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Old 05-03-2021, 05:51 PM   #1
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Bounder 35k starts without chassis battery hooked up?

I changed the 2-6 volt house batteries and the 12 volt chassis battery today and while checking for stray currents without the ground cable connected on the chassis battery, i was able to power up and start the motorhome. Why would the engine power up without the engine battery being connected? I didn't use the aux start switch. It was very odd to my thinking.
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Old 05-03-2021, 05:56 PM   #2
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Odd indeed. By “powered up” do you mean it started or just the dash lit up ?

IF it tried to start the motor, I would think the battery boost relay / isolator may be stuck engaged even with out the switch being depressed.
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Old 05-03-2021, 09:28 PM   #3
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I would check that someone hasn't moved both cables to the same side of the relay as a "temporary" repair and left it at that. Either that or the solenoid is frozen in the connected position.
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Old 05-04-2021, 04:35 AM   #4
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You said you removed the ground wire not the positive. In most motorhomes both chassis and house (coach/aux) batteries are ground to the actual chassis of the vehicle - which means you never have disconnected the ground unless both sets of batteries are disconnected. So your chassis battery just sent the ground to the house batteries and the vehicle started.
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Old 05-04-2021, 06:56 AM   #5
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Did you have shore power plugged in or the generator running ?

With many systems, the house battery charger also charges the chassis battery. That is done by a controller that sends a signal to the same solenoid as the Boost / Aux start switch does.

You started your engine thru the Bi-directional Isolatior Relay, another name for Boost solenoid.
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Old 05-04-2021, 01:15 PM   #6
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I received the battery wiring diagram and see how the two banks are connected at the Battery Control Center. I can see how the starting system would still be grounded through the house battery bank and the engine would actually be started on those batteries. (I think) I also read somewhere that as long as both banks are fully charged the BCC joins the two. Once the chassis battery voltage drops, it opens the circuit to ensure the house batteries do not loose charge. Since I just replaced all batteries, they were at 100% charge. This makes a bit of sense but have no idea if it is true.
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Old 05-04-2021, 01:25 PM   #7
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Twinboat: Thanks, I can't recall if I had it plugged into shore or not. That makes sense though.
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Old 05-04-2021, 02:12 PM   #8
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Quote:
Originally Posted by sandwip View Post
I would check that someone hasn't moved both cables to the same side of the relay as a "temporary" repair and left it at that. Either that or the solenoid is frozen in the connected position.
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Old 05-04-2021, 05:11 PM   #9
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Quote:
Originally Posted by edit4ever View Post
You said you removed the ground wire not the positive. In most motorhomes both chassis and house (coach/aux) batteries are ground to the actual chassis of the vehicle - which means you never have disconnected the ground unless both sets of batteries are disconnected. So your chassis battery just sent the ground to the house batteries and the vehicle started.
That is impossible. Look up the definition of " electrical circuit".
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