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Old 12-03-2017, 04:26 PM   #1
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Mystery switch #1: at starter battery

Vehicle is a 2002 E350 7.3L built by SMB Austin. As I recall, during the purchase handoff, the original owner mentioned to me that they use the pictured switch when plugged into to shore power. Turning this switch to 'off' was critical enough that they had rigged their steering wheel with a "remove before flight" banner to remind them to turn that switch off before hitting the road. Best I can tell in tracing all the wires is that it connects the house battery side of the isolator to the positive terminal on the starter battery. I've modified the diagram below with green wiring to show the path. SMB Fresno, when I was there last, said it not a SMB factory switch and suggested that I do not use it. The switch is black and bottom center of photo. The positive lead connection is the small terminal clamp. Wiring is light gauge. Does anyone know more about what this switch is for?
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Old 12-03-2017, 04:44 PM   #2
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I'm looking forward to seeing what the electrical geniuses on the site have to say, but here are my thoughts:

1. This is not a standard setup.

2. The key to understanding the intention behind this approach is that you have a battery isolator, not a battery separator.

3. When you are plugged into shore power, your house batteries take a charge from your charger, but the isolator prevents current from reaching your chassis battery.

4. Whomever installed this isolator bypass decided that they wanted to "trickle charge" your chassis battery when the van was plugged into shore power. Consequently, they installed light gauge wire between the house side and the chassis side, with the idea that charge would trickle over to the chassis side when the switch is turned on.

5. I'm guessing that this was done because the chassis battery was getting weak and/or the van was not driven regularly, and the previous owner figured a little bit of wire and a switch would be cheaper than a new battery and/or keeping the chassis battery charged.

6. I do not see this as a good solution. If you want to be able to send charge to your chassis battery when you are plugged into shore power, you can replace your isolator with a separator.
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Old 12-04-2017, 06:59 AM   #3
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Probably a cheap get around to charge the chassis battery as glider mentioned. Leaving it on while camping could result in dead starting and coach batteries by the morning if left in the on position and you had some nighttime loads. SMB quit using isolators...they only allow current to travel in one direction (alternator to coach battery) so the shore charger can't charge the starting battery. If the shore charger is a high amp model and you have a low or weak starting battery, the switch and wire needs to be rated for the amps the charger can deliver.
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Old 12-04-2017, 09:45 AM   #4
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I agree with all of the above, and this should be removed as soon as possible.

-greg
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Old 12-04-2017, 10:38 AM   #5
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I agree with all of the above, and this should be removed as soon as possible.

-greg
I have installed a similar setup. I have a relay that closes when it senses ignition voltage, combining both house and start batteries. When the engine is off, the relay is open, preventing my solar from connecting to the start battery. I didn't use a switch though, just a fused wire. When the fuse is installed, it allows my solar panels to charge my start battery. The only time I install the fuse is when I'll be gone from the van for extended periods of time, as my alarm system is wired to the start battery and will discharge it after a couple weeks. I'd say you could leave it as is, with the switch open, it's doing nothing but could prove useful some day.
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Old 12-04-2017, 11:06 AM   #6
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I agree with all of the above, and this should be removed as soon as possible.

-greg
I'll second this. This is a good idea to discuss "custom" wiring as many vans are changing hands these days. I was just lucky when I bought my van when I had a similar but different electric circuit melt down. It didn't start a fire but did fill the cab with smoke and panic. In my case the previous owner had several stereo shops do work and somebody installed several hidden relays and crossed up the rear stereo switch circuits. When I got the van the previous owner failed to inform me ( maybe he didn't even know) that you can not have the key on if the back stereo switch was in house battery position. After a few months of owning the van I unknowingly must have did this causing the alternator to try to charge the house battery through the small wire of the stereo switch and said relays. After I cleaned my pants I ripped it all out and installed something more simple and idiot proof. Don't keep any sketchy wiring from the first owner. There is a lot of info here in the forum to have reliable and safe electrical in your van.

- Eric

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Old 12-04-2017, 11:31 AM   #7
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Eric, that sounds scary as hell. No doubt the wires you fried didn't have a properly sized fuse installed. My set up is fused at both ends so there is no way the wire can overheat. A fuse sized properly for the ampacity of the wire will blow prior to the wire overheating. In the OP's photo, it looks like the wire is fused, and if so, it should be safe (assuming the fuse is properly sized that is). Still, tons of folks install sub-standard wiring, so any suspect circuits should be inspected closely or removed. Ramsey's van once fried a bunch of wires that started the paneling on fire, and it was factory wiring. I suspect someone may have installed the wrong fuse.
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Old 12-04-2017, 10:18 PM   #8
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I have installed a similar setup. I have a relay that closes when it senses ignition voltage, combining both house and start batteries. When the engine is off, the relay is open, preventing my solar from connecting to the start battery. I didn't use a switch though, just a fused wire. When the fuse is installed, it allows my solar panels to charge my start battery. The only time I install the fuse is when I'll be gone from the van for extended periods of time, as my alarm system is wired to the start battery and will discharge it after a couple weeks. I'd say you could leave it as is, with the switch open, it's doing nothing but could prove useful some day.
While I don't necessarily disagree with the intent of the OP's wiring change, I would say that it was poorly implemented at best. Using a clamp on terminal is fraught with issues, the wire for the fuse holder is butt joined with a different size wire. I also suspect that the switch itself is specked lower than the fuse or wire sized used.

In general, I would say any wiring change that you have to rely on the circuit protection, in the non specified usage model should be rethought.

-greg
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Old 12-05-2017, 09:06 AM   #9
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While I don't necessarily disagree with the intent of the OP's wiring change, I would say that it was poorly implemented at best. Using a clamp on terminal is fraught with issues, the wire for the fuse holder is butt joined with a different size wire. I also suspect that the switch itself is specked lower than the fuse or wire sized used.

In general, I would say any wiring change that you have to rely on the circuit protection, in the non specified usage model should be rethought.

-greg
Wow, I didn't notice that at first, but pretty poor idea for sure.
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