House Battery won't run lights, etc. Converter does.

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tmhudg

Active member
Joined
Jul 16, 2006
Posts
34
Hello,

I'm new to the forum but I did a cursory search for this and didn't find anything to help. I appologize in advance for the length of this post but I want to fully explain my problem.

I have a 25' 97 Coachmen Class C on a Ford E350 chassis. Just before my last trip, I noticed that my fridge would not run when on battery power. I got a very faint "auto" light on the fridge but it wouldn't get cold and none of my DC lights would work. I have a Battery disconnect switch and that would light faintly but it would not "switch" i.e. it would not disconnect - the light would just stay on faintly. If I plugged in 120, everything worked fine. The fridge would come on, the DC lights would work and the battery disconnect switch would turn everything on or off as expected.

I checked the battery and it seemed to have a charge but the cells were low on water. I bought another battery anyway and tried it but still had the same problem. I limped along with the generator and shore power when available.

So I've been trying to figure out what is wrong by poking around with the VOM. The battery sits in a box under the inside step. The ground goes to the frame and after some serious scraping, I measured 12 V from the positive terminal to the frame so I think my ground is OK. The Plus wire however, makes a rather complicated journey that I hope you guys can help me with.

From the battery terminal it makes a short run to what looks like kind of a power distribution "panel" although panel is too kind. It first goes to a terminal which appears to just be a place to make another connection to what I assume is an isolator. The connection between this terminal and the isolator is via a strange, flat piece of metal with a square object in the middle. I'm guessing that this is some kind of fuse???? There are two large terminals on the "isolator" (I'll call it an isolator but I'm not really sure). On the same terminal that the battery connects to (via the "fuse") there is another fairly thick wire. On the other terminal of the isoloator is another fairly thick wire. I'm guessing that one of these is from the converter and one is from the truck battery but I'm not clear which one is which. I know that the isolator keeps the house load from draining the truck battery but I don't really understand how the converter DC power fits in with everything. Would the converter POS be on the same terminal of the isolator as the battery Plus?There is also a small black wire that connects to the base of the isolator. Is that the emergency start line that connects the house battery to the truck battery for extra starting power?

Anyway, I was trying to follow voltage from the Pos battery through the wires of the isolator and getting nothing. Everything was so rusted and corroded that I could not get any readings. I first tried to take the battery cable off that first terminal and proceeded to break the bolt off - great! I tried removing the cables from the isolator and they were also rusted on. I basically ended up destroying the isolator in an effort to remove the cables.

Soooo, first, do the symptoms sound like a problem with the isolator? Are my assumptions about connections correct? Any suggestions on where to get another isolator?

Thanks for any help,

Tom
 

Ned

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Feb 1, 2005
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The amount of corrosion you describe could certainly cause the problems you're having.  It's hard to say what the device is that you call an isolator, could you post a photo of it?  It may be a fuse or could be a shunt for a DC ammeter.  Do you have a battery monitoring system that tells you the current usage?  A photo of that would help too.
 

Gary RV_Wizard

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Feb 2, 2005
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74,369
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At our Silver Springs FL home
In all likelihood the panel & "isolater" you describe serve a couple of functions.  Isolation is one of them, but there should also be a relay (or two) that enables/prevents multiple DC charging sources from attempting to charge the batteries at the same time. For example, if you started the engine while still connected to shore power, the relay might let the coverter charge the batteries but prevent the engine alternator from doing so. This protects the vehicle 12V system from conflicts with the house 12V system.  There will also be a relay that engages when you push the Emergency Start switch, connecting the house batteries in parallel to the engine battery.

Yes, the converter "positive" will have either one or two lines connected there somewhere.  A large one feeds the house 12V system just like the battery does. There may be a smaller one just for charging that connects on the battery side of the isolation.  There are many different was to wire these things, depending on the design of the isolation & power management panel and the converter/charger itself, so I hesitate to guess further without being able to see the whole thing and map out the connections.

The metal strip and square thing is probably a fusible link, which is essentially a fuse. Like a fuse, it literally burns out if there is a short.

The massive corrosion is undoubtedly the root cause of your electrical problem. You will need replacement cables (or cable ends) and the thing you destroyed.
 

John From Detroit

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Joined
Apr 12, 2005
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25,114
Location
Davison Michigan
If you have that much corrousion it is possible the corrousing extends to the switch contacts in the battery control panel, if so it's toast

Here is how to test  This in in two parts,  Identical for one thing.. First part "A"

Hook a fairly long, wire (standard lamp cord will work) to the negative battery terminal, an allegator or battery clip can be used.  The other end of this wire goes to the negative lead on your VOM, Firmly attach and/or tape.

now go around to various parts of the system, starting with the positive battery terminal, Measure voltage 12 volt or more, good

Go to the control box, (Converter off please) still have 12 volt or more good

Make sure the converter is turned on and measure the "load side" of it, still 12 volt good

(NOTE: I very strongly suspect you won't have 12 volts here) 

If you ever loose 12 volts then the problem is between the last 2 test points

Final step.. >Move  the negative clip to the frame or to the negative bus on the 12 volt distribution panel.

Part b: Turn on some 12 volt lamps and re-do all tests (This is the test that finds corrusion)

Same procedure, same results. 

Finally... Any rusted connections, clean and polish or replace Make them all shiey
 

tmhudg

Active member
Joined
Jul 16, 2006
Posts
34
Thanks everyone for all the help,

I've attached pictures of the isolator, the panel, and the fuse and labeled some points for reference.

The first pic is of the fuse. "A" was attached to a stand-alone terminal which had the POS connection to the battery (essentially a straight shot to the battery). The B side of the fuse was connected to the isolator at A. Also on A of the isolator was A shown in the panel pic (you can't really see the loose end that was connected to the isolator), and B coming from somewhere... The cable marked C in the panel pic was connected to the B side of the isolator. The wire marked D was connected to the C point on the isolator. I know, I should have taken a picture before I took everything apart...

As you can see, I've got bad corrosion on the remaining terminals of the panel. I'm not sure if those are relays under there nor what purpose they are serving. I probably also should find out where those other cables are going. I was kind of thinking that C and B were from the converter and alternator (not sure which was which however) and the two black cables with blue covers were running off to power lights, etc.

Again, I appreciate you guys taking the time to help me figure out what to do,

Tom
 

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Ned

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From the pictures, I'd say most of your problems are the corrosion.  What you're calling the isolator may actuall be the aux relay to bridge the house and chassis batteries for starting when the chassis batteries won't turn the engine over.  That would make terminal C the hot side of the relay coil with cable D going to the aux start switch.  That would mean cable C goes to the chassis battery as A goes to the house battery via the fuse.

I would definitely start by clean all of the terminals and testing the relay (if that's what it really is) before replacing any parts.  Of course, all this is guesswork based on your statements and the photos.  If you can trace any of the wires back to their source, it would probably resolve the problem.  If you can do that, be sure to draw a schematic so you don't have to do it the next time :)
 
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