish I would have said it like that.. and I like the idea of  running the jumper where needed  and just using a in line fuse. But I still don;t understand why it would stop working in the first place. OK  maybe one wire could go bad but 2?( feed and distribution) the 24vdc feed from stud 10 and the distribution from relay to stud 6( that feeds 55 in FNT J/B ..)

Thanks again for the help so far Rob
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Busted Knuckle
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« Reply #8 on: October 08, 2009, 03:17:18 PM »

Quote from: TNT3343
But I still don;t understand why it would stop working in the first place. [b
OK  maybe one wire could go bad but 2?( feed and distribution) [/b]the 24vdc feed from stud 10 and the distribution from relay to stud 6( that feeds 55 in FNT J/B ..)

Thanks again for the help so far Rob

Rob,
Ever hear of mice? They don't always finish just 1 wire. Sometimes the next one over looks "greener" or tastes better! FWIW Grin  BK  Grin
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Chuck Newman
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« Reply #9 on: October 08, 2009, 07:00:58 PM »

Well, I think I have the information Rob needs for this problem, but I can't figure out how to post a .pdf file of a schematic.  I did read the Help section.  When I click the "Insert Image" icon I get img img in brackets.  What do I do with that?  Can someone please tell me how to do this?

Thanks,

Chuck
« Last Edit: October 08, 2009, 07:03:02 PM by Chuck Newman » Logged

1989 MCI 102A3, Series 50, DDEC III, Allison 740D
Ob1quixote
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« Reply #10 on: October 08, 2009, 07:24:01 PM »

Probably easier to email him the PDF.

But, if you hosted the file online at a host site {google "free file hosting}, then you could get a URL to post in a reply {dont bother with the thing}
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