Stereo isolator

illking · 10-22-2015, 04:36 PM

I was reading the manual for my used SMB I just got. It mentions a switch that would change the stereo to house battery while parked at camp.

I cant find the switch anywhere and am assuming its an option that wasnt installed on my van. Is this a hard thing to set up? My van has amp and subwoofer so it might be too much drain on the house battery anyway, but im new to all this so I thought id ask the experts. For now ill just use my Jawbone Jambox while camped.

**daveb** · 10-22-2015, 05:24 PM

It is usually located by the water pump switch but with each build it may be different. With it “on”, the vehicles key does not have to be on. The way it works (IIRC) is SMB lets the stereo run off the starting battery like any normal vehicle but when parked you can activate the radio switch so the stereo runs off the house battery system. The way it switches is by activating a relay but the relay is powered off the starting system. So most of the amperage that is drawn by the stereo does get rerouted, but there is still a slight draw off the starting battery system. The problem is when you forget to turn it off and the vehicle sits for long periods of time. The slight draw can actually drain both battery systems.
At least that is what I’ve been told concerning my 2006 build. One note is that I have stereo amps that always run off the starting batteries. So no matter how I have it switched I watch not pulling my starting batteries too low while cranking out the tunes. I don’t know if SMB’s built before or after mine was done are configured the same as mine.

Scalf77 · 10-22-2015, 06:23 PM

Pretty much as Dave said, the relay should be powered by the house battery if done correctly. I think we were at the mercy of the person wiring the van. I have attached a diagram of the switch and with a mod to pull all power from the house.

greg

**daveb** · 10-22-2015, 06:38 PM

Very possible Greg, I just went off what SMB told me way back when. So it's possible the relay is wired off the house. I do know some have claimed to see a draw off the starting battery system that was offering up a lot of confusion over the years. Some of these new stereos have multiple power inputs to run clocks and presets and I don't know how those are wired. Being my main primary amp isn't wired through the relay anyway it doesn't matter in my case. I actually added a switch to my secondary sub-woofer amp because when outside I don't really need it and when inside using the stereo as a surround sound, the sub woofer amp supplies more bass than I like. The switch for that amp I added works out well for me so I keep the current draw down and also so I don't have to mess with the bass and sub woofer level controls on the stereo.

Scalf77 · 10-22-2015, 06:59 PM

Dave,

I am just giving the designer of the circuit the benefit of the doubt. If the intent of the circuit was to switch power to the house battery, it would be kind of senseless to have the relay power coming off of the starting battery to make that happen. I agree that many people had issue's with it being "wrong" and by that I mean how I would do it. I modified mine early on so I can't really tell you was it was originally wired.

Yes the other draw could have been from the secondary power source.

-greg

**daveb** · 10-23-2015, 10:35 AM

I agree Greg. You'd think all the load would come off a single source. In all honesty I don't know if SMB's sales people really knew how it was wired. I got a lot of double talk about it.

illking · 10-23-2015, 12:50 PM

Thanks guys, yah it does not seem to have been an added option on my van. Im horrible with electronics so maybe when i upgrade my stereo receiver ill have that relay wired in by a professional or any local SMB'ers in los angeles that are good with wiring.

boywonder · 10-25-2015, 09:08 PM

Here's how I wired my system to run off of house batteries when parked.....really simple....

The power amp gets its power from the house batteries.

The switched head unit power is supplied from a single pole double throw rocker switch. The center terminal of the switch is connected to the radio switched power wire. The other two switch terminals are connected as follows: one terminal to the switched power wire in the factory dash harness and the other switch terminal to the house batteries (with proper fusing)

With the switch in one position, the regular radio wiring harness is providing switched power to the head unit and in the other position the house batteries provide the power.

The blue remote wire from the head unit turns on the amp when the head unit is on.......in my case the amp is always powered by the house batteries.

The wires going to the head unit are tiny gauge, so no need for a relay.....

Viva · 10-26-2015, 01:00 AM

That sounds slick. Is the "always on" wire that powers the presets/memory and etc. on the start battery? Or?

JWA · 10-26-2015, 04:12 AM

Quote:

Originally Posted by Viva

That sounds slick. Is the "always on" wire that powers the presets/memory and etc. on the start battery? Or?

Most automotive-type radios have two power connections, one switched and one constant to the head unit which keeps memory and pre-sets alive.