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Author Topic: Urgent help - Bus won't start  (Read 1013 times)
paul102a3
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« on: June 10, 2009, 10:25:51 AM »

Went to start the bus today after it was sitting for 3 weeks and starting batteries were completely dead. When I say dead, I mean 0.7 volts on one battery and -0.2 volts on the other. Don't ask me how one ended up with reversed polarity, that is a good question for another post.

I replaced them with 2 new 8Ds and now the motor will not start.

The NOTGEN/DO NOT SHIFT light flashes continuesly when I turn the key to the on position which I don't remember it doing that (I thought it just stayed on until the bus aired up but I may be mistaken). When I engage the starter the  motor spins over with no problems but no fire in the hole.

The motor is a 1988 8V92 DDEC II with an Allison trans. The battery configuration is 2 8Ds hooked in series with a Vanner equalizer.

I have no idea how long the batteries were discharged but does this affect the DDEC and does it need to be reset?

Any help would be greatly appreciated as we were planning a trip on Friday.

Thanks

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2001 Prevost XL II
paul102a3
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« Reply #1 on: June 10, 2009, 11:20:53 AM »

Bus won't start - Update

Panic attack is over. Dummy me forgot to connect hot lead to ECM. I must need new glasses LOL.
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2001 Prevost XL II
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« Reply #2 on: June 10, 2009, 11:26:40 AM »

I knew there had to be a computer in the wood pile.
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RTS/Daytona
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« Reply #3 on: June 10, 2009, 11:53:59 AM »

Paul

When you turn the ignition "ON" the DDEC's "CHECK ENGINE" wlll flash on for few seconds - or stay on if there is a active fault.

bet that didn't happen - ask me how I know

Been three / done that / bought the T-shirt

Pete RTS/Daytona
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Don4107
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« Reply #4 on: June 10, 2009, 01:09:21 PM »

"Don't ask me how one ended up with reversed polarity, that is a good question for another post."

This happens because each battery or cell has a different capacity than the others it is in series with.  When the lower capacity one is discharge to zero volts and there is still current draw the now dead battery/cell is reverse charged by the those that still have some capacity left. 

If you draw out a series of cells, +/- +/- +/-, then assume the one in the middle has reached zero volts, you can see that the positive lead is connected to a negative point and the negative lead of the dead cell is connected to a positive point.  Continued current draw will thus reverse charge the middle cell.

This is what prematurely kills many battery packs.  When you continue to try to drive that last screw with your cordless drill until the chuck won't turn at all, you have just subjected most of the cells to reverse charging. Stop using the battery as soon as you notice it losing power as this is when the lowest capacity cells are going to zero volts. 

This also contributes to the death of lead acid batteries when they are run dead.  The lower capacity cells are reverse charged as they individually die, just as the battery you measure negative voltage on had less capacity/charge than the other. If you could measure each cell of both batteries you would probably find some positive and some negative.

Don 4107
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Don 4107 Eastern Washington
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paul102a3
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« Reply #5 on: June 10, 2009, 01:54:37 PM »

Glad to hear others have similar issues. Makes me feel right at home.

FWIW, the story goes like this. Bus has two shore power connections, one in the front of the bus and the other at the rear. Toggle switch in the dash energizes a contactor to select fwd or rear inputs. The coil in the contactor is 24 volts so creates a draw on the main batteries any time you want 120/240 house power.

Dummy goes away, leaves one of the HVAC units in dehumidify cycle, forgets to turn on battery charger and comes home to find two very dead batteries.

This was an expensive lesson and one that I will not soon forget.

I appreciate the explanation of how a single battery cell can become so discharged it creates a reverse polarity.

Thanks.
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2001 Prevost XL II
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