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Old 11-14-2011, 07:48 PM   #1
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What side of battery to isolate on chassis battery

Dealer just replaced my chassis batteries and talked me into placing a disconnect switch on the chassis battery. However he placed it on the ground side. Seems backwards to me. I think the disconnect should be on the positive terminal. I now have my cockpit instrumentation lights on when my chassis and house batteries are disconnected. I also have terrible static on my cameras.

Hew are Newmar chassis batteries normally isolated?
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Old 11-14-2011, 10:01 PM   #2
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Chassis battery, disconnect on my 99 DSDP,on a freightliner chassis, is a ground side disconnect, installed by freightliner, and refered to in the wiring diagrams as a mechanic's safety switch . Primary use is for the mechanic to disconnect the batteries when servicing the engine or trans. Ground side interupt is commom, because the load on the ground wiring is less, than the load on the positive cables, and re -connecting a ground produces less arcing. So the amp rating can be lower for the switch and it will last longer. All instructions for servicing a battery say to disconnect the ground wire first. Contact the installer on your two problems , they may have missed a ground wire , lights still work, or made a poor connection, static in B/U camera.
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Old 11-14-2011, 10:17 PM   #3
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Hmm, sounds strange to me but........ I have always been taught that you disconnect on the hot side, upstream of the components. I wonder if they install a light switch down stream of the light. JMHO. Your lights and camera are grounded to the chassis and so is your chassis battery.
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Old 11-15-2011, 04:39 AM   #4
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Hi Vegasguzzi,
Does your coach have a "store" button/switch? If so, what was the reason for installing the new switch? Are you trying to solve a problem by installing the switch?
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Old 11-15-2011, 07:10 AM   #5
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Wasn't there already a switch or circuit breaker for the chassis batteries? I thought all the Newmar diesel pushers had them.
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Old 11-15-2011, 08:35 AM   #6
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...because the load on the ground wiring is less, than the load on the positive cables...
Just an important point of clarification, pretty sure the physics books will NOT agree with this statement... The load on the positive will (has to) equal the load on the negative. (Aside from trivial internal leakage.)
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Old 11-16-2011, 08:00 PM   #7
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Dealer told me they suspected leakage from the chassis batteries. My Ventana has an isolation for the house battery. That's why it is so curious to isolate the positive for the house batteries and the negative side for the chassis. If my engineering classes memory is correct this sets up a "floating" dc system and would explain why my instrument cluster is still energized with both house and chassis batteries in the storage mode. Just wondering if this was standard on Newmars
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Old 11-16-2011, 08:19 PM   #8
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If my engineering classes memory is correct this sets up a "floating" dc system and would explain why my instrument cluster is still energized with both house and chassis batteries in the storage mode.
Your memory is serving you correctly. Wish mine would.
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Old 11-16-2011, 09:23 PM   #9
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Sounds like your dealer needs to fix something.
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Old 11-16-2011, 09:38 PM   #10
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Hmm, sounds strange to me but........ I have always been taught that you disconnect on the hot side, upstream of the components. I wonder if they install a light switch down stream of the light. JMHO. Your lights and camera are grounded to the chassis and so is your chassis battery.
As an experiment , USING CAUTION of course, disconnect the positve battery cable, leaving the ground cable hooked up. Then lay a wrench across from the frame to the positive battery terminal. Then with the positive terminal hooked up disconnect the ground cable , then lay a wrench across from the frame to the positive,or negative, terminal. Compare results.
In a 12 volt system a switch could be installed on either side of a load to complete the circuit, but because most load/lights are grounded directly to the frame, not wired back to the battery, the switch is installed in the power wire.
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Old 11-16-2011, 10:04 PM   #11
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Just an important point of clarification, pretty sure the physics books will NOT agree with this statement... The load on the positive will (has to) equal the load on the negative. (Aside from trivial internal leakage.)
Perhaps, not the best discription on my part , I never studied physics, just auto mechanics. In almost every vehicle I ever worked on the ground cable was smaller than the power cable, in high load situations; engine cranking for example ; the positive cable is subjected to more heat, than the ground. Could we agree on that ?
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Old 11-17-2011, 07:38 AM   #12
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What goes in must come out - period. Either in the form of power and/or heat etc.. The longer dedicated positive starter cord may explain why its bigger vs the many ground connections and their shorter lengths.
Just my opinion.
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Old 11-17-2011, 07:49 AM   #13
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Quote:
Originally Posted by Skip426 View Post
As an experiment , USING CAUTION of course, disconnect the positve battery cable, leaving the ground cable hooked up. Then lay a wrench across from the frame to the positive battery terminal. Then with the positive terminal hooked up disconnect the ground cable , then lay a wrench across from the frame to the positive,or negative, terminal. Compare results.
In a 12 volt system a switch could be installed on either side of a load to complete the circuit, but because most load/lights are grounded directly to the frame, not wired back to the battery, the switch is installed in the power wire.
Unless I have misunderstood you, and that of course is possible, DO NOT UNDER ANY CURCUMSTANCES PERFORM THIS TEST. As I am reading it with the ground still connected, you are saying lay a spanner from the chassis (ie the ground) to the positive terminal. Every body take cover.
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Old 11-17-2011, 08:27 AM   #14
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For my coach the separation of chassis 12 VDC and coach 12 VDC is a great theory, but in practice is not true. I can turn off either the coach store or the chassis kill switch and there will be some (small amount) of VDC on the side I turned off. I'm not sure if this the case on other coaches, but this is how my coach is. Because my coach is used year around this mystery has never made it to the top of my to do list.
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