View Full Version : Thor Sprinter battery/isolator

09-23-2015, 11:00 PM
I have a 2015 Siesta Sprinter, I left the lights on and killed the battery.

From page 96 in the 2015 siesta sprinter motorhomes owner's manual:

4. In the event the automotive battery is discharged, it will benecessary to press and hold the Start Switch located in the dash.

There is no switch..called Thor ,they say MB will not let the put in switch.
(generic owners manual is wrong)
I asked where the isolator was and was told it was under the hood..NOT
I think it under the drivers seat but can't look now. It should have a by-pass button.

Does any one know where the isolator is, and can it be wired to use the house batteries to start the sprinter? (I have cables and got a nice RV'r to help me jump it)

Also does anyone with a Sprinter have a switch than enables the house batteries?

Thanks in advance

09-23-2015, 11:23 PM
Also does anyone with a Sprinter have a switch than enables the house batteries?

I do--added it myself. Assuming you have a simple 3-wire (HOUSE/COIL/CHASSIS) isolation relay, all you need to do is add a switch that connects together HOUSE and COIL. I used a pushbutton switch rather than a toggle. Pretty straightforward. Just make sure the wires to the isolator are of adequate gauge to handle the start current (the battery wires, that is. The wires to the switch don't have to be big.)

Note also, that even without a switch, you can use one of the clips of your jumper cables to short HOUSE to CHASSIS.

01-18-2016, 12:41 AM
Stevehar or Avanti,

My Synergy doesn't have the battery boost switch either but I've seen Winnabegos (videos and forums for the Views or Navions) that have the switch on the dash below the radio and climate controls. Is the switch really that big of an issue for Thor and does Jayco or Forest River do one compared to another?

Also, is it possible to get a model-specific schematic for my Synergy or is Thor protective of that info? I have a wiring diagram for the TT24 that looks more like something an architect would draw up for a set of house plans. That is still good info but I would really like the schematics in conjunction with the wiring diagram. Besides other benefits I could then compare what Winnabego does and perhaps incorporate the design for my rig - once it's out of warranty.

It's good to see some forum activity for the Thor Sprinters and I hope to learn a bunch from more experienced owners.


01-18-2016, 12:53 AM
it is not that the switch itself is such a big deal. I think the upfitters who choose not to do it are worried about various failure modes. For example, suppose the chassis battery is completely dead and the user simply holds down the switch and starts cranking. For this to work safely, all the interconnect wiring and its fuses need to have the capacity to handle full cranking currents for extended periods. Not a trivial requirement, so they just don't bother.

But, as an informed end user, we can add the switch easily and then use common sense in using it. E.g., only use it as a brief "boost" for a weak battery, or use it to add some charge to the chassis battery, rather than as a crank current source.

01-18-2016, 01:06 AM
Thanks Avanti. I think the Winnabego prints are available online and I can do a bit of researching the matter to see what components I need to get and the proper precautions to take for a safe installation. Like all RV's ours is a work in progress!

01-18-2016, 07:53 PM
I don't know where the isolator and relay is on the Synergy, on my Siesta is it under the drivers seat. Thor crammed alot of wires there. They should have used the space under the passenger seat. You need external torx to remove the seat. I called Thor and requested the chassis wiring diagram which shows wire colors and lengths.
I am going to wait until the warranty is up before I permanently add a momentary switch, for now it is just a piece of wire....here is some info



01-18-2016, 08:20 PM
The Airstream Interstates, like mine, have had a "Battery Boost" switch since 2012 models. It is a momentary switch that forces a connection between the house and Sprinter starter batteries. This is done through the Battery Isolation Manager (Control & Relay), made by Precision Citcuits Inc. Mine has the 160A unit seen at this web page:

The BIM relay is located in the rear of my van under the lounge seat/bed. Here is copy of the manual.